[v5,5/6] media: subdev: add v4l2_subdev_call_state_active()

  Add v4l2_subdev_call_state_active() macro to help calling subdev ops
that take a subdev state as a parameter. Normally the v4l2 framework
will lock and pass the correct subdev state to the subdev ops, but there
are legacy situations where this is not the case (e.g. non-MC video
device driver calling set_fmt in a source subdev).

As this macro is only needed for legacy use cases, the macro is added in
a new header file, v4l2-subdev-legacy.h.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
---
 include/media/v4l2-subdev-legacy.h | 42 ++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)
 create mode 100644 include/media/v4l2-subdev-legacy.h
  

Hi Tomi,

Thank you for the patch.

On Tue, Mar 01, 2022 at 12:55:47PM +0200, Tomi Valkeinen wrote:
> Add v4l2_subdev_call_state_active() macro to help calling subdev ops
> that take a subdev state as a parameter. Normally the v4l2 framework
> will lock and pass the correct subdev state to the subdev ops, but there
> are legacy situations where this is not the case (e.g. non-MC video
> device driver calling set_fmt in a source subdev).
> 
> As this macro is only needed for legacy use cases, the macro is added in
> a new header file, v4l2-subdev-legacy.h.
> 
> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
> ---
>  include/media/v4l2-subdev-legacy.h | 42 ++++++++++++++++++++++++++++++
>  1 file changed, 42 insertions(+)
>  create mode 100644 include/media/v4l2-subdev-legacy.h
> 
> diff --git a/include/media/v4l2-subdev-legacy.h b/include/media/v4l2-subdev-legacy.h
> new file mode 100644
> index 000000000000..6a61e579b629
> --- /dev/null
> +++ b/include/media/v4l2-subdev-legacy.h
> @@ -0,0 +1,42 @@
> +/* SPDX-License-Identifier: GPL-2.0-or-later */
> +/*
> + *  V4L2 sub-device legacy support header.
> + *
> + *  Copyright (C) 2022  Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
> + */
> +
> +#ifndef _V4L2_SUBDEV_LEGACY_H
> +#define _V4L2_SUBDEV_LEGACY_H
> +
> +/**
> + * v4l2_subdev_call_state_active - call an operation of a v4l2_subdev which
> + *				   takes state as a parameter, passing the
> + *				   subdev its active state.
> + *
> + * @sd: pointer to the &struct v4l2_subdev
> + * @o: name of the element at &struct v4l2_subdev_ops that contains @f.
> + *     Each element there groups a set of callbacks functions.
> + * @f: callback function to be called.
> + *     The callback functions are defined in groups, according to
> + *     each element at &struct v4l2_subdev_ops.
> + * @args: arguments for @f.
> + *
> + * This is similar to v4l2_subdev_call(), except that this version can only be
> + * used for ops that take a subdev state as a parameter. The macro will get the
> + * active state and lock it before calling the op, and unlock it after the

s/active state and/active state,/

> + * call.
> + */
> +#define v4l2_subdev_call_state_active(sd, o, f, args...)		\
> +	({								\
> +		int __result;						\
> +		struct v4l2_subdev_state *state;			\
> +		state = v4l2_subdev_get_active_state(sd);		\
> +		if (state)						\
> +			v4l2_subdev_lock_state(state);			\
> +		__result = v4l2_subdev_call(sd, o, f, state, ##args);	\
> +		if (state)						\
> +			v4l2_subdev_unlock_state(state);		\
> +		__result;						\
> +	})
> +
> +#endif

I think

#endif /* _V4L2_SUBDEV_LEGACY_H */

is the usual pattern. Apart from that,

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
  

Moi,

On Wed, Mar 02, 2022 at 11:36:25AM +0200, Laurent Pinchart wrote:
> Hi Tomi,
> 
> Thank you for the patch.
> 
> On Tue, Mar 01, 2022 at 12:55:47PM +0200, Tomi Valkeinen wrote:
> > Add v4l2_subdev_call_state_active() macro to help calling subdev ops
> > that take a subdev state as a parameter. Normally the v4l2 framework
> > will lock and pass the correct subdev state to the subdev ops, but there
> > are legacy situations where this is not the case (e.g. non-MC video
> > device driver calling set_fmt in a source subdev).
> > 
> > As this macro is only needed for legacy use cases, the macro is added in
> > a new header file, v4l2-subdev-legacy.h.
> > 
> > Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
> > ---
> >  include/media/v4l2-subdev-legacy.h | 42 ++++++++++++++++++++++++++++++
> >  1 file changed, 42 insertions(+)
> >  create mode 100644 include/media/v4l2-subdev-legacy.h
> > 
> > diff --git a/include/media/v4l2-subdev-legacy.h b/include/media/v4l2-subdev-legacy.h
> > new file mode 100644
> > index 000000000000..6a61e579b629
> > --- /dev/null
> > +++ b/include/media/v4l2-subdev-legacy.h
> > @@ -0,0 +1,42 @@
> > +/* SPDX-License-Identifier: GPL-2.0-or-later */
> > +/*
> > + *  V4L2 sub-device legacy support header.
> > + *
> > + *  Copyright (C) 2022  Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
> > + */
> > +
> > +#ifndef _V4L2_SUBDEV_LEGACY_H
> > +#define _V4L2_SUBDEV_LEGACY_H
> > +
> > +/**
> > + * v4l2_subdev_call_state_active - call an operation of a v4l2_subdev which
> > + *				   takes state as a parameter, passing the
> > + *				   subdev its active state.
> > + *
> > + * @sd: pointer to the &struct v4l2_subdev
> > + * @o: name of the element at &struct v4l2_subdev_ops that contains @f.
> > + *     Each element there groups a set of callbacks functions.
> > + * @f: callback function to be called.
> > + *     The callback functions are defined in groups, according to
> > + *     each element at &struct v4l2_subdev_ops.
> > + * @args: arguments for @f.
> > + *
> > + * This is similar to v4l2_subdev_call(), except that this version can only be
> > + * used for ops that take a subdev state as a parameter. The macro will get the
> > + * active state and lock it before calling the op, and unlock it after the
> 
> s/active state and/active state,/
> 
> > + * call.
> > + */
> > +#define v4l2_subdev_call_state_active(sd, o, f, args...)		\
> > +	({								\
> > +		int __result;						\
> > +		struct v4l2_subdev_state *state;			\
> > +		state = v4l2_subdev_get_active_state(sd);		\
> > +		if (state)						\
> > +			v4l2_subdev_lock_state(state);			\
> > +		__result = v4l2_subdev_call(sd, o, f, state, ##args);	\
> > +		if (state)						\
> > +			v4l2_subdev_unlock_state(state);		\
> > +		__result;						\
> > +	})
> > +
> > +#endif
> 
> I think
> 
> #endif /* _V4L2_SUBDEV_LEGACY_H */

I'll do these (plus the code block type change) when applying the patches
if there are no other comments to the series.

> 
> is the usual pattern. Apart from that,
> 
> Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

Thanks!
  

Hi Tomi,

On 3/1/22 11:55, Tomi Valkeinen wrote:
> Add v4l2_subdev_call_state_active() macro to help calling subdev ops
> that take a subdev state as a parameter. Normally the v4l2 framework
> will lock and pass the correct subdev state to the subdev ops, but there
> are legacy situations where this is not the case (e.g. non-MC video
> device driver calling set_fmt in a source subdev).
> 
> As this macro is only needed for legacy use cases, the macro is added in
> a new header file, v4l2-subdev-legacy.h.
> 
> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
> ---
>  include/media/v4l2-subdev-legacy.h | 42 ++++++++++++++++++++++++++++++
>  1 file changed, 42 insertions(+)
>  create mode 100644 include/media/v4l2-subdev-legacy.h
> 
> diff --git a/include/media/v4l2-subdev-legacy.h b/include/media/v4l2-subdev-legacy.h
> new file mode 100644
> index 000000000000..6a61e579b629
> --- /dev/null
> +++ b/include/media/v4l2-subdev-legacy.h
> @@ -0,0 +1,42 @@
> +/* SPDX-License-Identifier: GPL-2.0-or-later */
> +/*
> + *  V4L2 sub-device legacy support header.
> + *
> + *  Copyright (C) 2022  Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
> + */
> +
> +#ifndef _V4L2_SUBDEV_LEGACY_H
> +#define _V4L2_SUBDEV_LEGACY_H
> +
> +/**
> + * v4l2_subdev_call_state_active - call an operation of a v4l2_subdev which
> + *				   takes state as a parameter, passing the
> + *				   subdev its active state.
> + *
> + * @sd: pointer to the &struct v4l2_subdev
> + * @o: name of the element at &struct v4l2_subdev_ops that contains @f.
> + *     Each element there groups a set of callbacks functions.
> + * @f: callback function to be called.
> + *     The callback functions are defined in groups, according to
> + *     each element at &struct v4l2_subdev_ops.
> + * @args: arguments for @f.
> + *
> + * This is similar to v4l2_subdev_call(), except that this version can only be
> + * used for ops that take a subdev state as a parameter. The macro will get the
> + * active state and lock it before calling the op, and unlock it after the
> + * call.
> + */

You should explain why this is a legacy macro and, ideally, what would need to
be done to get rid of it. The first is in the commit log, but nobody reads that :-)

But if just using it in a non-MC video device driver constitutes 'legacy' use,
then I disagree with that. There are many non-MC video device drivers, nothing
legacy about that.

Regards,

	Hans

> +#define v4l2_subdev_call_state_active(sd, o, f, args...)		\
> +	({								\
> +		int __result;						\
> +		struct v4l2_subdev_state *state;			\
> +		state = v4l2_subdev_get_active_state(sd);		\
> +		if (state)						\
> +			v4l2_subdev_lock_state(state);			\
> +		__result = v4l2_subdev_call(sd, o, f, state, ##args);	\
> +		if (state)						\
> +			v4l2_subdev_unlock_state(state);		\
> +		__result;						\
> +	})
> +
> +#endif
  

Hi Hans,

On 04/03/2022 15:34, Hans Verkuil wrote:
> Hi Tomi,
> 
> On 3/1/22 11:55, Tomi Valkeinen wrote:
>> Add v4l2_subdev_call_state_active() macro to help calling subdev ops
>> that take a subdev state as a parameter. Normally the v4l2 framework
>> will lock and pass the correct subdev state to the subdev ops, but there
>> are legacy situations where this is not the case (e.g. non-MC video
>> device driver calling set_fmt in a source subdev).
>>
>> As this macro is only needed for legacy use cases, the macro is added in
>> a new header file, v4l2-subdev-legacy.h.
>>
>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>> ---
>>   include/media/v4l2-subdev-legacy.h | 42 ++++++++++++++++++++++++++++++
>>   1 file changed, 42 insertions(+)
>>   create mode 100644 include/media/v4l2-subdev-legacy.h
>>
>> diff --git a/include/media/v4l2-subdev-legacy.h b/include/media/v4l2-subdev-legacy.h
>> new file mode 100644
>> index 000000000000..6a61e579b629
>> --- /dev/null
>> +++ b/include/media/v4l2-subdev-legacy.h
>> @@ -0,0 +1,42 @@
>> +/* SPDX-License-Identifier: GPL-2.0-or-later */
>> +/*
>> + *  V4L2 sub-device legacy support header.
>> + *
>> + *  Copyright (C) 2022  Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>> + */
>> +
>> +#ifndef _V4L2_SUBDEV_LEGACY_H
>> +#define _V4L2_SUBDEV_LEGACY_H
>> +
>> +/**
>> + * v4l2_subdev_call_state_active - call an operation of a v4l2_subdev which
>> + *				   takes state as a parameter, passing the
>> + *				   subdev its active state.
>> + *
>> + * @sd: pointer to the &struct v4l2_subdev
>> + * @o: name of the element at &struct v4l2_subdev_ops that contains @f.
>> + *     Each element there groups a set of callbacks functions.
>> + * @f: callback function to be called.
>> + *     The callback functions are defined in groups, according to
>> + *     each element at &struct v4l2_subdev_ops.
>> + * @args: arguments for @f.
>> + *
>> + * This is similar to v4l2_subdev_call(), except that this version can only be
>> + * used for ops that take a subdev state as a parameter. The macro will get the
>> + * active state and lock it before calling the op, and unlock it after the
>> + * call.
>> + */
> 
> You should explain why this is a legacy macro and, ideally, what would need to
> be done to get rid of it. The first is in the commit log, but nobody reads that :-)
> 
> But if just using it in a non-MC video device driver constitutes 'legacy' use,
> then I disagree with that. There are many non-MC video device drivers, nothing
> legacy about that.

It's difficult to define all the scenarios where this can be used, but 
the ones I can imagine fall under legacy (depending on how you define 
that, though).

I use this in CAL driver, which supports non-MC (legacy) and MC. CAL has 
a bunch of video devices (one for each DMA engine) and two CSI-2 PHY 
devices (v4l2 subdevs).

When operating in MC mode, the userspace will call, e.g., set_fmt in the 
PHY subdev, and so forth.

But in non-MC case the userspace calls VIDIOC_S_FMT in the video dev, 
and the video dev has to propagate that to the PHY subdev. I do this 
propagation using the v4l2_subdev_call_state_active macro.

I don't know if there are other drivers that support both non-MC and MC 
modes. I could also just move this macro to the CAL driver, and we could 
add this to the v4l2 framework if we see other drivers using similar 
constructs.

  Tomi
  

On 3/7/22 08:16, Tomi Valkeinen wrote:
> Hi Hans,
> 
> On 04/03/2022 15:34, Hans Verkuil wrote:
>> Hi Tomi,
>>
>> On 3/1/22 11:55, Tomi Valkeinen wrote:
>>> Add v4l2_subdev_call_state_active() macro to help calling subdev ops
>>> that take a subdev state as a parameter. Normally the v4l2 framework
>>> will lock and pass the correct subdev state to the subdev ops, but there
>>> are legacy situations where this is not the case (e.g. non-MC video
>>> device driver calling set_fmt in a source subdev).
>>>
>>> As this macro is only needed for legacy use cases, the macro is added in
>>> a new header file, v4l2-subdev-legacy.h.
>>>
>>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>>> ---
>>>   include/media/v4l2-subdev-legacy.h | 42 ++++++++++++++++++++++++++++++
>>>   1 file changed, 42 insertions(+)
>>>   create mode 100644 include/media/v4l2-subdev-legacy.h
>>>
>>> diff --git a/include/media/v4l2-subdev-legacy.h b/include/media/v4l2-subdev-legacy.h
>>> new file mode 100644
>>> index 000000000000..6a61e579b629
>>> --- /dev/null
>>> +++ b/include/media/v4l2-subdev-legacy.h
>>> @@ -0,0 +1,42 @@
>>> +/* SPDX-License-Identifier: GPL-2.0-or-later */
>>> +/*
>>> + *  V4L2 sub-device legacy support header.
>>> + *
>>> + *  Copyright (C) 2022  Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>>> + */
>>> +
>>> +#ifndef _V4L2_SUBDEV_LEGACY_H
>>> +#define _V4L2_SUBDEV_LEGACY_H
>>> +
>>> +/**
>>> + * v4l2_subdev_call_state_active - call an operation of a v4l2_subdev which
>>> + *                   takes state as a parameter, passing the
>>> + *                   subdev its active state.
>>> + *
>>> + * @sd: pointer to the &struct v4l2_subdev
>>> + * @o: name of the element at &struct v4l2_subdev_ops that contains @f.
>>> + *     Each element there groups a set of callbacks functions.
>>> + * @f: callback function to be called.
>>> + *     The callback functions are defined in groups, according to
>>> + *     each element at &struct v4l2_subdev_ops.
>>> + * @args: arguments for @f.
>>> + *
>>> + * This is similar to v4l2_subdev_call(), except that this version can only be
>>> + * used for ops that take a subdev state as a parameter. The macro will get the
>>> + * active state and lock it before calling the op, and unlock it after the
>>> + * call.
>>> + */
>>
>> You should explain why this is a legacy macro and, ideally, what would need to
>> be done to get rid of it. The first is in the commit log, but nobody reads that :-)
>>
>> But if just using it in a non-MC video device driver constitutes 'legacy' use,
>> then I disagree with that. There are many non-MC video device drivers, nothing
>> legacy about that.
> 
> It's difficult to define all the scenarios where this can be used, but the ones I can imagine fall under legacy (depending on how you define that, though).
> 
> I use this in CAL driver, which supports non-MC (legacy) and MC. CAL has a bunch of video devices (one for each DMA engine) and two CSI-2 PHY devices (v4l2 subdevs).
> 
> When operating in MC mode, the userspace will call, e.g., set_fmt in the PHY subdev, and so forth.
> 
> But in non-MC case the userspace calls VIDIOC_S_FMT in the video dev, and the video dev has to propagate that to the PHY subdev. I do this propagation using the v4l2_subdev_call_state_active macro.
> 
> I don't know if there are other drivers that support both non-MC and MC modes. I could also just move this macro to the CAL driver, and we could add this to the v4l2 framework if we see other drivers using similar constructs.

It is common to have non-MC drivers that call set_fmt of a subdev.
Wouldn't they all need to use this helper macro? If so, then this is NOT a
legacy use, it's just a non-MC driver use.

Am I missing something?

Regards,

	Hans

> 
>  Tomi
  

Hi Hans,

On 07/03/2022 10:36, Hans Verkuil wrote:
> 
> 
> On 3/7/22 08:16, Tomi Valkeinen wrote:
>> Hi Hans,
>>
>> On 04/03/2022 15:34, Hans Verkuil wrote:
>>> Hi Tomi,
>>>
>>> On 3/1/22 11:55, Tomi Valkeinen wrote:
>>>> Add v4l2_subdev_call_state_active() macro to help calling subdev ops
>>>> that take a subdev state as a parameter. Normally the v4l2 framework
>>>> will lock and pass the correct subdev state to the subdev ops, but there
>>>> are legacy situations where this is not the case (e.g. non-MC video
>>>> device driver calling set_fmt in a source subdev).
>>>>
>>>> As this macro is only needed for legacy use cases, the macro is added in
>>>> a new header file, v4l2-subdev-legacy.h.
>>>>
>>>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>>>> ---
>>>>    include/media/v4l2-subdev-legacy.h | 42 ++++++++++++++++++++++++++++++
>>>>    1 file changed, 42 insertions(+)
>>>>    create mode 100644 include/media/v4l2-subdev-legacy.h
>>>>
>>>> diff --git a/include/media/v4l2-subdev-legacy.h b/include/media/v4l2-subdev-legacy.h
>>>> new file mode 100644
>>>> index 000000000000..6a61e579b629
>>>> --- /dev/null
>>>> +++ b/include/media/v4l2-subdev-legacy.h
>>>> @@ -0,0 +1,42 @@
>>>> +/* SPDX-License-Identifier: GPL-2.0-or-later */
>>>> +/*
>>>> + *  V4L2 sub-device legacy support header.
>>>> + *
>>>> + *  Copyright (C) 2022  Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>>>> + */
>>>> +
>>>> +#ifndef _V4L2_SUBDEV_LEGACY_H
>>>> +#define _V4L2_SUBDEV_LEGACY_H
>>>> +
>>>> +/**
>>>> + * v4l2_subdev_call_state_active - call an operation of a v4l2_subdev which
>>>> + *                   takes state as a parameter, passing the
>>>> + *                   subdev its active state.
>>>> + *
>>>> + * @sd: pointer to the &struct v4l2_subdev
>>>> + * @o: name of the element at &struct v4l2_subdev_ops that contains @f.
>>>> + *     Each element there groups a set of callbacks functions.
>>>> + * @f: callback function to be called.
>>>> + *     The callback functions are defined in groups, according to
>>>> + *     each element at &struct v4l2_subdev_ops.
>>>> + * @args: arguments for @f.
>>>> + *
>>>> + * This is similar to v4l2_subdev_call(), except that this version can only be
>>>> + * used for ops that take a subdev state as a parameter. The macro will get the
>>>> + * active state and lock it before calling the op, and unlock it after the
>>>> + * call.
>>>> + */
>>>
>>> You should explain why this is a legacy macro and, ideally, what would need to
>>> be done to get rid of it. The first is in the commit log, but nobody reads that :-)
>>>
>>> But if just using it in a non-MC video device driver constitutes 'legacy' use,
>>> then I disagree with that. There are many non-MC video device drivers, nothing
>>> legacy about that.
>>
>> It's difficult to define all the scenarios where this can be used, but the ones I can imagine fall under legacy (depending on how you define that, though).
>>
>> I use this in CAL driver, which supports non-MC (legacy) and MC. CAL has a bunch of video devices (one for each DMA engine) and two CSI-2 PHY devices (v4l2 subdevs).
>>
>> When operating in MC mode, the userspace will call, e.g., set_fmt in the PHY subdev, and so forth.
>>
>> But in non-MC case the userspace calls VIDIOC_S_FMT in the video dev, and the video dev has to propagate that to the PHY subdev. I do this propagation using the v4l2_subdev_call_state_active macro.
>>
>> I don't know if there are other drivers that support both non-MC and MC modes. I could also just move this macro to the CAL driver, and we could add this to the v4l2 framework if we see other drivers using similar constructs.
> 
> It is common to have non-MC drivers that call set_fmt of a subdev.
> Wouldn't they all need to use this helper macro? If so, then this is NOT a
> legacy use, it's just a non-MC driver use.

These non-MC drivers that call set_fmt of a subdev, they're video device 
drivers, right? In other words, there are no subdev drivers that call 
set_fmt on other subdevs?

This does get a bit complex, keeping the old and new code working 
together. In this context, I think we have three different "classes":

1. non-MC
2. MC, no state support
3. MC, state support

We have classes 1 and 2 in upstream, and 3 will be enabled with this 
series (and expanded with the streams series).

Classes 1 and 2 continue working as before. If you have a pipeline with 
only class 3 drivers, it works without any legacy "hacks". The problems 
come when you combine 1 or 2 with 3. Or possibly the problems appear 
only when combining class 1 and class 3, as class 2 drivers are not 
supposed to call subdev ops which take a state parameter on other subdevs.

A class 3 driver expects to get either a try or an active state as a 
parameter, but class 1 drivers pass NULL for the active state. If you 
write a class 3 driver and want it to work with class 1 (without any 
changes to those drivers), you must do extra plumbing in the ops 
functions, to catch the NULL state case and get & lock the state 
yourself. If you do that, this macro is not needed.

Alternatively, class 1 drivers could be changed to use this macro, so 
that a possible class 3 driver in the pipeline would work without 
additional code. But there are a lot of class 1 drivers, and thus 
modifications, and I wasn't planning to go that way.

The CAL driver I mentioned supports both class 1 and class 3 (via a 
module parameter) in the video dev driver, and the class 1 mode uses 
this macro as CAL's PHY subdev (part of the same driver) is a class 3 
subdev. The class 1 support is legacy support in CAL's case.

So... Depending on what kind of driver combinations we want to support, 
this may or may not be legacy, depending on how you define legacy =).

  Tomi
  

Hi Tomi,

On 3/7/22 10:16, Tomi Valkeinen wrote:
> Hi Hans,
> 
> On 07/03/2022 10:36, Hans Verkuil wrote:
>>
>>
>> On 3/7/22 08:16, Tomi Valkeinen wrote:
>>> Hi Hans,
>>>
>>> On 04/03/2022 15:34, Hans Verkuil wrote:
>>>> Hi Tomi,
>>>>
>>>> On 3/1/22 11:55, Tomi Valkeinen wrote:
>>>>> Add v4l2_subdev_call_state_active() macro to help calling subdev ops
>>>>> that take a subdev state as a parameter. Normally the v4l2 framework
>>>>> will lock and pass the correct subdev state to the subdev ops, but there
>>>>> are legacy situations where this is not the case (e.g. non-MC video
>>>>> device driver calling set_fmt in a source subdev).
>>>>>
>>>>> As this macro is only needed for legacy use cases, the macro is added in
>>>>> a new header file, v4l2-subdev-legacy.h.
>>>>>
>>>>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>>>>> ---
>>>>>    include/media/v4l2-subdev-legacy.h | 42 ++++++++++++++++++++++++++++++
>>>>>    1 file changed, 42 insertions(+)
>>>>>    create mode 100644 include/media/v4l2-subdev-legacy.h
>>>>>
>>>>> diff --git a/include/media/v4l2-subdev-legacy.h b/include/media/v4l2-subdev-legacy.h
>>>>> new file mode 100644
>>>>> index 000000000000..6a61e579b629
>>>>> --- /dev/null
>>>>> +++ b/include/media/v4l2-subdev-legacy.h
>>>>> @@ -0,0 +1,42 @@
>>>>> +/* SPDX-License-Identifier: GPL-2.0-or-later */
>>>>> +/*
>>>>> + *  V4L2 sub-device legacy support header.
>>>>> + *
>>>>> + *  Copyright (C) 2022  Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>>>>> + */
>>>>> +
>>>>> +#ifndef _V4L2_SUBDEV_LEGACY_H
>>>>> +#define _V4L2_SUBDEV_LEGACY_H
>>>>> +
>>>>> +/**
>>>>> + * v4l2_subdev_call_state_active - call an operation of a v4l2_subdev which
>>>>> + *                   takes state as a parameter, passing the
>>>>> + *                   subdev its active state.
>>>>> + *
>>>>> + * @sd: pointer to the &struct v4l2_subdev
>>>>> + * @o: name of the element at &struct v4l2_subdev_ops that contains @f.
>>>>> + *     Each element there groups a set of callbacks functions.
>>>>> + * @f: callback function to be called.
>>>>> + *     The callback functions are defined in groups, according to
>>>>> + *     each element at &struct v4l2_subdev_ops.
>>>>> + * @args: arguments for @f.
>>>>> + *
>>>>> + * This is similar to v4l2_subdev_call(), except that this version can only be
>>>>> + * used for ops that take a subdev state as a parameter. The macro will get the
>>>>> + * active state and lock it before calling the op, and unlock it after the
>>>>> + * call.
>>>>> + */
>>>>
>>>> You should explain why this is a legacy macro and, ideally, what would need to
>>>> be done to get rid of it. The first is in the commit log, but nobody reads that :-)
>>>>
>>>> But if just using it in a non-MC video device driver constitutes 'legacy' use,
>>>> then I disagree with that. There are many non-MC video device drivers, nothing
>>>> legacy about that.
>>>
>>> It's difficult to define all the scenarios where this can be used, but the ones I can imagine fall under legacy (depending on how you define that, though).
>>>
>>> I use this in CAL driver, which supports non-MC (legacy) and MC. CAL has a bunch of video devices (one for each DMA engine) and two CSI-2 PHY devices (v4l2 subdevs).
>>>
>>> When operating in MC mode, the userspace will call, e.g., set_fmt in the PHY subdev, and so forth.
>>>
>>> But in non-MC case the userspace calls VIDIOC_S_FMT in the video dev, and the video dev has to propagate that to the PHY subdev. I do this propagation using the v4l2_subdev_call_state_active macro.
>>>
>>> I don't know if there are other drivers that support both non-MC and MC modes. I could also just move this macro to the CAL driver, and we could add this to the v4l2 framework if we see other drivers using similar constructs.
>>
>> It is common to have non-MC drivers that call set_fmt of a subdev.
>> Wouldn't they all need to use this helper macro? If so, then this is NOT a
>> legacy use, it's just a non-MC driver use.
> 
> These non-MC drivers that call set_fmt of a subdev, they're video device drivers, right? In other words, there are no subdev drivers that call set_fmt on other subdevs?

Probably not, but I am not 100% certain. There are a few nested subdev cases, but I don't remember which.

> 
> This does get a bit complex, keeping the old and new code working together. In this context, I think we have three different "classes":
> 
> 1. non-MC
> 2. MC, no state support
> 3. MC, state support

Are you talking about subdev drivers or bridge drivers? It's a bit confusing. I'm assuming it can be either.

> 
> We have classes 1 and 2 in upstream, and 3 will be enabled with this series (and expanded with the streams series).
> 
> Classes 1 and 2 continue working as before. If you have a pipeline with only class 3 drivers, it works without any legacy "hacks". The problems come when you combine 1 or 2 with 3. Or possibly the problems appear only when combining class 1 and class 3, as class 2 drivers are not supposed to call subdev ops which take a state parameter on other subdevs.
> 
> A class 3 driver expects to get either a try or an active state as a parameter, but class 1 drivers pass NULL for the active state. If you write a class 3 driver and want it to work with class 1 (without any changes to those drivers), you must do extra plumbing in the ops functions, to catch the NULL state case and get & lock the state yourself. If you do that, this macro is not needed.
> 
> Alternatively, class 1 drivers could be changed to use this macro, so that a possible class 3 driver in the pipeline would work without additional code. But there are a lot of class 1 drivers, and thus modifications, and I wasn't planning to go that way.
> 
> The CAL driver I mentioned supports both class 1 and class 3 (via a module parameter) in the video dev driver, and the class 1 mode uses this macro as CAL's PHY subdev (part of the same driver) is a class 3 subdev. The class 1 support is legacy support in CAL's case.
> 
> So... Depending on what kind of driver combinations we want to support, this may or may not be legacy, depending on how you define legacy =).

Let me try to explain what my concerns are. Eventually I would really like all subdevs to be capable of working with MC bridge drivers, i.e. have state support. Bridge drivers can be either MC or non-MC.

So I would like to know:

1.1) How to convert a subdev driver to a MC state-aware subdev driver?
1.2) What is the legacy code that such a MC state-aware subdev driver has to keep in order to work with older bridge drivers that do not support such subdev drivers? (i.e. they pass NULL as the state)

2.1) How to convert a bridge driver (either non-MC or MC, but no state support) to properly support a fully converted subdev (MC state-aware) driver?
2.2) What is the legacy code that such a bridge driver has to keep in order to work with older subdev drivers that are not yet MC state-aware?

The code needed for 1.2 and 2.2 (helper functions/macros) is legacy code, and can be marked as such.

If this is clear, then we can work towards converting both subdev and bridge drivers and eventually (might take years!) get rid of the legacy code.

Removing support for case 2 is probably something that we want to do sooner than later.

For the CAL driver I do not consider non-MC support as legacy. It's legacy in the context of the CAL driver only, but API-wise it is not since there are many non-MC bridge drivers.

Converting all subdev drivers to support the same MC state mechanism is time consuming, but we've done such things before.

Regards,

	Hans
  

On 07/03/2022 11:51, Hans Verkuil wrote:
> Hi Tomi,
> 
> On 3/7/22 10:16, Tomi Valkeinen wrote:
>> Hi Hans,
>>
>> On 07/03/2022 10:36, Hans Verkuil wrote:
>>>
>>>
>>> On 3/7/22 08:16, Tomi Valkeinen wrote:
>>>> Hi Hans,
>>>>
>>>> On 04/03/2022 15:34, Hans Verkuil wrote:
>>>>> Hi Tomi,
>>>>>
>>>>> On 3/1/22 11:55, Tomi Valkeinen wrote:
>>>>>> Add v4l2_subdev_call_state_active() macro to help calling subdev ops
>>>>>> that take a subdev state as a parameter. Normally the v4l2 framework
>>>>>> will lock and pass the correct subdev state to the subdev ops, but there
>>>>>> are legacy situations where this is not the case (e.g. non-MC video
>>>>>> device driver calling set_fmt in a source subdev).
>>>>>>
>>>>>> As this macro is only needed for legacy use cases, the macro is added in
>>>>>> a new header file, v4l2-subdev-legacy.h.
>>>>>>
>>>>>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>>>>>> ---
>>>>>>     include/media/v4l2-subdev-legacy.h | 42 ++++++++++++++++++++++++++++++
>>>>>>     1 file changed, 42 insertions(+)
>>>>>>     create mode 100644 include/media/v4l2-subdev-legacy.h
>>>>>>
>>>>>> diff --git a/include/media/v4l2-subdev-legacy.h b/include/media/v4l2-subdev-legacy.h
>>>>>> new file mode 100644
>>>>>> index 000000000000..6a61e579b629
>>>>>> --- /dev/null
>>>>>> +++ b/include/media/v4l2-subdev-legacy.h
>>>>>> @@ -0,0 +1,42 @@
>>>>>> +/* SPDX-License-Identifier: GPL-2.0-or-later */
>>>>>> +/*
>>>>>> + *  V4L2 sub-device legacy support header.
>>>>>> + *
>>>>>> + *  Copyright (C) 2022  Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>>>>>> + */
>>>>>> +
>>>>>> +#ifndef _V4L2_SUBDEV_LEGACY_H
>>>>>> +#define _V4L2_SUBDEV_LEGACY_H
>>>>>> +
>>>>>> +/**
>>>>>> + * v4l2_subdev_call_state_active - call an operation of a v4l2_subdev which
>>>>>> + *                   takes state as a parameter, passing the
>>>>>> + *                   subdev its active state.
>>>>>> + *
>>>>>> + * @sd: pointer to the &struct v4l2_subdev
>>>>>> + * @o: name of the element at &struct v4l2_subdev_ops that contains @f.
>>>>>> + *     Each element there groups a set of callbacks functions.
>>>>>> + * @f: callback function to be called.
>>>>>> + *     The callback functions are defined in groups, according to
>>>>>> + *     each element at &struct v4l2_subdev_ops.
>>>>>> + * @args: arguments for @f.
>>>>>> + *
>>>>>> + * This is similar to v4l2_subdev_call(), except that this version can only be
>>>>>> + * used for ops that take a subdev state as a parameter. The macro will get the
>>>>>> + * active state and lock it before calling the op, and unlock it after the
>>>>>> + * call.
>>>>>> + */
>>>>>
>>>>> You should explain why this is a legacy macro and, ideally, what would need to
>>>>> be done to get rid of it. The first is in the commit log, but nobody reads that :-)
>>>>>
>>>>> But if just using it in a non-MC video device driver constitutes 'legacy' use,
>>>>> then I disagree with that. There are many non-MC video device drivers, nothing
>>>>> legacy about that.
>>>>
>>>> It's difficult to define all the scenarios where this can be used, but the ones I can imagine fall under legacy (depending on how you define that, though).
>>>>
>>>> I use this in CAL driver, which supports non-MC (legacy) and MC. CAL has a bunch of video devices (one for each DMA engine) and two CSI-2 PHY devices (v4l2 subdevs).
>>>>
>>>> When operating in MC mode, the userspace will call, e.g., set_fmt in the PHY subdev, and so forth.
>>>>
>>>> But in non-MC case the userspace calls VIDIOC_S_FMT in the video dev, and the video dev has to propagate that to the PHY subdev. I do this propagation using the v4l2_subdev_call_state_active macro.
>>>>
>>>> I don't know if there are other drivers that support both non-MC and MC modes. I could also just move this macro to the CAL driver, and we could add this to the v4l2 framework if we see other drivers using similar constructs.
>>>
>>> It is common to have non-MC drivers that call set_fmt of a subdev.
>>> Wouldn't they all need to use this helper macro? If so, then this is NOT a
>>> legacy use, it's just a non-MC driver use.
>>
>> These non-MC drivers that call set_fmt of a subdev, they're video device drivers, right? In other words, there are no subdev drivers that call set_fmt on other subdevs?
> 
> Probably not, but I am not 100% certain. There are a few nested subdev cases, but I don't remember which.
> 
>>
>> This does get a bit complex, keeping the old and new code working together. In this context, I think we have three different "classes":
>>
>> 1. non-MC
>> 2. MC, no state support
>> 3. MC, state support
> 
> Are you talking about subdev drivers or bridge drivers? It's a bit confusing. I'm assuming it can be either.

Yes, I mean either one.

>> We have classes 1 and 2 in upstream, and 3 will be enabled with this series (and expanded with the streams series).
>>
>> Classes 1 and 2 continue working as before. If you have a pipeline with only class 3 drivers, it works without any legacy "hacks". The problems come when you combine 1 or 2 with 3. Or possibly the problems appear only when combining class 1 and class 3, as class 2 drivers are not supposed to call subdev ops which take a state parameter on other subdevs.
>>
>> A class 3 driver expects to get either a try or an active state as a parameter, but class 1 drivers pass NULL for the active state. If you write a class 3 driver and want it to work with class 1 (without any changes to those drivers), you must do extra plumbing in the ops functions, to catch the NULL state case and get & lock the state yourself. If you do that, this macro is not needed.
>>
>> Alternatively, class 1 drivers could be changed to use this macro, so that a possible class 3 driver in the pipeline would work without additional code. But there are a lot of class 1 drivers, and thus modifications, and I wasn't planning to go that way.
>>
>> The CAL driver I mentioned supports both class 1 and class 3 (via a module parameter) in the video dev driver, and the class 1 mode uses this macro as CAL's PHY subdev (part of the same driver) is a class 3 subdev. The class 1 support is legacy support in CAL's case.
>>
>> So... Depending on what kind of driver combinations we want to support, this may or may not be legacy, depending on how you define legacy =).
> 
> Let me try to explain what my concerns are. Eventually I would really like all subdevs to be capable of working with MC bridge drivers, i.e. have state support. Bridge drivers can be either MC or non-MC.
> 
> So I would like to know:
> 
> 1.1) How to convert a subdev driver to a MC state-aware subdev driver?

I presume you mean how to convert an MC subdev driver to state-aware MC 
driver. If you have a non-MC subdev driver, then that first needs to be 
converted to an MC driver, which is out of scope here.

Here's an example commit where I convert OV10635 to streams:

https://git.kernel.org/pub/scm/linux/kernel/git/tomba/linux.git/commit/?h=streams/work-v11&id=ffb08af15f04ef2ce0a00bb356e957c839d6bccb

However, the commit is on top of the streams series and I add support 
for multiple streams in addition to the active state, so 1) it's not as 
simple as it could be (e.g. get_frame_desc() and set_routing() ops could 
be left out), and 2) it's not using the old-style v4l2_subdev_pad_config 
(which is the only option on top of this series), but the new routes & 
streams.

So the example doesn't quite answer to your question... I haven't looked 
at implementing a driver which would be state-aware but not 
streams-aware, but I think it's essentially just:

- Use v4l2_subdev_init_finalize() to create the active state storage
- Always use state->pads instead of something stored in the driver's 
private data for active case.

> 1.2) What is the legacy code that such a MC state-aware subdev driver has to keep in order to work with older bridge drivers that do not support such subdev drivers? (i.e. they pass NULL as the state)

I believe the only extra code needed is to handle the state == NULL 
case. This means adding code to each subdev op which has the state as a 
parameter, and doing, perhaps, something like this:

my_set_fmt(sd, _state, fmt)
{
	state = _state

	if (!_state)
		state = v4l2_subdev_lock_and_get_active_state(sd);

	... use 'state' here ...

	if (!_state)
		v4l2_subdev_unlock_state(state);
}

Maybe we can somehow macro-ify the above, which creates a wrapper for 
the op. Or, as I mentioned, we could try to change all the drivers that 
do those calls, so that they use the macro in this patch instead.

> 2.1) How to convert a bridge driver (either non-MC or MC, but no state support) to properly support a fully converted subdev (MC state-aware) driver?

Converting non-MC driver to an MC driver is out of the context here, as 
it's not related to the active state. An MC bridge driver should work 
fine with state-aware subdev drivers, as the bridge driver should not 
call any of the subdev's state-related ops.

To make a non-MC bridge driver support state-aware subdev drivers, they 
can use the macro in this patch.

> 2.2) What is the legacy code that such a bridge driver has to keep in order to work with older subdev drivers that are not yet MC state-aware?

The older subdev drivers should keep working without any extra code.

> The code needed for 1.2 and 2.2 (helper functions/macros) is legacy code, and can be marked as such.
> > If this is clear, then we can work towards converting both subdev and 
bridge drivers and eventually (might take years!) get rid of the legacy 
code.
> 
> Removing support for case 2 is probably something that we want to do sooner than later.
> 
> For the CAL driver I do not consider non-MC support as legacy. It's legacy in the context of the CAL driver only, but API-wise it is not since there are many non-MC bridge drivers.

That's true, but also, non-MC bridge drivers do not need to use this 
function if the subdev drivers use the method shown in 1.2. I think this 
is the question here:

- Change all the callers and use the macro in this patch. Then the macro 
is not legacy.
- Change the callees, in which case this macro is needed only in some 
cases where, for whatever reason, a specific callee has not been changed 
(yet?). In this case it's legacy.

Changing the callers would be a nicer option, I think, but I also fear 
that it's very difficult and easily brings in bugs. I haven't looked 
closely, but I think it would be a big patch.

And it's not clear to me if there's a benefit: do all those drivers ever 
need to interact a state-aware subdev driver? If they do, maybe there 
are only a few such subdev drivers, and it's not a big issue to have the 
lock/unlock code in those state-aware subdev drivers. All the other 
state-aware subdev drivers would not need the legacy support code as 
they're used only in more modern pipelines.

  Tomi
  

Hi Hans,

On 07/03/2022 16:00, Tomi Valkeinen wrote:
> On 07/03/2022 11:51, Hans Verkuil wrote:
>> Hi Tomi,
>>
>> On 3/7/22 10:16, Tomi Valkeinen wrote:
>>> Hi Hans,
>>>
>>> On 07/03/2022 10:36, Hans Verkuil wrote:
>>>>
>>>>
>>>> On 3/7/22 08:16, Tomi Valkeinen wrote:
>>>>> Hi Hans,
>>>>>
>>>>> On 04/03/2022 15:34, Hans Verkuil wrote:
>>>>>> Hi Tomi,
>>>>>>
>>>>>> On 3/1/22 11:55, Tomi Valkeinen wrote:
>>>>>>> Add v4l2_subdev_call_state_active() macro to help calling subdev ops
>>>>>>> that take a subdev state as a parameter. Normally the v4l2 framework
>>>>>>> will lock and pass the correct subdev state to the subdev ops, 
>>>>>>> but there
>>>>>>> are legacy situations where this is not the case (e.g. non-MC video
>>>>>>> device driver calling set_fmt in a source subdev).
>>>>>>>
>>>>>>> As this macro is only needed for legacy use cases, the macro is 
>>>>>>> added in
>>>>>>> a new header file, v4l2-subdev-legacy.h.
>>>>>>>
>>>>>>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>>>>>>> ---
>>>>>>>     include/media/v4l2-subdev-legacy.h | 42 
>>>>>>> ++++++++++++++++++++++++++++++
>>>>>>>     1 file changed, 42 insertions(+)
>>>>>>>     create mode 100644 include/media/v4l2-subdev-legacy.h
>>>>>>>
>>>>>>> diff --git a/include/media/v4l2-subdev-legacy.h 
>>>>>>> b/include/media/v4l2-subdev-legacy.h
>>>>>>> new file mode 100644
>>>>>>> index 000000000000..6a61e579b629
>>>>>>> --- /dev/null
>>>>>>> +++ b/include/media/v4l2-subdev-legacy.h
>>>>>>> @@ -0,0 +1,42 @@
>>>>>>> +/* SPDX-License-Identifier: GPL-2.0-or-later */
>>>>>>> +/*
>>>>>>> + *  V4L2 sub-device legacy support header.
>>>>>>> + *
>>>>>>> + *  Copyright (C) 2022  Tomi Valkeinen 
>>>>>>> <tomi.valkeinen@ideasonboard.com>
>>>>>>> + */
>>>>>>> +
>>>>>>> +#ifndef _V4L2_SUBDEV_LEGACY_H
>>>>>>> +#define _V4L2_SUBDEV_LEGACY_H
>>>>>>> +
>>>>>>> +/**
>>>>>>> + * v4l2_subdev_call_state_active - call an operation of a 
>>>>>>> v4l2_subdev which
>>>>>>> + *                   takes state as a parameter, passing the
>>>>>>> + *                   subdev its active state.
>>>>>>> + *
>>>>>>> + * @sd: pointer to the &struct v4l2_subdev
>>>>>>> + * @o: name of the element at &struct v4l2_subdev_ops that 
>>>>>>> contains @f.
>>>>>>> + *     Each element there groups a set of callbacks functions.
>>>>>>> + * @f: callback function to be called.
>>>>>>> + *     The callback functions are defined in groups, according to
>>>>>>> + *     each element at &struct v4l2_subdev_ops.
>>>>>>> + * @args: arguments for @f.
>>>>>>> + *
>>>>>>> + * This is similar to v4l2_subdev_call(), except that this 
>>>>>>> version can only be
>>>>>>> + * used for ops that take a subdev state as a parameter. The 
>>>>>>> macro will get the
>>>>>>> + * active state and lock it before calling the op, and unlock it 
>>>>>>> after the
>>>>>>> + * call.
>>>>>>> + */
>>>>>>
>>>>>> You should explain why this is a legacy macro and, ideally, what 
>>>>>> would need to
>>>>>> be done to get rid of it. The first is in the commit log, but 
>>>>>> nobody reads that :-)
>>>>>>
>>>>>> But if just using it in a non-MC video device driver constitutes 
>>>>>> 'legacy' use,
>>>>>> then I disagree with that. There are many non-MC video device 
>>>>>> drivers, nothing
>>>>>> legacy about that.
>>>>>
>>>>> It's difficult to define all the scenarios where this can be used, 
>>>>> but the ones I can imagine fall under legacy (depending on how you 
>>>>> define that, though).
>>>>>
>>>>> I use this in CAL driver, which supports non-MC (legacy) and MC. 
>>>>> CAL has a bunch of video devices (one for each DMA engine) and two 
>>>>> CSI-2 PHY devices (v4l2 subdevs).
>>>>>
>>>>> When operating in MC mode, the userspace will call, e.g., set_fmt 
>>>>> in the PHY subdev, and so forth.
>>>>>
>>>>> But in non-MC case the userspace calls VIDIOC_S_FMT in the video 
>>>>> dev, and the video dev has to propagate that to the PHY subdev. I 
>>>>> do this propagation using the v4l2_subdev_call_state_active macro.
>>>>>
>>>>> I don't know if there are other drivers that support both non-MC 
>>>>> and MC modes. I could also just move this macro to the CAL driver, 
>>>>> and we could add this to the v4l2 framework if we see other drivers 
>>>>> using similar constructs.
>>>>
>>>> It is common to have non-MC drivers that call set_fmt of a subdev.
>>>> Wouldn't they all need to use this helper macro? If so, then this is 
>>>> NOT a
>>>> legacy use, it's just a non-MC driver use.
>>>
>>> These non-MC drivers that call set_fmt of a subdev, they're video 
>>> device drivers, right? In other words, there are no subdev drivers 
>>> that call set_fmt on other subdevs?
>>
>> Probably not, but I am not 100% certain. There are a few nested subdev 
>> cases, but I don't remember which.
>>
>>>
>>> This does get a bit complex, keeping the old and new code working 
>>> together. In this context, I think we have three different "classes":
>>>
>>> 1. non-MC
>>> 2. MC, no state support
>>> 3. MC, state support
>>
>> Are you talking about subdev drivers or bridge drivers? It's a bit 
>> confusing. I'm assuming it can be either.
> 
> Yes, I mean either one.
> 
>>> We have classes 1 and 2 in upstream, and 3 will be enabled with this 
>>> series (and expanded with the streams series).
>>>
>>> Classes 1 and 2 continue working as before. If you have a pipeline 
>>> with only class 3 drivers, it works without any legacy "hacks". The 
>>> problems come when you combine 1 or 2 with 3. Or possibly the 
>>> problems appear only when combining class 1 and class 3, as class 2 
>>> drivers are not supposed to call subdev ops which take a state 
>>> parameter on other subdevs.
>>>
>>> A class 3 driver expects to get either a try or an active state as a 
>>> parameter, but class 1 drivers pass NULL for the active state. If you 
>>> write a class 3 driver and want it to work with class 1 (without any 
>>> changes to those drivers), you must do extra plumbing in the ops 
>>> functions, to catch the NULL state case and get & lock the state 
>>> yourself. If you do that, this macro is not needed.
>>>
>>> Alternatively, class 1 drivers could be changed to use this macro, so 
>>> that a possible class 3 driver in the pipeline would work without 
>>> additional code. But there are a lot of class 1 drivers, and thus 
>>> modifications, and I wasn't planning to go that way.
>>>
>>> The CAL driver I mentioned supports both class 1 and class 3 (via a 
>>> module parameter) in the video dev driver, and the class 1 mode uses 
>>> this macro as CAL's PHY subdev (part of the same driver) is a class 3 
>>> subdev. The class 1 support is legacy support in CAL's case.
>>>
>>> So... Depending on what kind of driver combinations we want to 
>>> support, this may or may not be legacy, depending on how you define 
>>> legacy =).
>>
>> Let me try to explain what my concerns are. Eventually I would really 
>> like all subdevs to be capable of working with MC bridge drivers, i.e. 
>> have state support. Bridge drivers can be either MC or non-MC.
>>
>> So I would like to know:
>>
>> 1.1) How to convert a subdev driver to a MC state-aware subdev driver?
> 
> I presume you mean how to convert an MC subdev driver to state-aware MC 
> driver. If you have a non-MC subdev driver, then that first needs to be 
> converted to an MC driver, which is out of scope here.
> 
> Here's an example commit where I convert OV10635 to streams:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/tomba/linux.git/commit/?h=streams/work-v11&id=ffb08af15f04ef2ce0a00bb356e957c839d6bccb 
> 
> 
> However, the commit is on top of the streams series and I add support 
> for multiple streams in addition to the active state, so 1) it's not as 
> simple as it could be (e.g. get_frame_desc() and set_routing() ops could 
> be left out), and 2) it's not using the old-style v4l2_subdev_pad_config 
> (which is the only option on top of this series), but the new routes & 
> streams.
> 
> So the example doesn't quite answer to your question... I haven't looked 
> at implementing a driver which would be state-aware but not 
> streams-aware, but I think it's essentially just:
> 
> - Use v4l2_subdev_init_finalize() to create the active state storage
> - Always use state->pads instead of something stored in the driver's 
> private data for active case.
> 
>> 1.2) What is the legacy code that such a MC state-aware subdev driver 
>> has to keep in order to work with older bridge drivers that do not 
>> support such subdev drivers? (i.e. they pass NULL as the state)
> 
> I believe the only extra code needed is to handle the state == NULL 
> case. This means adding code to each subdev op which has the state as a 
> parameter, and doing, perhaps, something like this:
> 
> my_set_fmt(sd, _state, fmt)
> {
>      state = _state
> 
>      if (!_state)
>          state = v4l2_subdev_lock_and_get_active_state(sd);
> 
>      ... use 'state' here ...
> 
>      if (!_state)
>          v4l2_subdev_unlock_state(state);
> }
> 
> Maybe we can somehow macro-ify the above, which creates a wrapper for 
> the op. Or, as I mentioned, we could try to change all the drivers that 
> do those calls, so that they use the macro in this patch instead.
> 
>> 2.1) How to convert a bridge driver (either non-MC or MC, but no state 
>> support) to properly support a fully converted subdev (MC state-aware) 
>> driver?
> 
> Converting non-MC driver to an MC driver is out of the context here, as 
> it's not related to the active state. An MC bridge driver should work 
> fine with state-aware subdev drivers, as the bridge driver should not 
> call any of the subdev's state-related ops.
> 
> To make a non-MC bridge driver support state-aware subdev drivers, they 
> can use the macro in this patch.
> 
>> 2.2) What is the legacy code that such a bridge driver has to keep in 
>> order to work with older subdev drivers that are not yet MC state-aware?
> 
> The older subdev drivers should keep working without any extra code.
> 
>> The code needed for 1.2 and 2.2 (helper functions/macros) is legacy 
>> code, and can be marked as such.
>> > If this is clear, then we can work towards converting both subdev and 
> bridge drivers and eventually (might take years!) get rid of the legacy 
> code.
>>
>> Removing support for case 2 is probably something that we want to do 
>> sooner than later.
>>
>> For the CAL driver I do not consider non-MC support as legacy. It's 
>> legacy in the context of the CAL driver only, but API-wise it is not 
>> since there are many non-MC bridge drivers.
> 
> That's true, but also, non-MC bridge drivers do not need to use this 
> function if the subdev drivers use the method shown in 1.2. I think this 
> is the question here:
> 
> - Change all the callers and use the macro in this patch. Then the macro 
> is not legacy.
> - Change the callees, in which case this macro is needed only in some 
> cases where, for whatever reason, a specific callee has not been changed 
> (yet?). In this case it's legacy.
> 
> Changing the callers would be a nicer option, I think, but I also fear 
> that it's very difficult and easily brings in bugs. I haven't looked 
> closely, but I think it would be a big patch.
> 
> And it's not clear to me if there's a benefit: do all those drivers ever 
> need to interact a state-aware subdev driver? If they do, maybe there 
> are only a few such subdev drivers, and it's not a big issue to have the 
> lock/unlock code in those state-aware subdev drivers. All the other 
> state-aware subdev drivers would not need the legacy support code as 
> they're used only in more modern pipelines.

Did the above explain the situation enough? I'm ok with both options 
(changing the callers or callees), but I also think we don't need to 
make the decision know.

  Tomi
  

On 07/03/2022 15:00, Tomi Valkeinen wrote:
> On 07/03/2022 11:51, Hans Verkuil wrote:
>> Hi Tomi,
>>
>> On 3/7/22 10:16, Tomi Valkeinen wrote:
>>> Hi Hans,
>>>
>>> On 07/03/2022 10:36, Hans Verkuil wrote:
>>>>
>>>>
>>>> On 3/7/22 08:16, Tomi Valkeinen wrote:
>>>>> Hi Hans,
>>>>>
>>>>> On 04/03/2022 15:34, Hans Verkuil wrote:
>>>>>> Hi Tomi,
>>>>>>
>>>>>> On 3/1/22 11:55, Tomi Valkeinen wrote:
>>>>>>> Add v4l2_subdev_call_state_active() macro to help calling subdev ops
>>>>>>> that take a subdev state as a parameter. Normally the v4l2 framework
>>>>>>> will lock and pass the correct subdev state to the subdev ops, but there
>>>>>>> are legacy situations where this is not the case (e.g. non-MC video
>>>>>>> device driver calling set_fmt in a source subdev).
>>>>>>>
>>>>>>> As this macro is only needed for legacy use cases, the macro is added in
>>>>>>> a new header file, v4l2-subdev-legacy.h.
>>>>>>>
>>>>>>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>>>>>>> ---
>>>>>>>     include/media/v4l2-subdev-legacy.h | 42 ++++++++++++++++++++++++++++++
>>>>>>>     1 file changed, 42 insertions(+)
>>>>>>>     create mode 100644 include/media/v4l2-subdev-legacy.h
>>>>>>>
>>>>>>> diff --git a/include/media/v4l2-subdev-legacy.h b/include/media/v4l2-subdev-legacy.h
>>>>>>> new file mode 100644
>>>>>>> index 000000000000..6a61e579b629
>>>>>>> --- /dev/null
>>>>>>> +++ b/include/media/v4l2-subdev-legacy.h
>>>>>>> @@ -0,0 +1,42 @@
>>>>>>> +/* SPDX-License-Identifier: GPL-2.0-or-later */
>>>>>>> +/*
>>>>>>> + *  V4L2 sub-device legacy support header.
>>>>>>> + *
>>>>>>> + *  Copyright (C) 2022  Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>>>>>>> + */
>>>>>>> +
>>>>>>> +#ifndef _V4L2_SUBDEV_LEGACY_H
>>>>>>> +#define _V4L2_SUBDEV_LEGACY_H
>>>>>>> +
>>>>>>> +/**
>>>>>>> + * v4l2_subdev_call_state_active - call an operation of a v4l2_subdev which
>>>>>>> + *                   takes state as a parameter, passing the
>>>>>>> + *                   subdev its active state.
>>>>>>> + *
>>>>>>> + * @sd: pointer to the &struct v4l2_subdev
>>>>>>> + * @o: name of the element at &struct v4l2_subdev_ops that contains @f.
>>>>>>> + *     Each element there groups a set of callbacks functions.
>>>>>>> + * @f: callback function to be called.
>>>>>>> + *     The callback functions are defined in groups, according to
>>>>>>> + *     each element at &struct v4l2_subdev_ops.
>>>>>>> + * @args: arguments for @f.
>>>>>>> + *
>>>>>>> + * This is similar to v4l2_subdev_call(), except that this version can only be
>>>>>>> + * used for ops that take a subdev state as a parameter. The macro will get the
>>>>>>> + * active state and lock it before calling the op, and unlock it after the
>>>>>>> + * call.
>>>>>>> + */
>>>>>>
>>>>>> You should explain why this is a legacy macro and, ideally, what would need to
>>>>>> be done to get rid of it. The first is in the commit log, but nobody reads that :-)
>>>>>>
>>>>>> But if just using it in a non-MC video device driver constitutes 'legacy' use,
>>>>>> then I disagree with that. There are many non-MC video device drivers, nothing
>>>>>> legacy about that.
>>>>>
>>>>> It's difficult to define all the scenarios where this can be used, but the ones I can imagine fall under legacy (depending on how you define that, though).
>>>>>
>>>>> I use this in CAL driver, which supports non-MC (legacy) and MC. CAL has a bunch of video devices (one for each DMA engine) and two CSI-2 PHY devices (v4l2 subdevs).
>>>>>
>>>>> When operating in MC mode, the userspace will call, e.g., set_fmt in the PHY subdev, and so forth.
>>>>>
>>>>> But in non-MC case the userspace calls VIDIOC_S_FMT in the video dev, and the video dev has to propagate that to the PHY subdev. I do this propagation using the v4l2_subdev_call_state_active macro.
>>>>>
>>>>> I don't know if there are other drivers that support both non-MC and MC modes. I could also just move this macro to the CAL driver, and we could add this to the v4l2 framework if we see other
>>>>> drivers using similar constructs.
>>>>
>>>> It is common to have non-MC drivers that call set_fmt of a subdev.
>>>> Wouldn't they all need to use this helper macro? If so, then this is NOT a
>>>> legacy use, it's just a non-MC driver use.
>>>
>>> These non-MC drivers that call set_fmt of a subdev, they're video device drivers, right? In other words, there are no subdev drivers that call set_fmt on other subdevs?
>>
>> Probably not, but I am not 100% certain. There are a few nested subdev cases, but I don't remember which.
>>
>>>
>>> This does get a bit complex, keeping the old and new code working together. In this context, I think we have three different "classes":
>>>
>>> 1. non-MC
>>> 2. MC, no state support
>>> 3. MC, state support
>>
>> Are you talking about subdev drivers or bridge drivers? It's a bit confusing. I'm assuming it can be either.
> 
> Yes, I mean either one.
> 
>>> We have classes 1 and 2 in upstream, and 3 will be enabled with this series (and expanded with the streams series).
>>>
>>> Classes 1 and 2 continue working as before. If you have a pipeline with only class 3 drivers, it works without any legacy "hacks". The problems come when you combine 1 or 2 with 3. Or possibly the
>>> problems appear only when combining class 1 and class 3, as class 2 drivers are not supposed to call subdev ops which take a state parameter on other subdevs.
>>>
>>> A class 3 driver expects to get either a try or an active state as a parameter, but class 1 drivers pass NULL for the active state. If you write a class 3 driver and want it to work with class 1
>>> (without any changes to those drivers), you must do extra plumbing in the ops functions, to catch the NULL state case and get & lock the state yourself. If you do that, this macro is not needed.
>>>
>>> Alternatively, class 1 drivers could be changed to use this macro, so that a possible class 3 driver in the pipeline would work without additional code. But there are a lot of class 1 drivers, and
>>> thus modifications, and I wasn't planning to go that way.
>>>
>>> The CAL driver I mentioned supports both class 1 and class 3 (via a module parameter) in the video dev driver, and the class 1 mode uses this macro as CAL's PHY subdev (part of the same driver) is
>>> a class 3 subdev. The class 1 support is legacy support in CAL's case.
>>>
>>> So... Depending on what kind of driver combinations we want to support, this may or may not be legacy, depending on how you define legacy =).
>>
>> Let me try to explain what my concerns are. Eventually I would really like all subdevs to be capable of working with MC bridge drivers, i.e. have state support. Bridge drivers can be either MC or
>> non-MC.
>>
>> So I would like to know:
>>
>> 1.1) How to convert a subdev driver to a MC state-aware subdev driver?
> 
> I presume you mean how to convert an MC subdev driver to state-aware MC driver. If you have a non-MC subdev driver, then that first needs to be converted to an MC driver, which is out of scope here.
> 
> Here's an example commit where I convert OV10635 to streams:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/tomba/linux.git/commit/?h=streams/work-v11&id=ffb08af15f04ef2ce0a00bb356e957c839d6bccb
> 
> However, the commit is on top of the streams series and I add support for multiple streams in addition to the active state, so 1) it's not as simple as it could be (e.g. get_frame_desc() and
> set_routing() ops could be left out), and 2) it's not using the old-style v4l2_subdev_pad_config (which is the only option on top of this series), but the new routes & streams.
> 
> So the example doesn't quite answer to your question... I haven't looked at implementing a driver which would be state-aware but not streams-aware, but I think it's essentially just:
> 
> - Use v4l2_subdev_init_finalize() to create the active state storage
> - Always use state->pads instead of something stored in the driver's private data for active case.

Can you make a patch that converts ov5648.c (or a similar driver) to use
sd->active_state? I'd like to see how that looks.

> 
>> 1.2) What is the legacy code that such a MC state-aware subdev driver has to keep in order to work with older bridge drivers that do not support such subdev drivers? (i.e. they pass NULL as the state)
> 
> I believe the only extra code needed is to handle the state == NULL case. This means adding code to each subdev op which has the state as a parameter, and doing, perhaps, something like this:
> 
> my_set_fmt(sd, _state, fmt)
> {
>     state = _state
> 
>     if (!_state)
>         state = v4l2_subdev_lock_and_get_active_state(sd);
> 
>     ... use 'state' here ...
> 
>     if (!_state)
>         v4l2_subdev_unlock_state(state);
> }
> 
> Maybe we can somehow macro-ify the above, which creates a wrapper for the op. Or, as I mentioned, we could try to change all the drivers that do those calls, so that they use the macro in this patch
> instead.

Right. So to confirm: the only reason a state-ware subdev set_fmt op is called
with a NULL state pointer is if it is called from non-MC bridge drivers? And if
we would update those bridge drivers to always pass a non-NULL state pointer,
then such subdevs no longer need to care about this and can just use the state.

If so, then that's the way forward.

> 
>> 2.1) How to convert a bridge driver (either non-MC or MC, but no state support) to properly support a fully converted subdev (MC state-aware) driver?
> 
> Converting non-MC driver to an MC driver is out of the context here, as it's not related to the active state. An MC bridge driver should work fine with state-aware subdev drivers, as the bridge driver
> should not call any of the subdev's state-related ops.
> 
> To make a non-MC bridge driver support state-aware subdev drivers, they can use the macro in this patch.

That sounds good to me.

The only legacy bit about the macro, though, is the fact that v4l2_subdev_get_active_state()
can return NULL: that indicates that the subdev driver isn't properly state-aware.

So I think this should be a regular macro in v4l2-subdev.h.

Perhaps with a comment mentioning that v4l2_subdev_get_active_state() can be replaced by
v4l2_subdev_lock_and_get_active_state() once all subdevs are state-aware.

> 
>> 2.2) What is the legacy code that such a bridge driver has to keep in order to work with older subdev drivers that are not yet MC state-aware?
> 
> The older subdev drivers should keep working without any extra code.
> 
>> The code needed for 1.2 and 2.2 (helper functions/macros) is legacy code, and can be marked as such.
>> > If this is clear, then we can work towards converting both subdev and 
> bridge drivers and eventually (might take years!) get rid of the legacy code.
>>
>> Removing support for case 2 is probably something that we want to do sooner than later.
>>
>> For the CAL driver I do not consider non-MC support as legacy. It's legacy in the context of the CAL driver only, but API-wise it is not since there are many non-MC bridge drivers.
> 
> That's true, but also, non-MC bridge drivers do not need to use this function if the subdev drivers use the method shown in 1.2. I think this is the question here:
> 
> - Change all the callers and use the macro in this patch. Then the macro is not legacy.
> - Change the callees, in which case this macro is needed only in some cases where, for whatever reason, a specific callee has not been changed (yet?). In this case it's legacy.
> 
> Changing the callers would be a nicer option, I think, but I also fear that it's very difficult and easily brings in bugs. I haven't looked closely, but I think it would be a big patch.

I believe changing the callers is the correct approach. Next to that I want to slowly
convert all subdevs to be state-aware (i.e. use sd->active_state etc). Based on past
experience it is a really bad idea to have all these variations in how subdevs work.
That should be minimized.

I suspect that modifying the callers isn't as bad as you might think. It is also the
sane approach: passing the active_state to the subdev really *is* what you want to do.
It is not a hack, it is the right thing. Adding a workaround in a subdev where a
NULL state is handled separately is, however, a hack, and a hack that is all to easy
to forget to implement in new drivers.

> 
> And it's not clear to me if there's a benefit: do all those drivers ever need to interact a state-aware subdev driver? If they do, maybe there are only a few such subdev drivers, and it's not a big
> issue to have the lock/unlock code in those state-aware subdev drivers. All the other state-aware subdev drivers would not need the legacy support code as they're used only in more modern pipelines.

You never know that: anyone can hook any sensor to a non-MC driver in a product.
Just because there are no such boards in the mainline kernel doesn't mean that
such boards don't exist.

I hope this helps, and apologies for the delay in replying.

Regards,

	Hans
  

On 22/03/2022 11:23, Hans Verkuil wrote:

> Can you make a patch that converts ov5648.c (or a similar driver) to use
> sd->active_state? I'd like to see how that looks.

I have ov5640, I can convert that one.
> Right. So to confirm: the only reason a state-ware subdev set_fmt op is called
> with a NULL state pointer is if it is called from non-MC bridge drivers? And if

I hope so. There could be MC bridge or subdev drivers that call set_fmt 
(or get_fmt) on other subdevs. I cannot figure out why that would be 
done, though, and it sounds wrong to me. Afaik, with MC, set_fmt (and 
other such calls) always come from the userspace, and each subdev is 
configured separately from the others.

But if there are such drivers, they'll get fixed along the non-MC bridge 
drivers.

> we would update those bridge drivers to always pass a non-NULL state pointer,
> then such subdevs no longer need to care about this and can just use the state.

That's correct.

>>> 2.1) How to convert a bridge driver (either non-MC or MC, but no state support) to properly support a fully converted subdev (MC state-aware) driver?
>>
>> Converting non-MC driver to an MC driver is out of the context here, as it's not related to the active state. An MC bridge driver should work fine with state-aware subdev drivers, as the bridge driver
>> should not call any of the subdev's state-related ops.
>>
>> To make a non-MC bridge driver support state-aware subdev drivers, they can use the macro in this patch.
> 
> That sounds good to me.
> 
> The only legacy bit about the macro, though, is the fact that v4l2_subdev_get_active_state()
> can return NULL: that indicates that the subdev driver isn't properly state-aware.
> 
> So I think this should be a regular macro in v4l2-subdev.h.
> 
> Perhaps with a comment mentioning that v4l2_subdev_get_active_state() can be replaced by
> v4l2_subdev_lock_and_get_active_state() once all subdevs are state-aware.

Yes, that's true. I'll add the comment, and move this back to v4l2-subdev.h.

>>> 2.2) What is the legacy code that such a bridge driver has to keep in order to work with older subdev drivers that are not yet MC state-aware?
>>
>> The older subdev drivers should keep working without any extra code.
>>
>>> The code needed for 1.2 and 2.2 (helper functions/macros) is legacy code, and can be marked as such.
>>>> If this is clear, then we can work towards converting both subdev and
>> bridge drivers and eventually (might take years!) get rid of the legacy code.
>>>
>>> Removing support for case 2 is probably something that we want to do sooner than later.
>>>
>>> For the CAL driver I do not consider non-MC support as legacy. It's legacy in the context of the CAL driver only, but API-wise it is not since there are many non-MC bridge drivers.
>>
>> That's true, but also, non-MC bridge drivers do not need to use this function if the subdev drivers use the method shown in 1.2. I think this is the question here:
>>
>> - Change all the callers and use the macro in this patch. Then the macro is not legacy.
>> - Change the callees, in which case this macro is needed only in some cases where, for whatever reason, a specific callee has not been changed (yet?). In this case it's legacy.
>>
>> Changing the callers would be a nicer option, I think, but I also fear that it's very difficult and easily brings in bugs. I haven't looked closely, but I think it would be a big patch.
> 
> I believe changing the callers is the correct approach. Next to that I want to slowly
> convert all subdevs to be state-aware (i.e. use sd->active_state etc). Based on past
> experience it is a really bad idea to have all these variations in how subdevs work.
> That should be minimized.
> 
> I suspect that modifying the callers isn't as bad as you might think. It is also the
> sane approach: passing the active_state to the subdev really *is* what you want to do.
> It is not a hack, it is the right thing. Adding a workaround in a subdev where a
> NULL state is handled separately is, however, a hack, and a hack that is all to easy
> to forget to implement in new drivers.

Ok.

I'll send a v6 soon with the changes we've discussed, and I'll take a 
look at ov5640.c and changing the callers.

  Tomi
  

Hi Tomi,

On Mon, Mar 07, 2022 at 04:00:53PM +0200, Tomi Valkeinen wrote:
> On 07/03/2022 11:51, Hans Verkuil wrote:
> > On 3/7/22 10:16, Tomi Valkeinen wrote:
> >> On 07/03/2022 10:36, Hans Verkuil wrote:
> >>> On 3/7/22 08:16, Tomi Valkeinen wrote:
> >>>> On 04/03/2022 15:34, Hans Verkuil wrote:
> >>>>> On 3/1/22 11:55, Tomi Valkeinen wrote:
> >>>>>> Add v4l2_subdev_call_state_active() macro to help calling subdev ops
> >>>>>> that take a subdev state as a parameter. Normally the v4l2 framework
> >>>>>> will lock and pass the correct subdev state to the subdev ops, but there
> >>>>>> are legacy situations where this is not the case (e.g. non-MC video
> >>>>>> device driver calling set_fmt in a source subdev).
> >>>>>>
> >>>>>> As this macro is only needed for legacy use cases, the macro is added in
> >>>>>> a new header file, v4l2-subdev-legacy.h.
> >>>>>>
> >>>>>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
> >>>>>> ---
> >>>>>>     include/media/v4l2-subdev-legacy.h | 42 ++++++++++++++++++++++++++++++
> >>>>>>     1 file changed, 42 insertions(+)
> >>>>>>     create mode 100644 include/media/v4l2-subdev-legacy.h
> >>>>>>
> >>>>>> diff --git a/include/media/v4l2-subdev-legacy.h b/include/media/v4l2-subdev-legacy.h
> >>>>>> new file mode 100644
> >>>>>> index 000000000000..6a61e579b629
> >>>>>> --- /dev/null
> >>>>>> +++ b/include/media/v4l2-subdev-legacy.h
> >>>>>> @@ -0,0 +1,42 @@
> >>>>>> +/* SPDX-License-Identifier: GPL-2.0-or-later */
> >>>>>> +/*
> >>>>>> + *  V4L2 sub-device legacy support header.
> >>>>>> + *
> >>>>>> + *  Copyright (C) 2022  Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
> >>>>>> + */
> >>>>>> +
> >>>>>> +#ifndef _V4L2_SUBDEV_LEGACY_H
> >>>>>> +#define _V4L2_SUBDEV_LEGACY_H
> >>>>>> +
> >>>>>> +/**
> >>>>>> + * v4l2_subdev_call_state_active - call an operation of a v4l2_subdev which
> >>>>>> + *                   takes state as a parameter, passing the
> >>>>>> + *                   subdev its active state.
> >>>>>> + *
> >>>>>> + * @sd: pointer to the &struct v4l2_subdev
> >>>>>> + * @o: name of the element at &struct v4l2_subdev_ops that contains @f.
> >>>>>> + *     Each element there groups a set of callbacks functions.
> >>>>>> + * @f: callback function to be called.
> >>>>>> + *     The callback functions are defined in groups, according to
> >>>>>> + *     each element at &struct v4l2_subdev_ops.
> >>>>>> + * @args: arguments for @f.
> >>>>>> + *
> >>>>>> + * This is similar to v4l2_subdev_call(), except that this version can only be
> >>>>>> + * used for ops that take a subdev state as a parameter. The macro will get the
> >>>>>> + * active state and lock it before calling the op, and unlock it after the
> >>>>>> + * call.
> >>>>>> + */
> >>>>>
> >>>>> You should explain why this is a legacy macro and, ideally, what would need to
> >>>>> be done to get rid of it. The first is in the commit log, but nobody reads that :-)
> >>>>>
> >>>>> But if just using it in a non-MC video device driver constitutes 'legacy' use,
> >>>>> then I disagree with that. There are many non-MC video device drivers, nothing
> >>>>> legacy about that.
> >>>>
> >>>> It's difficult to define all the scenarios where this can be
> >>>> used, but the ones I can imagine fall under legacy (depending on
> >>>> how you define that, though).
> >>>>
> >>>> I use this in CAL driver, which supports non-MC (legacy) and MC.
> >>>> CAL has a bunch of video devices (one for each DMA engine) and
> >>>> two CSI-2 PHY devices (v4l2 subdevs).
> >>>>
> >>>> When operating in MC mode, the userspace will call, e.g., set_fmt
> >>>> in the PHY subdev, and so forth.
> >>>>
> >>>> But in non-MC case the userspace calls VIDIOC_S_FMT in the video
> >>>> dev, and the video dev has to propagate that to the PHY subdev. I
> >>>> do this propagation using the v4l2_subdev_call_state_active
> >>>> macro.
> >>>>
> >>>> I don't know if there are other drivers that support both non-MC
> >>>> and MC modes. I could also just move this macro to the CAL
> >>>> driver, and we could add this to the v4l2 framework if we see
> >>>> other drivers using similar constructs.
> >>>
> >>> It is common to have non-MC drivers that call set_fmt of a subdev.
> >>> Wouldn't they all need to use this helper macro? If so, then this is NOT a
> >>> legacy use, it's just a non-MC driver use.
> >>
> >> These non-MC drivers that call set_fmt of a subdev, they're video
> >> device drivers, right? In other words, there are no subdev drivers
> >> that call set_fmt on other subdevs?
> > 
> > Probably not, but I am not 100% certain. There are a few nested
> > subdev cases, but I don't remember which.
> > 
> >> This does get a bit complex, keeping the old and new code working
> >> together. In this context, I think we have three different
> >> "classes":
> >>
> >> 1. non-MC
> >> 2. MC, no state support
> >> 3. MC, state support
> > 
> > Are you talking about subdev drivers or bridge drivers? It's a bit
> > confusing. I'm assuming it can be either.
> 
> Yes, I mean either one.
> 
> >> We have classes 1 and 2 in upstream, and 3 will be enabled with
> >> this series (and expanded with the streams series).
> >>
> >> Classes 1 and 2 continue working as before. If you have a pipeline
> >> with only class 3 drivers, it works without any legacy "hacks". The
> >> problems come when you combine 1 or 2 with 3. Or possibly the
> >> problems appear only when combining class 1 and class 3, as class 2
> >> drivers are not supposed to call subdev ops which take a state
> >> parameter on other subdevs.
> >>
> >> A class 3 driver expects to get either a try or an active state as
> >> a parameter, but class 1 drivers pass NULL for the active state. If
> >> you write a class 3 driver and want it to work with class 1
> >> (without any changes to those drivers), you must do extra plumbing
> >> in the ops functions, to catch the NULL state case and get & lock
> >> the state yourself. If you do that, this macro is not needed.
> >>
> >> Alternatively, class 1 drivers could be changed to use this macro,
> >> so that a possible class 3 driver in the pipeline would work
> >> without additional code. But there are a lot of class 1 drivers,
> >> and thus modifications, and I wasn't planning to go that way.
> >>
> >> The CAL driver I mentioned supports both class 1 and class 3 (via a
> >> module parameter) in the video dev driver, and the class 1 mode
> >> uses this macro as CAL's PHY subdev (part of the same driver) is a
> >> class 3 subdev. The class 1 support is legacy support in CAL's
> >> case.
> >>
> >> So... Depending on what kind of driver combinations we want to
> >> support, this may or may not be legacy, depending on how you define
> >> legacy =).
> > 
> > Let me try to explain what my concerns are. Eventually I would
> > really like all subdevs to be capable of working with MC bridge
> > drivers, i.e. have state support. Bridge drivers can be either MC or
> > non-MC.
> > 
> > So I would like to know:
> > 
> > 1.1) How to convert a subdev driver to a MC state-aware subdev driver?
> 
> I presume you mean how to convert an MC subdev driver to state-aware MC 
> driver. If you have a non-MC subdev driver, then that first needs to be 
> converted to an MC driver, which is out of scope here.
> 
> Here's an example commit where I convert OV10635 to streams:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/tomba/linux.git/commit/?h=streams/work-v11&id=ffb08af15f04ef2ce0a00bb356e957c839d6bccb
> 
> However, the commit is on top of the streams series and I add support 
> for multiple streams in addition to the active state, so 1) it's not as 
> simple as it could be (e.g. get_frame_desc() and set_routing() ops could 
> be left out), and 2) it's not using the old-style v4l2_subdev_pad_config 
> (which is the only option on top of this series), but the new routes & 
> streams.
> 
> So the example doesn't quite answer to your question... I haven't looked 
> at implementing a driver which would be state-aware but not 
> streams-aware, but I think it's essentially just:
> 
> - Use v4l2_subdev_init_finalize() to create the active state storage
> - Always use state->pads instead of something stored in the driver's 
> private data for active case.
> 
> > 1.2) What is the legacy code that such a MC state-aware subdev
> > driver has to keep in order to work with older bridge drivers that
> > do not support such subdev drivers? (i.e. they pass NULL as the
> > state)
> 
> I believe the only extra code needed is to handle the state == NULL 
> case. This means adding code to each subdev op which has the state as a 
> parameter, and doing, perhaps, something like this:
> 
> my_set_fmt(sd, _state, fmt)
> {
> 	state = _state
> 
> 	if (!_state)
> 		state = v4l2_subdev_lock_and_get_active_state(sd);
> 
> 	... use 'state' here ...
> 
> 	if (!_state)
> 		v4l2_subdev_unlock_state(state);
> }
> 
> Maybe we can somehow macro-ify the above, which creates a wrapper for 
> the op. Or, as I mentioned, we could try to change all the drivers that 
> do those calls, so that they use the macro in this patch instead.
>
> > 2.1) How to convert a bridge driver (either non-MC or MC, but no
> > state support) to properly support a fully converted subdev (MC
> > state-aware) driver?
> 
> Converting non-MC driver to an MC driver is out of the context here, as 
> it's not related to the active state. An MC bridge driver should work 
> fine with state-aware subdev drivers, as the bridge driver should not 
> call any of the subdev's state-related ops.
> 
> To make a non-MC bridge driver support state-aware subdev drivers, they 
> can use the macro in this patch.
> 
> > 2.2) What is the legacy code that such a bridge driver has to keep
> > in order to work with older subdev drivers that are not yet MC
> > state-aware?
> 
> The older subdev drivers should keep working without any extra code.
> 
> > The code needed for 1.2 and 2.2 (helper functions/macros) is legacy
> > code, and can be marked as such.
> > 
> > > If this is clear, then we can work towards converting both subdev
> > > and bridge drivers and eventually (might take years!) get rid of
> > > the legacy code.
> > 
> > Removing support for case 2 is probably something that we want to do
> > sooner than later.
> > 
> > For the CAL driver I do not consider non-MC support as legacy. It's
> > legacy in the context of the CAL driver only, but API-wise it is not
> > since there are many non-MC bridge drivers.
> 
> That's true, but also, non-MC bridge drivers do not need to use this 
> function if the subdev drivers use the method shown in 1.2. I think this 
> is the question here:
> 
> - Change all the callers and use the macro in this patch. Then the macro 
> is not legacy.
> - Change the callees, in which case this macro is needed only in some 
> cases where, for whatever reason, a specific callee has not been changed 
> (yet?). In this case it's legacy.
> 
> Changing the callers would be a nicer option, I think, but I also fear 
> that it's very difficult and easily brings in bugs. I haven't looked 
> closely, but I think it would be a big patch.

Could this be done in the existing wrappers (v4l2_subdev_call_wrappers,
in drivers/media/v4l2-core/v4l2-subdev.c) ?

> And it's not clear to me if there's a benefit: do all those drivers ever 
> need to interact a state-aware subdev driver? If they do, maybe there 
> are only a few such subdev drivers, and it's not a big issue to have the 
> lock/unlock code in those state-aware subdev drivers. All the other 
> state-aware subdev drivers would not need the legacy support code as 
> they're used only in more modern pipelines.