[v2,1/2] media: v4l2-subdev: Provide const-aware subdev state accessors

  It would be useful to mark instances of v4l2_subdev_state structures as
const when code needs to access them read-only. This isn't currently
possible, as the v4l2_subdev_state_get_*() accessor functions take a
non-const pointer to the state.

Use _Generic() to provide two different versions of the accessors, for
const and non-const states respectively. The former returns a const
pointer to the requested format, rectangle or interval, implementing
const-correctness. The latter returns a non-const pointer, preserving
the current behaviour for drivers.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
---
Changes since v1:

- Wrap the accessors with _Generic() using a single macro instead of
  adding a _Generic() statement in each of them.
---
 include/media/v4l2-subdev.h | 40 +++++++++++++++++++++++++------------
 1 file changed, 27 insertions(+), 13 deletions(-)
  

Hi,

On 07/05/2024 09:03, Laurent Pinchart wrote:
> It would be useful to mark instances of v4l2_subdev_state structures as
> const when code needs to access them read-only. This isn't currently
> possible, as the v4l2_subdev_state_get_*() accessor functions take a
> non-const pointer to the state.
> 
> Use _Generic() to provide two different versions of the accessors, for
> const and non-const states respectively. The former returns a const
> pointer to the requested format, rectangle or interval, implementing
> const-correctness. The latter returns a non-const pointer, preserving
> the current behaviour for drivers.
> 
> Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
> ---
> Changes since v1:
> 
> - Wrap the accessors with _Generic() using a single macro instead of
>    adding a _Generic() statement in each of them.
> ---
>   include/media/v4l2-subdev.h | 40 +++++++++++++++++++++++++------------
>   1 file changed, 27 insertions(+), 13 deletions(-)
> 
> diff --git a/include/media/v4l2-subdev.h b/include/media/v4l2-subdev.h
> index e30c463d90e5..c8cfa1eaa82f 100644
> --- a/include/media/v4l2-subdev.h
> +++ b/include/media/v4l2-subdev.h
> @@ -1326,6 +1326,16 @@ void v4l2_subdev_cleanup(struct v4l2_subdev *sd);
>   #define __v4l2_subdev_state_gen_call(NAME, _1, ARG, ...)	\
>   	__v4l2_subdev_state_get_ ## NAME ## ARG
>   
> +/*
> + * A macro to constify the return value of the state accessors when the state
> + * parameter is const.
> + */
> +#define __v4l2_subdev_state_constify_call(state, value)			\
> +	_Generic(state,							\
> +		const typeof(*(state)) *: (const typeof(value))value,	\

Why is the first typeof not typeof(state)?

> +		default: value						\
> +	)

For non-const state this expands to:

_Generic(state,
	const struct v4l2_subdev_state *: (const struct valuetype *)value,
	default: value
)

And for const state:

_Generic(state,
	const const struct v4l2_subdev_state *: (const struct valuetype *)value,
	default: value
)

Afaics, that works as the double-const is just considered a single-const, but the
macro looks like as if you use typeof(state) thinking that it would always produce
the non-const base type. Also, the macro would not work if the default case was
"typeof(*(state)) *".

Is there a reason to use the typeof()? As the result of typeof depends on the state
type, the macro also changes depending on the state type, which I feel is an extra,
unnecessary, complication in an already not-so-clear system.

Why not just use "struct v4l2_subdev_state" directly? Or if typeof() is used, maybe
explain in a comment that the "double-constifying" is intended, and it happens to
work because we have the default case (instead of explicit non-const case).

  Tomi

>   /**
>    * v4l2_subdev_state_get_format() - Get pointer to a stream format
>    * @state: subdevice state
> @@ -1340,16 +1350,17 @@ void v4l2_subdev_cleanup(struct v4l2_subdev *sd);
>    */
>   /*
>    * Wrap v4l2_subdev_state_get_format(), allowing the function to be called with
> - * two or three arguments. The purpose of the __v4l2_subdev_state_get_format()
> + * two or three arguments. The purpose of the __v4l2_subdev_state_gen_call()
>    * macro below is to come up with the name of the function or macro to call,
>    * using the last two arguments (_stream and _pad). The selected function or
>    * macro is then called using the arguments specified by the caller. A similar
>    * arrangement is used for v4l2_subdev_state_crop() and
>    * v4l2_subdev_state_compose() below.
>    */
> -#define v4l2_subdev_state_get_format(state, pad, ...)			\
> -	__v4l2_subdev_state_gen_call(format, ##__VA_ARGS__, , _pad)	\
> -		(state, pad, ##__VA_ARGS__)
> +#define v4l2_subdev_state_get_format(state, pad, ...)				\
> +	__v4l2_subdev_state_constify_call(state,				\
> +		__v4l2_subdev_state_gen_call(format, ##__VA_ARGS__, , _pad)	\
> +			((struct v4l2_subdev_state *)state, pad, ##__VA_ARGS__))
>   #define __v4l2_subdev_state_get_format_pad(state, pad)	\
>   	__v4l2_subdev_state_get_format(state, pad, 0)
>   struct v4l2_mbus_framefmt *
> @@ -1368,9 +1379,10 @@ __v4l2_subdev_state_get_format(struct v4l2_subdev_state *state,
>    * For stream-unaware drivers the crop rectangle for the corresponding pad is
>    * returned. If the pad does not exist, NULL is returned.
>    */
> -#define v4l2_subdev_state_get_crop(state, pad, ...)			\
> -	__v4l2_subdev_state_gen_call(crop, ##__VA_ARGS__, , _pad)	\
> -		(state, pad, ##__VA_ARGS__)
> +#define v4l2_subdev_state_get_crop(state, pad, ...)				\
> +	__v4l2_subdev_state_constify_call(state,				\
> +		__v4l2_subdev_state_gen_call(crop, ##__VA_ARGS__, , _pad)	\
> +			((struct v4l2_subdev_state *)state, pad, ##__VA_ARGS__))
>   #define __v4l2_subdev_state_get_crop_pad(state, pad)	\
>   	__v4l2_subdev_state_get_crop(state, pad, 0)
>   struct v4l2_rect *
> @@ -1389,9 +1401,10 @@ __v4l2_subdev_state_get_crop(struct v4l2_subdev_state *state, unsigned int pad,
>    * For stream-unaware drivers the compose rectangle for the corresponding pad is
>    * returned. If the pad does not exist, NULL is returned.
>    */
> -#define v4l2_subdev_state_get_compose(state, pad, ...)			\
> -	__v4l2_subdev_state_gen_call(compose, ##__VA_ARGS__, , _pad)	\
> -		(state, pad, ##__VA_ARGS__)
> +#define v4l2_subdev_state_get_compose(state, pad, ...)				\
> +	__v4l2_subdev_state_constify_call(state,				\
> +		__v4l2_subdev_state_gen_call(compose, ##__VA_ARGS__, , _pad)	\
> +			((struct v4l2_subdev_state *)state, pad, ##__VA_ARGS__))
>   #define __v4l2_subdev_state_get_compose_pad(state, pad)	\
>   	__v4l2_subdev_state_get_compose(state, pad, 0)
>   struct v4l2_rect *
> @@ -1410,9 +1423,10 @@ __v4l2_subdev_state_get_compose(struct v4l2_subdev_state *state,
>    * For stream-unaware drivers the frame interval for the corresponding pad is
>    * returned. If the pad does not exist, NULL is returned.
>    */
> -#define v4l2_subdev_state_get_interval(state, pad, ...)			\
> -	__v4l2_subdev_state_gen_call(interval, ##__VA_ARGS__, , _pad)	\
> -		(state, pad, ##__VA_ARGS__)
> +#define v4l2_subdev_state_get_interval(state, pad, ...)				\
> +	__v4l2_subdev_state_constify_call(state,				\
> +		__v4l2_subdev_state_gen_call(interval, ##__VA_ARGS__, , _pad)	\
> +			((struct v4l2_subdev_state *)state, pad, ##__VA_ARGS__))
>   #define __v4l2_subdev_state_get_interval_pad(state, pad)	\
>   	__v4l2_subdev_state_get_interval(state, pad, 0)
>   struct v4l2_fract *
  

On Tue, May 07, 2024 at 09:46:08AM +0300, Tomi Valkeinen wrote:
> Hi,
> 
> On 07/05/2024 09:03, Laurent Pinchart wrote:
> > It would be useful to mark instances of v4l2_subdev_state structures as
> > const when code needs to access them read-only. This isn't currently
> > possible, as the v4l2_subdev_state_get_*() accessor functions take a
> > non-const pointer to the state.
> > 
> > Use _Generic() to provide two different versions of the accessors, for
> > const and non-const states respectively. The former returns a const
> > pointer to the requested format, rectangle or interval, implementing
> > const-correctness. The latter returns a non-const pointer, preserving
> > the current behaviour for drivers.
> > 
> > Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
> > ---
> > Changes since v1:
> > 
> > - Wrap the accessors with _Generic() using a single macro instead of
> >    adding a _Generic() statement in each of them.
> > ---
> >   include/media/v4l2-subdev.h | 40 +++++++++++++++++++++++++------------
> >   1 file changed, 27 insertions(+), 13 deletions(-)
> > 
> > diff --git a/include/media/v4l2-subdev.h b/include/media/v4l2-subdev.h
> > index e30c463d90e5..c8cfa1eaa82f 100644
> > --- a/include/media/v4l2-subdev.h
> > +++ b/include/media/v4l2-subdev.h
> > @@ -1326,6 +1326,16 @@ void v4l2_subdev_cleanup(struct v4l2_subdev *sd);
> >   #define __v4l2_subdev_state_gen_call(NAME, _1, ARG, ...)	\
> >   	__v4l2_subdev_state_get_ ## NAME ## ARG
> >   
> > +/*
> > + * A macro to constify the return value of the state accessors when the state
> > + * parameter is const.
> > + */
> > +#define __v4l2_subdev_state_constify_call(state, value)			\
> > +	_Generic(state,							\
> > +		const typeof(*(state)) *: (const typeof(value))value,	\
> 
> Why is the first typeof not typeof(state)?

Wouldn't that be "const const struct v4l2_subdev_state *" when the state
is const ?

> > +		default: value						\
> > +	)
> 
> For non-const state this expands to:
> 
> _Generic(state,
> 	const struct v4l2_subdev_state *: (const struct valuetype *)value,
> 	default: value
> )
> 
> And for const state:
> 
> _Generic(state,
> 	const const struct v4l2_subdev_state *: (const struct valuetype *)value,
> 	default: value
> )

Indeed it would :-)

> 
> Afaics, that works as the double-const is just considered a single-const, but the
> macro looks like as if you use typeof(state) thinking that it would always produce
> the non-const base type. Also, the macro would not work if the default case was
> "typeof(*(state)) *".

Indeed.

> Is there a reason to use the typeof()? As the result of typeof depends on the state
> type, the macro also changes depending on the state type, which I feel is an extra,
> unnecessary, complication in an already not-so-clear system.
> 
> Why not just use "struct v4l2_subdev_state" directly? Or if typeof() is used, maybe
> explain in a comment that the "double-constifying" is intended, and it happens to
> work because we have the default case (instead of explicit non-const case).

Using the explicit type works for me.

> >   /**
> >    * v4l2_subdev_state_get_format() - Get pointer to a stream format
> >    * @state: subdevice state
> > @@ -1340,16 +1350,17 @@ void v4l2_subdev_cleanup(struct v4l2_subdev *sd);
> >    */
> >   /*
> >    * Wrap v4l2_subdev_state_get_format(), allowing the function to be called with
> > - * two or three arguments. The purpose of the __v4l2_subdev_state_get_format()
> > + * two or three arguments. The purpose of the __v4l2_subdev_state_gen_call()
> >    * macro below is to come up with the name of the function or macro to call,
> >    * using the last two arguments (_stream and _pad). The selected function or
> >    * macro is then called using the arguments specified by the caller. A similar
> >    * arrangement is used for v4l2_subdev_state_crop() and
> >    * v4l2_subdev_state_compose() below.
> >    */
> > -#define v4l2_subdev_state_get_format(state, pad, ...)			\
> > -	__v4l2_subdev_state_gen_call(format, ##__VA_ARGS__, , _pad)	\
> > -		(state, pad, ##__VA_ARGS__)
> > +#define v4l2_subdev_state_get_format(state, pad, ...)				\
> > +	__v4l2_subdev_state_constify_call(state,				\
> > +		__v4l2_subdev_state_gen_call(format, ##__VA_ARGS__, , _pad)	\
> > +			((struct v4l2_subdev_state *)state, pad, ##__VA_ARGS__))
> >   #define __v4l2_subdev_state_get_format_pad(state, pad)	\
> >   	__v4l2_subdev_state_get_format(state, pad, 0)
> >   struct v4l2_mbus_framefmt *
> > @@ -1368,9 +1379,10 @@ __v4l2_subdev_state_get_format(struct v4l2_subdev_state *state,
> >    * For stream-unaware drivers the crop rectangle for the corresponding pad is
> >    * returned. If the pad does not exist, NULL is returned.
> >    */
> > -#define v4l2_subdev_state_get_crop(state, pad, ...)			\
> > -	__v4l2_subdev_state_gen_call(crop, ##__VA_ARGS__, , _pad)	\
> > -		(state, pad, ##__VA_ARGS__)
> > +#define v4l2_subdev_state_get_crop(state, pad, ...)				\
> > +	__v4l2_subdev_state_constify_call(state,				\
> > +		__v4l2_subdev_state_gen_call(crop, ##__VA_ARGS__, , _pad)	\
> > +			((struct v4l2_subdev_state *)state, pad, ##__VA_ARGS__))
> >   #define __v4l2_subdev_state_get_crop_pad(state, pad)	\
> >   	__v4l2_subdev_state_get_crop(state, pad, 0)
> >   struct v4l2_rect *
> > @@ -1389,9 +1401,10 @@ __v4l2_subdev_state_get_crop(struct v4l2_subdev_state *state, unsigned int pad,
> >    * For stream-unaware drivers the compose rectangle for the corresponding pad is
> >    * returned. If the pad does not exist, NULL is returned.
> >    */
> > -#define v4l2_subdev_state_get_compose(state, pad, ...)			\
> > -	__v4l2_subdev_state_gen_call(compose, ##__VA_ARGS__, , _pad)	\
> > -		(state, pad, ##__VA_ARGS__)
> > +#define v4l2_subdev_state_get_compose(state, pad, ...)				\
> > +	__v4l2_subdev_state_constify_call(state,				\
> > +		__v4l2_subdev_state_gen_call(compose, ##__VA_ARGS__, , _pad)	\
> > +			((struct v4l2_subdev_state *)state, pad, ##__VA_ARGS__))
> >   #define __v4l2_subdev_state_get_compose_pad(state, pad)	\
> >   	__v4l2_subdev_state_get_compose(state, pad, 0)
> >   struct v4l2_rect *
> > @@ -1410,9 +1423,10 @@ __v4l2_subdev_state_get_compose(struct v4l2_subdev_state *state,
> >    * For stream-unaware drivers the frame interval for the corresponding pad is
> >    * returned. If the pad does not exist, NULL is returned.
> >    */
> > -#define v4l2_subdev_state_get_interval(state, pad, ...)			\
> > -	__v4l2_subdev_state_gen_call(interval, ##__VA_ARGS__, , _pad)	\
> > -		(state, pad, ##__VA_ARGS__)
> > +#define v4l2_subdev_state_get_interval(state, pad, ...)				\
> > +	__v4l2_subdev_state_constify_call(state,				\
> > +		__v4l2_subdev_state_gen_call(interval, ##__VA_ARGS__, , _pad)	\
> > +			((struct v4l2_subdev_state *)state, pad, ##__VA_ARGS__))
> >   #define __v4l2_subdev_state_get_interval_pad(state, pad)	\
> >   	__v4l2_subdev_state_get_interval(state, pad, 0)
> >   struct v4l2_fract *