[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>
---
Bike-shedding about names and the 4 leading underscores is allowed (and
expected).
---
 drivers/media/v4l2-core/v4l2-subdev.c | 24 +++++-----
 include/media/v4l2-subdev.h           | 66 ++++++++++++++++++---------
 2 files changed, 57 insertions(+), 33 deletions(-)
  

Hi Laurent,

Thanks for the patch.

On Mon, May 06, 2024 at 08:52:54PM +0300, 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>
> ---
> Bike-shedding about names and the 4 leading underscores is allowed (and
> expected).
> ---
>  drivers/media/v4l2-core/v4l2-subdev.c | 24 +++++-----
>  include/media/v4l2-subdev.h           | 66 ++++++++++++++++++---------
>  2 files changed, 57 insertions(+), 33 deletions(-)
> 
> diff --git a/drivers/media/v4l2-core/v4l2-subdev.c b/drivers/media/v4l2-core/v4l2-subdev.c
> index 83c6d1e36871..af1fbc28c8f0 100644
> --- a/drivers/media/v4l2-core/v4l2-subdev.c
> +++ b/drivers/media/v4l2-core/v4l2-subdev.c
> @@ -1616,8 +1616,8 @@ void v4l2_subdev_cleanup(struct v4l2_subdev *sd)
>  EXPORT_SYMBOL_GPL(v4l2_subdev_cleanup);
>  
>  struct v4l2_mbus_framefmt *
> -__v4l2_subdev_state_get_format(struct v4l2_subdev_state *state,
> -			       unsigned int pad, u32 stream)
> +____v4l2_subdev_state_get_format(struct v4l2_subdev_state *state,
> +				 unsigned int pad, u32 stream)
>  {
>  	struct v4l2_subdev_stream_configs *stream_configs;
>  	unsigned int i;
> @@ -1647,11 +1647,11 @@ __v4l2_subdev_state_get_format(struct v4l2_subdev_state *state,
>  
>  	return NULL;
>  }
> -EXPORT_SYMBOL_GPL(__v4l2_subdev_state_get_format);
> +EXPORT_SYMBOL_GPL(____v4l2_subdev_state_get_format);
>  
>  struct v4l2_rect *
> -__v4l2_subdev_state_get_crop(struct v4l2_subdev_state *state, unsigned int pad,
> -			     u32 stream)
> +____v4l2_subdev_state_get_crop(struct v4l2_subdev_state *state, unsigned int pad,
> +			       u32 stream)
>  {
>  	struct v4l2_subdev_stream_configs *stream_configs;
>  	unsigned int i;
> @@ -1681,11 +1681,11 @@ __v4l2_subdev_state_get_crop(struct v4l2_subdev_state *state, unsigned int pad,
>  
>  	return NULL;
>  }
> -EXPORT_SYMBOL_GPL(__v4l2_subdev_state_get_crop);
> +EXPORT_SYMBOL_GPL(____v4l2_subdev_state_get_crop);
>  
>  struct v4l2_rect *
> -__v4l2_subdev_state_get_compose(struct v4l2_subdev_state *state,
> -				unsigned int pad, u32 stream)
> +____v4l2_subdev_state_get_compose(struct v4l2_subdev_state *state,
> +				  unsigned int pad, u32 stream)
>  {
>  	struct v4l2_subdev_stream_configs *stream_configs;
>  	unsigned int i;
> @@ -1715,11 +1715,11 @@ __v4l2_subdev_state_get_compose(struct v4l2_subdev_state *state,
>  
>  	return NULL;
>  }
> -EXPORT_SYMBOL_GPL(__v4l2_subdev_state_get_compose);
> +EXPORT_SYMBOL_GPL(____v4l2_subdev_state_get_compose);
>  
>  struct v4l2_fract *
> -__v4l2_subdev_state_get_interval(struct v4l2_subdev_state *state,
> -				 unsigned int pad, u32 stream)
> +____v4l2_subdev_state_get_interval(struct v4l2_subdev_state *state,
> +				   unsigned int pad, u32 stream)
>  {
>  	struct v4l2_subdev_stream_configs *stream_configs;
>  	unsigned int i;
> @@ -1751,7 +1751,7 @@ __v4l2_subdev_state_get_interval(struct v4l2_subdev_state *state,
>  
>  	return NULL;
>  }
> -EXPORT_SYMBOL_GPL(__v4l2_subdev_state_get_interval);
> +EXPORT_SYMBOL_GPL(____v4l2_subdev_state_get_interval);
>  
>  #if defined(CONFIG_VIDEO_V4L2_SUBDEV_API)
>  
> diff --git a/include/media/v4l2-subdev.h b/include/media/v4l2-subdev.h
> index e30c463d90e5..ba722ab501c1 100644
> --- a/include/media/v4l2-subdev.h
> +++ b/include/media/v4l2-subdev.h
> @@ -1340,21 +1340,27 @@ 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)	\
> +#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_pad(state, pad)	\
> +#define __v4l2_subdev_state_get_format_pad(state, pad)				\
>  	__v4l2_subdev_state_get_format(state, pad, 0)
> +#define __v4l2_subdev_state_get_format(state, pad, stream)			\
> +	_Generic(state,								\
> +		const struct v4l2_subdev_state *: ((const struct v4l2_mbus_framefmt *)	\
> +			____v4l2_subdev_state_get_format((struct v4l2_subdev_state *)state, pad, stream)), \
> +		default: ____v4l2_subdev_state_get_format((struct v4l2_subdev_state *)state, pad, stream) \
> +	)
>  struct v4l2_mbus_framefmt *
> -__v4l2_subdev_state_get_format(struct v4l2_subdev_state *state,
> -			       unsigned int pad, u32 stream);
> +____v4l2_subdev_state_get_format(struct v4l2_subdev_state *state,
> +				 unsigned int pad, u32 stream);
>  
>  /**
>   * v4l2_subdev_state_get_crop() - Get pointer to a stream crop rectangle
> @@ -1368,14 +1374,20 @@ __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)	\
> +#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_pad(state, pad)	\
> +#define __v4l2_subdev_state_get_crop_pad(state, pad)				\
>  	__v4l2_subdev_state_get_crop(state, pad, 0)
> +#define __v4l2_subdev_state_get_crop(state, pad, stream)			\
> +	_Generic(state,								\
> +		const struct v4l2_subdev_state *: ((const struct v4l2_rect *)		\
> +			____v4l2_subdev_state_get_crop((struct v4l2_subdev_state *)state, pad, stream)), \
> +		default: ____v4l2_subdev_state_get_crop((struct v4l2_subdev_state *)state, pad, stream) \
> +	)
>  struct v4l2_rect *
> -__v4l2_subdev_state_get_crop(struct v4l2_subdev_state *state, unsigned int pad,
> -			     u32 stream);
> +____v4l2_subdev_state_get_crop(struct v4l2_subdev_state *state, unsigned int pad,
> +			       u32 stream);
>  
>  /**
>   * v4l2_subdev_state_get_compose() - Get pointer to a stream compose rectangle
> @@ -1389,14 +1401,20 @@ __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)	\
> +#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_pad(state, pad)	\
> +#define __v4l2_subdev_state_get_compose_pad(state, pad)				\
>  	__v4l2_subdev_state_get_compose(state, pad, 0)
> +#define __v4l2_subdev_state_get_compose(state, pad, stream)			\
> +	_Generic(state,								\
> +		const struct v4l2_subdev_state *: ((const struct v4l2_rect *)		\
> +			____v4l2_subdev_state_get_compose((struct v4l2_subdev_state *)state, pad, stream)), \
> +		default: ____v4l2_subdev_state_get_compose((struct v4l2_subdev_state *)state, pad, stream) \
> +	)
>  struct v4l2_rect *
> -__v4l2_subdev_state_get_compose(struct v4l2_subdev_state *state,
> -				unsigned int pad, u32 stream);
> +____v4l2_subdev_state_get_compose(struct v4l2_subdev_state *state,
> +				  unsigned int pad, u32 stream);
>  
>  /**
>   * v4l2_subdev_state_get_interval() - Get pointer to a stream frame interval
> @@ -1410,14 +1428,20 @@ __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)	\
> +#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_pad(state, pad)	\
> +#define __v4l2_subdev_state_get_interval_pad(state, pad)			\
>  	__v4l2_subdev_state_get_interval(state, pad, 0)
> +#define __v4l2_subdev_state_get_interval(state, pad, stream)			\
> +	_Generic(state,								\
> +		const struct v4l2_subdev_state *: ((const struct v4l2_frac *)		\
> +			____v4l2_subdev_state_get_interval((struct v4l2_subdev_state *)state, pad, stream)), \
> +		default: ____v4l2_subdev_state_get_interval((struct v4l2_subdev_state *)state, pad, stream) \

Is the cast necessary in the latter case? It would seem like we lose most
of the type safety checks as a result.

Could this be implemented using a similar arrangement than the gen_call
macro above, so that we wouldn't have three very similar sets of macros
here?

> +	)
>  struct v4l2_fract *
> -__v4l2_subdev_state_get_interval(struct v4l2_subdev_state *state,
> -				 unsigned int pad, u32 stream);
> +____v4l2_subdev_state_get_interval(struct v4l2_subdev_state *state,
> +				   unsigned int pad, u32 stream);
>  
>  #if defined(CONFIG_VIDEO_V4L2_SUBDEV_API)
>
  

On Mon, May 06, 2024 at 09:53:18PM +0000, Sakari Ailus wrote:
> Hi Laurent,
> 
> Thanks for the patch.
> 
> On Mon, May 06, 2024 at 08:52:54PM +0300, 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>
> > ---
> > Bike-shedding about names and the 4 leading underscores is allowed (and
> > expected).
> > ---
> >  drivers/media/v4l2-core/v4l2-subdev.c | 24 +++++-----
> >  include/media/v4l2-subdev.h           | 66 ++++++++++++++++++---------
> >  2 files changed, 57 insertions(+), 33 deletions(-)
> > 
> > diff --git a/drivers/media/v4l2-core/v4l2-subdev.c b/drivers/media/v4l2-core/v4l2-subdev.c
> > index 83c6d1e36871..af1fbc28c8f0 100644
> > --- a/drivers/media/v4l2-core/v4l2-subdev.c
> > +++ b/drivers/media/v4l2-core/v4l2-subdev.c
> > @@ -1616,8 +1616,8 @@ void v4l2_subdev_cleanup(struct v4l2_subdev *sd)
> >  EXPORT_SYMBOL_GPL(v4l2_subdev_cleanup);
> >  
> >  struct v4l2_mbus_framefmt *
> > -__v4l2_subdev_state_get_format(struct v4l2_subdev_state *state,
> > -			       unsigned int pad, u32 stream)
> > +____v4l2_subdev_state_get_format(struct v4l2_subdev_state *state,
> > +				 unsigned int pad, u32 stream)
> >  {
> >  	struct v4l2_subdev_stream_configs *stream_configs;
> >  	unsigned int i;
> > @@ -1647,11 +1647,11 @@ __v4l2_subdev_state_get_format(struct v4l2_subdev_state *state,
> >  
> >  	return NULL;
> >  }
> > -EXPORT_SYMBOL_GPL(__v4l2_subdev_state_get_format);
> > +EXPORT_SYMBOL_GPL(____v4l2_subdev_state_get_format);
> >  
> >  struct v4l2_rect *
> > -__v4l2_subdev_state_get_crop(struct v4l2_subdev_state *state, unsigned int pad,
> > -			     u32 stream)
> > +____v4l2_subdev_state_get_crop(struct v4l2_subdev_state *state, unsigned int pad,
> > +			       u32 stream)
> >  {
> >  	struct v4l2_subdev_stream_configs *stream_configs;
> >  	unsigned int i;
> > @@ -1681,11 +1681,11 @@ __v4l2_subdev_state_get_crop(struct v4l2_subdev_state *state, unsigned int pad,
> >  
> >  	return NULL;
> >  }
> > -EXPORT_SYMBOL_GPL(__v4l2_subdev_state_get_crop);
> > +EXPORT_SYMBOL_GPL(____v4l2_subdev_state_get_crop);
> >  
> >  struct v4l2_rect *
> > -__v4l2_subdev_state_get_compose(struct v4l2_subdev_state *state,
> > -				unsigned int pad, u32 stream)
> > +____v4l2_subdev_state_get_compose(struct v4l2_subdev_state *state,
> > +				  unsigned int pad, u32 stream)
> >  {
> >  	struct v4l2_subdev_stream_configs *stream_configs;
> >  	unsigned int i;
> > @@ -1715,11 +1715,11 @@ __v4l2_subdev_state_get_compose(struct v4l2_subdev_state *state,
> >  
> >  	return NULL;
> >  }
> > -EXPORT_SYMBOL_GPL(__v4l2_subdev_state_get_compose);
> > +EXPORT_SYMBOL_GPL(____v4l2_subdev_state_get_compose);
> >  
> >  struct v4l2_fract *
> > -__v4l2_subdev_state_get_interval(struct v4l2_subdev_state *state,
> > -				 unsigned int pad, u32 stream)
> > +____v4l2_subdev_state_get_interval(struct v4l2_subdev_state *state,
> > +				   unsigned int pad, u32 stream)
> >  {
> >  	struct v4l2_subdev_stream_configs *stream_configs;
> >  	unsigned int i;
> > @@ -1751,7 +1751,7 @@ __v4l2_subdev_state_get_interval(struct v4l2_subdev_state *state,
> >  
> >  	return NULL;
> >  }
> > -EXPORT_SYMBOL_GPL(__v4l2_subdev_state_get_interval);
> > +EXPORT_SYMBOL_GPL(____v4l2_subdev_state_get_interval);
> >  
> >  #if defined(CONFIG_VIDEO_V4L2_SUBDEV_API)
> >  
> > diff --git a/include/media/v4l2-subdev.h b/include/media/v4l2-subdev.h
> > index e30c463d90e5..ba722ab501c1 100644
> > --- a/include/media/v4l2-subdev.h
> > +++ b/include/media/v4l2-subdev.h
> > @@ -1340,21 +1340,27 @@ 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)	\
> > +#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_pad(state, pad)	\
> > +#define __v4l2_subdev_state_get_format_pad(state, pad)				\
> >  	__v4l2_subdev_state_get_format(state, pad, 0)
> > +#define __v4l2_subdev_state_get_format(state, pad, stream)			\
> > +	_Generic(state,								\
> > +		const struct v4l2_subdev_state *: ((const struct v4l2_mbus_framefmt *)	\
> > +			____v4l2_subdev_state_get_format((struct v4l2_subdev_state *)state, pad, stream)), \
> > +		default: ____v4l2_subdev_state_get_format((struct v4l2_subdev_state *)state, pad, stream) \
> > +	)
> >  struct v4l2_mbus_framefmt *
> > -__v4l2_subdev_state_get_format(struct v4l2_subdev_state *state,
> > -			       unsigned int pad, u32 stream);
> > +____v4l2_subdev_state_get_format(struct v4l2_subdev_state *state,
> > +				 unsigned int pad, u32 stream);
> >  
> >  /**
> >   * v4l2_subdev_state_get_crop() - Get pointer to a stream crop rectangle
> > @@ -1368,14 +1374,20 @@ __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)	\
> > +#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_pad(state, pad)	\
> > +#define __v4l2_subdev_state_get_crop_pad(state, pad)				\
> >  	__v4l2_subdev_state_get_crop(state, pad, 0)
> > +#define __v4l2_subdev_state_get_crop(state, pad, stream)			\
> > +	_Generic(state,								\
> > +		const struct v4l2_subdev_state *: ((const struct v4l2_rect *)		\
> > +			____v4l2_subdev_state_get_crop((struct v4l2_subdev_state *)state, pad, stream)), \
> > +		default: ____v4l2_subdev_state_get_crop((struct v4l2_subdev_state *)state, pad, stream) \
> > +	)
> >  struct v4l2_rect *
> > -__v4l2_subdev_state_get_crop(struct v4l2_subdev_state *state, unsigned int pad,
> > -			     u32 stream);
> > +____v4l2_subdev_state_get_crop(struct v4l2_subdev_state *state, unsigned int pad,
> > +			       u32 stream);
> >  
> >  /**
> >   * v4l2_subdev_state_get_compose() - Get pointer to a stream compose rectangle
> > @@ -1389,14 +1401,20 @@ __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)	\
> > +#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_pad(state, pad)	\
> > +#define __v4l2_subdev_state_get_compose_pad(state, pad)				\
> >  	__v4l2_subdev_state_get_compose(state, pad, 0)
> > +#define __v4l2_subdev_state_get_compose(state, pad, stream)			\
> > +	_Generic(state,								\
> > +		const struct v4l2_subdev_state *: ((const struct v4l2_rect *)		\
> > +			____v4l2_subdev_state_get_compose((struct v4l2_subdev_state *)state, pad, stream)), \
> > +		default: ____v4l2_subdev_state_get_compose((struct v4l2_subdev_state *)state, pad, stream) \
> > +	)
> >  struct v4l2_rect *
> > -__v4l2_subdev_state_get_compose(struct v4l2_subdev_state *state,
> > -				unsigned int pad, u32 stream);
> > +____v4l2_subdev_state_get_compose(struct v4l2_subdev_state *state,
> > +				  unsigned int pad, u32 stream);
> >  
> >  /**
> >   * v4l2_subdev_state_get_interval() - Get pointer to a stream frame interval
> > @@ -1410,14 +1428,20 @@ __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)	\
> > +#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_pad(state, pad)	\
> > +#define __v4l2_subdev_state_get_interval_pad(state, pad)			\
> >  	__v4l2_subdev_state_get_interval(state, pad, 0)
> > +#define __v4l2_subdev_state_get_interval(state, pad, stream)			\
> > +	_Generic(state,								\
> > +		const struct v4l2_subdev_state *: ((const struct v4l2_frac *)		\
> > +			____v4l2_subdev_state_get_interval((struct v4l2_subdev_state *)state, pad, stream)), \
> > +		default: ____v4l2_subdev_state_get_interval((struct v4l2_subdev_state *)state, pad, stream) \
> 
> Is the cast necessary in the latter case? It would seem like we lose most
> of the type safety checks as a result.

It is, and I was surprised by it. The reason is that all branches of the
_Generic statement must compile, even the ones that are not selected.
When the macro is called with a const state, the first branch is taken,
but the second branch must compile too. Without the cast, the compiler
complains that the state argument passed to
____v4l2_subdev_state_get_interval() is const.

> Could this be implemented using a similar arrangement than the gen_call
> macro above, so that we wouldn't have three very similar sets of macros
> here?

I've sent a v2 that does that.

> > +	)
> >  struct v4l2_fract *
> > -__v4l2_subdev_state_get_interval(struct v4l2_subdev_state *state,
> > -				 unsigned int pad, u32 stream);
> > +____v4l2_subdev_state_get_interval(struct v4l2_subdev_state *state,
> > +				   unsigned int pad, u32 stream);
> >  
> >  #if defined(CONFIG_VIDEO_V4L2_SUBDEV_API)
> >