[PATCH v3 1/2] media: v4l2-subdev: Provide const-aware subdev state accessors

[Laurent Pinchart]

On Wed, May 08, 2024 at 06:44:40PM +0000, Sakari Ailus wrote:
> Hi Laurent,
> 
> Thanks for the update.
> 
> This is starting to look very nice. A few comments below...

Thanks :-)

> On Tue, May 07, 2024 at 07:19:06PM +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 at ideasonboard.com>
> > ---
> > Changes since v2:
> > 
> > - Specify types explicitly in _Generic() expression
> > - Fix cast of value to const pointer
> > 
> > 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..cd3e9e2ebe4d 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)				\
> 
> This function just changes the return value constness. How about calling it
> e.g. __v4l2_subdev_state_constify_ret instead?

Fine with me.

> Either way,
> 
> Reviewed-by: Sakari Ailus <sakari.ailus at linux.intel.com>
> 
> > +	_Generic(state,								\
> > +		const struct v4l2_subdev_state *: (const typeof(*(value)) *)(value), \
> > +		struct v4l2_subdev_state *: (value)				\
> > +	)
> > +
> >  /**
> >   * 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.
> 
> These function names are missing _get after the prefix.
> 
> Could you also add the interval function here, too?
> 
> The would fit well for a separate patch.

I'll drop the change above and will fix the whole paragraph in a
separate patch.

> >   */
> > -#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 *

-- 
Regards,

Laurent Pinchart