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@xxxxxxxxxxxxxxxx>
> > ---
> > 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)
> >
--
Regards,
Laurent Pinchart
