| Message ID | 20240507060323.26950-2-laurent.pinchart+renesas@ideasonboard.com (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | Kieran Bingham |
| Headers | show |
| Series | media: v4l2-subdev: Support const-awareness in state accessors | expand |
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 *
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, \ + default: 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. */ -#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 *
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(-)