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

Tomi Valkeinen tomi.valkeinen at ideasonboard.com
Mon May 6 23:46:08 PDT 2024


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 at 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 *




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