[PATCH v4 2/3] media: v4l2-subdev: Provide const-aware subdev state accessors

[Laurent Pinchart]

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>
Reviewed-by: Sakari Ailus <sakari.ailus at linux.intel.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen at ideasonboard.com>
---
Changes since v3:

- Rename __v4l2_subdev_state_constify_call() to
  __v4l2_subdev_state_constify_ret()
- Update documentation

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 | 48 +++++++++++++++++++++++++------------
 1 file changed, 33 insertions(+), 15 deletions(-)

diff --git a/include/media/v4l2-subdev.h b/include/media/v4l2-subdev.h
index b25b6e97ecbd..e1a6c49a966d 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_ret(state, value)				\
+	_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
@@ -1343,13 +1353,18 @@ void v4l2_subdev_cleanup(struct v4l2_subdev *sd);
  * 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(), v4l2_subdev_state_compose()
- * and v4l2_subdev_state_get_interval() below.
+ * macro is then called using the arguments specified by the caller. The
+ * __v4l2_subdev_state_constify_ret() macro constifies the returned pointer
+ * when the state is const, allowing the state accessors to guarantee
+ * const-correctness in all cases.
+ *
+ * A similar arrangement is used for v4l2_subdev_state_crop(),
+ * v4l2_subdev_state_compose() and v4l2_subdev_state_get_interval() 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_ret(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 +1383,10 @@ __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_ret(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 +1405,10 @@ __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_ret(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 +1427,10 @@ __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_ret(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