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@xxxxxxxxxxxxxxxx>
---
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 *
