Re: [PATCH 1/4] media: v4l2-subdev: Add cleanup macros for active state

Tomi Valkeinen
2024-09-24T17:53:49Z

On 24/09/2024 20:17, Laurent Pinchart wrote:
> Hi Tomi,
>
> Thank you for the patch.
>
> On Tue, Sep 17, 2024 at 05:09:29PM +0300, Tomi Valkeinen wrote:
>> From: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
>>
>> Add cleanup macros for active state. These can be used to call
>> v4l2_subdev_lock_and_get_active_state() and manage the unlocking either
>> in unscoped or scoped fashion:
>>
>> This locks the state, gets it to the 'state' variable, and unlocks when
>> exiting the surrounding scope:
>>
>> CLASS(v4l2_subdev_lock_and_get_active_state, state)(subdev);
>>
>> This does the same, but inside the given scope:
>>
>> scoped_v4l2_subdev_lock_and_get_active_state(subdev) {
>> }
>>
>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
>> ---
>> include/media/v4l2-subdev.h | 10 ++++++++++
>> 1 file changed, 10 insertions(+)
>>
>> diff --git a/include/media/v4l2-subdev.h b/include/media/v4l2-subdev.h
>> index bd235d325ff9..699007cfffd7 100644
>> --- a/include/media/v4l2-subdev.h
>> +++ b/include/media/v4l2-subdev.h
>> @@ -8,6 +8,7 @@
>> #ifndef _V4L2_SUBDEV_H
>> #define _V4L2_SUBDEV_H
>>
>> +#include <linux/cleanup.h>
>> #include <linux/types.h>
>> #include <linux/v4l2-subdev.h>
>> #include <media/media-entity.h>
>> @@ -1854,6 +1855,15 @@ v4l2_subdev_lock_and_get_active_state(struct v4l2_subdev *sd)
>> return sd->active_state;
>> }
>>
>> +DEFINE_CLASS(v4l2_subdev_lock_and_get_active_state, struct v4l2_subdev_state *,
>> + v4l2_subdev_unlock_state(_T),
>> + v4l2_subdev_lock_and_get_active_state(sd), struct v4l2_subdev *sd);
>> +
>> +#define scoped_v4l2_subdev_lock_and_get_active_state(sd) \
>> + for (CLASS(v4l2_subdev_lock_and_get_active_state, state)(sd), \
>> + *done = NULL; \
>> + !done; done = (void *)1)
>
> That a very long name :-S Could this be done using the scoped_guard()
> macro instead ? For instance, with spinlocks you can do
>
> scoped_guard(spinlock_irqsave, &dev->lock) {
> ...
> }
>
> It would be nice to be able to write
>
> scoped_guard(v4l2_subdev_state, sd) {

This can be done but then you need to pass the state to it, not sd. Or
perhaps it would be possible to implicitly turn the sd into active
state, but I don't like that at all...

Or maybe:

scoped_guard(v4l2_subdev_state, v4l2_subdev_get_unlocked_active_state(sd))

Not very short either...

> ...
> }
>
> This being said, we would then end up with the state variable being
> named scope, which wouldn't be great.

No, it wouldn't.

> This is actually one of my issues with the above macros, and especially
> scoped_v4l2_subdev_lock_and_get_active_state(). It creates a local state
> variable in the scope, which makes the code less readable in my opinion.

It's trivial to add a variable name there, as mentioned in the intro letter.

You mentioned the const/non-const state issue in the other email. I
wonder if some macro-magic could be done for that... Or we can always
just add "scoped_v4l2_subdev_lock_and_get_active_state_const()"! =)

Also, it's not like we have to use these _everywhere_. So maybe if you
want a const state, don't use the scoped or the class.

> We could keep the class and drop
> scoped_v4l2_subdev_lock_and_get_active_state(). I think I would like to
> shorten the class name then.
>
> Another option is to use DEFINE_FREE() and __free() instead.

That can be added too. I had them, but I didn't consider them as useful
when I already added the class and scoped.

I have to say I don't particularly like the look of either the scoped or
the class, or even the free. But they're so useful wrt. error handling
that I don't care if the syntax annoys me =).

Also, I think in theory one should always just use the scoped macro,
never the class. But as the scoped one adds indentation, in some cases
using the class keeps the code formatting nicer.

Tomi