Re: [PATCH v8 11/38] media: v4l: subdev: Add a function to lock two sub-device states, use it

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Sakari,

Thank you for the patch.

On Wed, Mar 13, 2024 at 09:24:49AM +0200, Sakari Ailus wrote:
> Add two new functions, v4l2_subdev_lock_states() and
> v4l2_subdev_unclock_states(), to acquire and release the state of two
> sub-devices. They differ from calling v4l2_subdev_{un,}lock_state() so
> that if the two states share the same lock, the lock is acquired only
> once.

A bit of a hack, but I think we can live with it for now. A better long
term implementation will be to acquire all state locks from
media_pipeline_start() before doing any validation, and always using
different locks for different subdevs.

Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>

> Also use the new functions in v4l2_subdev_link_validate().
> 
> Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> ---
>  drivers/media/v4l2-core/v4l2-subdev.c | 12 +++-----
>  include/media/v4l2-subdev.h           | 40 +++++++++++++++++++++++++++
>  2 files changed, 44 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/media/v4l2-core/v4l2-subdev.c b/drivers/media/v4l2-core/v4l2-subdev.c
> index 4c6198c48dd6..6c34df19ea2f 100644
> --- a/drivers/media/v4l2-core/v4l2-subdev.c
> +++ b/drivers/media/v4l2-core/v4l2-subdev.c
> @@ -1400,17 +1400,13 @@ int v4l2_subdev_link_validate(struct media_link *link)
>  
>  	states_locked = sink_state && source_state;
>  
> -	if (states_locked) {
> -		v4l2_subdev_lock_state(sink_state);
> -		v4l2_subdev_lock_state(source_state);
> -	}
> +	if (states_locked)
> +		v4l2_subdev_lock_states(sink_state, source_state);
>  
>  	ret = v4l2_subdev_link_validate_locked(link, states_locked);
>  
> -	if (states_locked) {
> -		v4l2_subdev_unlock_state(sink_state);
> -		v4l2_subdev_unlock_state(source_state);
> -	}
> +	if (states_locked)
> +		v4l2_subdev_unlock_states(sink_state, source_state);
>  
>  	return ret;
>  }
> diff --git a/include/media/v4l2-subdev.h b/include/media/v4l2-subdev.h
> index a9e6b8146279..9cce48365975 100644
> --- a/include/media/v4l2-subdev.h
> +++ b/include/media/v4l2-subdev.h
> @@ -1724,6 +1724,46 @@ static inline void v4l2_subdev_unlock_state(struct v4l2_subdev_state *state)
>  	mutex_unlock(state->lock);
>  }
>  
> +/**
> + * v4l2_subdev_lock_states - Lock two sub-device states
> + * @state1: One subdevice state
> + * @state2: The other subdevice state
> + *
> + * Locks the state of two sub-devices.
> + *
> + * The states must be unlocked with v4l2_subdev_unlock_states() after use.
> + *
> + * This differs from calling v4l2_subdev_lock_state() on both states so that if
> + * the states share the same lock, the lock is acquired only once (so no
> + * deadlock occurs). The caller is responsible for ensuring the locks will
> + * always be acquired in the same order.
> + */
> +static inline void v4l2_subdev_lock_states(struct v4l2_subdev_state *state1,
> +					   struct v4l2_subdev_state *state2)
> +{
> +	mutex_lock(state1->lock);
> +	if (state1->lock != state2->lock)
> +		mutex_lock(state2->lock);
> +}
> +
> +/**
> + * v4l2_subdev_unlock_states() - Unlock two sub-device states
> + * @state1: One subdevice state
> + * @state2: The other subdevice state
> + *
> + * Unlocks the state of two sub-devices.
> + *
> + * This differs from calling v4l2_subdev_unlock_state() on both states so that
> + * if the states share the same lock, the lock is released only once.
> + */
> +static inline void v4l2_subdev_unlock_states(struct v4l2_subdev_state *state1,
> +					     struct v4l2_subdev_state *state2)
> +{
> +	mutex_unlock(state1->lock);
> +	if (state1->lock != state2->lock)
> +		mutex_unlock(state2->lock);
> +}
> +
>  /**
>   * v4l2_subdev_get_unlocked_active_state() - Checks that the active subdev state
>   *					     is unlocked and returns it

-- 
Regards,

Laurent Pinchart