linux-kernel - Re: [PATCH v9 06/13] media: atmel: atmel-isc-base: use mutex to lock awb workqueue from streaming

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Date:   Fri, 29 Apr 2022 09:41:56 +0200
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Eugen Hristev <eugen.hristev@...rochip.com>,
        linux-media@...r.kernel.org, jacopo@...ndi.org
Cc:     devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, claudiu.beznea@...rochip.com,
        robh+dt@...nel.org, nicolas.ferre@...rochip.com
Subject: Re: [PATCH v9 06/13] media: atmel: atmel-isc-base: use mutex to lock
 awb workqueue from streaming

Hi Eugen,

On 10/03/2022 10:51, Eugen Hristev wrote:
> The AWB workqueue runs in a kernel thread and needs to be synchronized
> w.r.t. the streaming status.
> It is possible that streaming is stopped while the AWB workq is running.
> In this case it is likely that the check for vb2_start_streaming_called is done
> at one point in time, but the AWB computations are done later, including a call
> to isc_update_profile, which requires streaming to be started.
> Thus , isc_update_profile will fail if during this operation sequence the
> streaming was stopped.
> To solve this issue, a mutex is added, that will serialize the awb work and
> streaming stopping, with the mention that either streaming is stopped
> completely including termination of the last frame is done, and after that
> the AWB work can check stream status and stop; either first AWB work is
> completed and after that the streaming can stop correctly.
> The awb spin lock cannot be used since this spinlock is taken in the same
> context and using it in the stop streaming will result in a recursion BUG.

Please keep the line length in a commit log to no more than 75.

> 
> Signed-off-by: Eugen Hristev <eugen.hristev@...rochip.com>
> Reviewed-by: Jacopo Mondi <jacopo@...ndi.org>
> ---
>  drivers/media/platform/atmel/atmel-isc-base.c | 29 ++++++++++++++++---
>  drivers/media/platform/atmel/atmel-isc.h      |  2 ++
>  2 files changed, 27 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/media/platform/atmel/atmel-isc-base.c b/drivers/media/platform/atmel/atmel-isc-base.c

<snip>

> @@ -1548,6 +1564,7 @@ static int isc_s_awb_ctrl(struct v4l2_ctrl *ctrl)
>  			 */
>  			v4l2_ctrl_activate(isc->do_wb_ctrl, false);
>  		}
> +		mutex_unlock(&isc->awb_mutex);

Huh? What is this unlock doing here? Am I missing something?

Regards,

	Hans

>  
>  		/* if we have autowhitebalance on, start histogram procedure */
>  		if (ctrls->awb == ISC_WB_AUTO &&
> @@ -1740,6 +1757,7 @@ static void isc_async_unbind(struct v4l2_async_notifier *notifier,
>  {
>  	struct isc_device *isc = container_of(notifier->v4l2_dev,
>  					      struct isc_device, v4l2_dev);
> +	mutex_destroy(&isc->awb_mutex);
>  	cancel_work_sync(&isc->awb_work);
>  	video_unregister_device(&isc->video_dev);
>  	v4l2_ctrl_handler_free(&isc->ctrls.handler);
> @@ -1850,6 +1868,8 @@ static int isc_async_complete(struct v4l2_async_notifier *notifier)
>  	isc->current_subdev = container_of(notifier,
>  					   struct isc_subdev_entity, notifier);
>  	mutex_init(&isc->lock);
> +	mutex_init(&isc->awb_mutex);
> +
>  	init_completion(&isc->comp);
>  
>  	/* Initialize videobuf2 queue */
> @@ -1930,6 +1950,7 @@ static int isc_async_complete(struct v4l2_async_notifier *notifier)
>  	video_unregister_device(vdev);
>  
>  isc_async_complete_err:
> +	mutex_destroy(&isc->awb_mutex);
>  	mutex_destroy(&isc->lock);
>  	return ret;
>  }
> diff --git a/drivers/media/platform/atmel/atmel-isc.h b/drivers/media/platform/atmel/atmel-isc.h
> index 9cc69c3ae26d..f98f25a55e73 100644
> --- a/drivers/media/platform/atmel/atmel-isc.h
> +++ b/drivers/media/platform/atmel/atmel-isc.h
> @@ -229,6 +229,7 @@ enum isc_scaler_pads {
>   *
>   * @lock:		lock for serializing userspace file operations
>   *			with ISC operations
> + * @awb_mutex:		serialize access to streaming status from awb work queue
>   * @awb_lock:		lock for serializing awb work queue operations
>   *			with DMA/buffer operations
>   *
> @@ -307,6 +308,7 @@ struct isc_device {
>  	struct work_struct	awb_work;
>  
>  	struct mutex		lock;
> +	struct mutex		awb_mutex;
>  	spinlock_t		awb_lock;
>  
>  	struct regmap_field	*pipeline[ISC_PIPE_LINE_NODE_NUM];