Re: [PATCH v3 17/23] media: atmel: atmel-isc-base: use mutex to lock awb workqueue from streaming

[<Eugen.Hristev@microchip.com>]

On 1/12/22 10:58 AM, Jacopo Mondi wrote:
> Hi Eugen
> 
> On Mon, Dec 13, 2021 at 03:49:34PM +0200, 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 isc->stop 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.
>>
>> Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
>> ---
>>   drivers/media/platform/atmel/atmel-isc-base.c | 31 ++++++++++++++++---
>>   drivers/media/platform/atmel/atmel-isc.h      |  1 +
>>   2 files changed, 28 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/media/platform/atmel/atmel-isc-base.c b/drivers/media/platform/atmel/atmel-isc-base.c
>> index b0c3ed21f372..53cac1aac0fd 100644
>> --- a/drivers/media/platform/atmel/atmel-isc-base.c
>> +++ b/drivers/media/platform/atmel/atmel-isc-base.c
>> @@ -401,6 +401,7 @@ static void isc_stop_streaming(struct vb2_queue *vq)
>>        struct isc_buffer *buf;
>>        int ret;
>>
>> +     mutex_lock(&isc->awb_mutex);
>>        v4l2_ctrl_activate(isc->do_wb_ctrl, false);
>>
>>        isc->stop = true;
>> @@ -410,6 +411,8 @@ static void isc_stop_streaming(struct vb2_queue *vq)
>>                v4l2_err(&isc->v4l2_dev,
>>                         "Timeout waiting for end of the capture\n");
>>
>> +     mutex_unlock(&isc->awb_mutex);
>> +
>>        /* Disable DMA interrupt */
>>        regmap_write(isc->regmap, ISC_INTDIS, ISC_INT_DDONE);
>>
>> @@ -1416,10 +1419,6 @@ static void isc_awb_work(struct work_struct *w)
>>        u32 min, max;
>>        int ret;
>>
>> -     /* streaming is not active anymore */
>> -     if (isc->stop)
>> -             return;
>> -
>>        if (ctrls->hist_stat != HIST_ENABLED)
>>                return;
>>
>> @@ -1470,7 +1469,24 @@ static void isc_awb_work(struct work_struct *w)
>>        }
>>        regmap_write(regmap, ISC_HIS_CFG + isc->offsets.his,
>>                     hist_id | baysel | ISC_HIS_CFG_RAR);
> 
> isc_stop_streaming() calls runtime_put and here you access the hw.

Hi Jacopo,

That is correct. However the awb routine will call resume and get here 
(before accessing the hardware) :

https://elixir.bootlin.com/linux/latest/source/drivers/media/platform/atmel/atmel-isc-base.c#L1722

So I think that we are good as we are now

> 
> Feels like it's safer to hold the mutex for the whole duration of the
> AWB routine ?
> 

I prefer to have the critical section as little as possible. As we are 
only synchronizing the 'streaming status' , only this should be under 
the lock.

If you have a different opinion, let me know.

Eugen

>> +
>> +     /*
>> +      * We have to make sure the streaming has not stopped meanwhile.
>> +      * ISC requires a frame to clock the internal profile update.
>> +      * To avoid issues, lock the sequence with a mutex
>> +      */
>> +     mutex_lock(&isc->awb_mutex);
>> +
>> +     /* streaming is not active anymore */
>> +     if (isc->stop) {
>> +             mutex_unlock(&isc->awb_mutex);
>> +             return;
>> +     };
>> +
>>        isc_update_profile(isc);
>> +
>> +     mutex_unlock(&isc->awb_mutex);
>> +
>>        /* if awb has been disabled, we don't need to start another histogram */
>>        if (ctrls->awb)
>>                regmap_write(regmap, ISC_CTRLEN, ISC_CTRL_HISREQ);
>> @@ -1549,6 +1565,8 @@ static int isc_s_awb_ctrl(struct v4l2_ctrl *ctrl)
>>
>>                isc_update_awb_ctrls(isc);
>>
>> +             mutex_lock(&isc->awb_mutex);
>> +
>>                if (!isc->stop) {
>>                        /*
>>                         * If we are streaming, we can update profile to
>> @@ -1563,6 +1581,7 @@ static int isc_s_awb_ctrl(struct v4l2_ctrl *ctrl)
>>                         */
>>                        v4l2_ctrl_activate(isc->do_wb_ctrl, false);
>>                }
>> +             mutex_unlock(&isc->awb_mutex);
>>
>>                /* if we have autowhitebalance on, start histogram procedure */
>>                if (ctrls->awb == ISC_WB_AUTO && !isc->stop &&
>> @@ -1754,6 +1773,7 @@ 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);
>> @@ -1866,6 +1886,8 @@ 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 */
>> @@ -1941,6 +1963,7 @@ 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 0b6370d7775f..c2cb805faff3 100644
>> --- a/drivers/media/platform/atmel/atmel-isc.h
>> +++ b/drivers/media/platform/atmel/atmel-isc.h
>> @@ -307,6 +307,7 @@ struct isc_device {
>>        struct work_struct      awb_work;
>>
>>        struct mutex            lock; /* serialize access to file operations */
>> +     struct mutex            awb_mutex; /* serialize access to streaming status from awb work queue */
>>        spinlock_t              awb_lock; /* serialize access to DMA buffers from awb work queue */
>>
>>        struct regmap_field     *pipeline[ISC_PIPE_LINE_NODE_NUM];
>> --
>> 2.25.1
>>