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Subject: Re: [RFC PATCH v9 6/9] media: tegra: Add Tegra210 Video input driver
To:     Dmitry Osipenko <digetx@gmail.com>, <thierry.reding@gmail.com>,
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From:   Sowjanya Komatineni <skomatineni@nvidia.com>
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On 4/23/20 5:42 PM, Dmitry Osipenko wrote:
> External email: Use caution opening links or attachments
>
>
> 24.04.2020 02:50, Sowjanya Komatineni =D0=BF=D0=B8=D1=88=D0=B5=D1=82:
>> On 4/23/20 4:25 PM, Dmitry Osipenko wrote:
>>> External email: Use caution opening links or attachments
>>>
>>>
>>> 24.04.2020 02:20, Sowjanya Komatineni =D0=BF=D0=B8=D1=88=D0=B5=D1=82:
>>>> On 4/23/20 4:19 PM, Sowjanya Komatineni wrote:
>>>>> On 4/23/20 4:16 PM, Dmitry Osipenko wrote:
>>>>>> External email: Use caution opening links or attachments
>>>>>>
>>>>>>
>>>>>> 22.04.2020 09:18, Sowjanya Komatineni =D0=BF=D0=B8=D1=88=D0=B5=D1=82=
:
>>>>>>> +static int chan_capture_kthread_start(void *data)
>>>>>>> +{
>>>>>>> +     struct tegra_vi_channel *chan =3D data;
>>>>>>> +     struct tegra_channel_buffer *buf;
>>>>>>> +     int err =3D 0;
>>>>>>> +
>>>>>>> +     set_freezable();
>>>>>>> +
>>>>>>> +     while (1) {
>>>>>>> +             try_to_freeze();
>>>>>>> +
>>>>>>> +             wait_event_interruptible(chan->start_wait,
>>>>>>> + !list_empty(&chan->capture) ||
>>>>>>> +                                      kthread_should_stop());
>>>>>>> +
>>>>>>> +             if (kthread_should_stop())
>>>>>>> +                     break;
>>>>>>> +
>>>>>>> +             /*
>>>>>>> +              * Source is not streaming if error is non-zero.
>>>>>>> +              * So, do not dequeue buffers on capture error.
>>>>>>> +              */
>>>>>>> +             if (err)
>>>>>>> +                     continue;
>>>>>> This will result in an endless loop, I suppose it wasn't the
>>>>>> intention.
>>>>> no it will not. on error we report vb2_queue_error which will do
>>>>> streaming stop request.
>>>>>
>>>>> So thread will be stopped on streaming stop request thru kthread stop
>>>>> signal
>>>> To be clear on error it reports vb2 queue error and waits for stop
>>>> streaming to happen
>>> If thread should exit on error, then it should do it on the actual
>>> error. Otherwise it looks very error-prone.
>> When v4l2 drivers indicate fatal error through vb2_queue_error, queue
>> error flag  is set and wakes up all processes waiting on queue along
>> with polling reporting  EPOLLERR and also reporting error for queuing
>> and dequeuing buffers. Stream stop will surely happen which stops the
>> thread.
> This doesn't explain what is the point of continuing to loop instead of
> exiting immediately on error.

We are using 2 threads and when capture start error happens, we can stop=20
capture_start thread immediately but capture_finish thread will still=20
run for any outstanding buffers.

So, as it makes no diff stopping both threads during stream stop which=20
will definitely happen on error and when we don't dequeue buffers