linux-kernel - Re: [RFC PATCH v9 6/9] media: tegra: Add Tegra210 Video input driver

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Date:   Fri, 24 Apr 2020 05:09:00 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Sowjanya Komatineni <skomatineni@...dia.com>,
        thierry.reding@...il.com, jonathanh@...dia.com, frankc@...dia.com,
        hverkuil@...all.nl, sakari.ailus@....fi, helen.koike@...labora.com
Cc:     sboyd@...nel.org, linux-media@...r.kernel.org,
        devicetree@...r.kernel.org, linux-clk@...r.kernel.org,
        linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v9 6/9] media: tegra: Add Tegra210 Video input driver

24.04.2020 04:08, Sowjanya Komatineni пишет:
> 
> On 4/23/20 5:51 PM, Sowjanya Komatineni wrote:
>>
>> 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 пишет:
>>>> 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 пишет:
>>>>>> 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 пишет:
>>>>>>>>> +static int chan_capture_kthread_start(void *data)
>>>>>>>>> +{
>>>>>>>>> +     struct tegra_vi_channel *chan = data;
>>>>>>>>> +     struct tegra_channel_buffer *buf;
>>>>>>>>> +     int err = 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 capture_start thread immediately but capture_finish thread will
>> still run for any outstanding buffers.
>>
>> So, as it makes no diff stopping both threads during stream stop which
>> will definitely happen on error and when we don't dequeue buffers
>>
> Also there will be an issue if we break on error immediately during
> stop_streaming -> kthread_stop()
> 
> As stop streaming can happen any time, we do kthread_stop and in case of
> error if we stop thread and on stop streaming kthread_stop might crash
> as kthread_stop can only be called on running thread
> 

This a better explanation, but still it's not good that there could be a
busy loop within the thread.

Should be better to sleep if error is set.

wait_event_interruptible(chan->start_wait,
			(!err && !list_empty(&chan->capture)) ||
			kthread_should_stop());