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

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Message-ID: <8c8c0154-f790-f90f-e117-0d3df6823f20@nvidia.com>
Date:   Thu, 23 Apr 2020 19:12:07 -0700
From:   Sowjanya Komatineni <skomatineni@...dia.com>
To:     Dmitry Osipenko <digetx@...il.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


On 4/23/20 7:09 PM, Dmitry Osipenko wrote:
> External email: Use caution opening links or attachments
>
>
> 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());
ok, will add err to wait condition to let it sleep