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

[Sowjanya Komatineni]

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 ÐÐÑÐÑ:
> > > 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 ÐÐÑÐÑ:
> > > > > 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 ÐÐÑÐÑ:
> > > > > > > > +static int chan_capture_kthread_start(void *data)
> > > > > > > > +{
> > > > > > > > +ÂÂÂÂ struct tegra_vi_channel *chan = data;
> > > > > > > > +ÂÂÂÂ struct tegra_channel_buffer *buf;
> > > > > > > > +ÂÂÂÂ int err = 0;
> > > > > > > > +
> > > > > > > > +ÂÂÂÂ set_freezable();
> > > > > > > > +
> > > > > > > > +ÂÂÂÂ while (1) {
> > > > > > > > +ÂÂÂÂÂÂÂÂÂÂÂÂ try_to_freeze();
> > > > > > > > +
> > > > > > > > + wait_event_interruptible(chan->start_wait,
> > > > > > > > + !list_empty(&chan->capture) ||
> > > > > > > > + kthread_should_stop());
> > > > > > > > +
> > > > > > > > +ÂÂÂÂÂÂÂÂÂÂÂÂ if (kthread_should_stop())
> > > > > > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ break;
> > > > > > > > +
> > > > > > > > +ÂÂÂÂÂÂÂÂÂÂÂÂ /*
> > > > > > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂ * Source is not streaming if error is non-zero.
> > > > > > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂ * So, do not dequeue buffers on capture error.
> > > > > > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂ */
> > > > > > > > +ÂÂÂÂÂÂÂÂÂÂÂÂ if (err)
> > > > > > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 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