Re: [RFC PATCH v11 6/9] media: tegra: Add Tegra210 Video input driver

[Sowjanya Komatineni <skomatineni@nvidia.com>]

On 4/30/20 12:33 PM, Dmitry Osipenko wrote:
> 30.04.2020 22:09, Sowjanya Komatineni пишет:
>> On 4/30/20 11:18 AM, Sowjanya Komatineni wrote:
>>> On 4/30/20 10:06 AM, Sowjanya Komatineni wrote:
>>>> On 4/30/20 9:29 AM, Sowjanya Komatineni wrote:
>>>>> On 4/30/20 9:04 AM, Sowjanya Komatineni wrote:
>>>>>> On 4/30/20 7:13 AM, Dmitry Osipenko wrote:
>>>>>>> 30.04.2020 17:02, Dmitry Osipenko пишет:
>>>>>>>> 30.04.2020 16:56, Dmitry Osipenko пишет:
>>>>>>>>> 30.04.2020 01:00, Sowjanya Komatineni пишет:
>>>>>>>>>> +static int chan_capture_kthread_finish(void *data)
>>>>>>>>>> +{
>>>>>>>>>> +    struct tegra_vi_channel *chan = data;
>>>>>>>>>> +    struct tegra_channel_buffer *buf;
>>>>>>>>>> +
>>>>>>>>>> +    set_freezable();
>>>>>>>>>> +
>>>>>>>>>> +    while (1) {
>>>>>>>>>> +        try_to_freeze();
>>>>>>>>> I guess it won't be great to freeze in the middle of a capture
>>>>>>>>> process, so:
>>>>>>>>>          if (list_empty(&chan->done))
>>>>>>>>>              try_to_freeze();
>>>>>>>> And here should be some locking protection in order not race with
>>>>>>>> the
>>>>>>>> chan_capture_kthread_start because kthread_finish could freeze
>>>>>>>> before
>>>>>>>> kthread_start.
>>>>>>> Or maybe both start / finish threads should simply be allowed to
>>>>>>> freeze
>>>>>>> only when both capture and done lists are empty.
>>>>>>>
>>>>>>> if (list_empty(&chan->capture) &&
>>>>>>>       list_empty(&chan->done))
>>>>>>>      try_to_freeze();
>>>>>> good to freeze when not in middle of the frame capture but why
>>>>>> should we not allow freeze in between captures?
>>>>>>
>>>>>> Other drivers do allow freeze in between frame captures.
>>>>>>
>>>>>> I guess we can freeze before dequeue for capture and in finish
>>>>>> thread we can freeze after capture done. This also don't need to
>>>>>> check for list_empty with freeze to allow between frame captures.
>>>>>>
>>>>> Also if we add check for both lists empty, freeze is not allowed as
>>>>> long as streaming is going on and in case of continuous streaming
>>>>> freeze will never happen.
>>> To allow freeze b/w frames (but not in middle of a frame),
>>>
>>> for capture_start thread, probably we can do unconditional
>>> try_to_freeze()
> Is it possible to use wait_event_freezable()?
>
> https://www.kernel.org/doc/Documentation/power/freezing-of-tasks.txt
>
> Will the wait_event_interruptible() be woken up when system freezes?

Based on wait_event_freezable implementation, looks like it similar to 
wait_event_interruptible + try_to_free() as it does freezable_schedule 
unlike schedule with wait_event_interruptible.

So using this for capture_start may be ok to allow freeze before start 
of frame. But can't use for capture_finish as this is same as 
wait_event_interruptible followed by unconditional try_to_freeze.

>
>>> for capture_finish thread, at end of capture done we can do
>>> try_to_freeze() only when done list is empty
> This doesn't prevent situation where the done-list is empty and the
> "finish" thread freezes, in the same time the "start" thread issues new
> capture and then freezes too.
>
> 1. "start" thread issues capture
>
> 2. "finish" thread wakes and waits for the capture to complete
>
> 3. "start" thread begins another capture, waits for FRAME_START
>
> 4. system freezing activates
>
> 5. "finish" thread completes the capture and freezes because done-list
> is empty
>
> 6. "start" thread gets FRAME_START, issues another capture and freezes

This will not happen as we allow double buffering done list will not be 
empty till stream stop happens

There will always be 1 outstanding frame in done list

>> My understanding is buffer updates/release should not happen after
>> frozen state. So we should let frame capture of outstanding buffer to
>> finish before freezing in capture_finish thread.
>>
>> But for capture_start thread we can unconditionally freeze before
>> dequeuing next buffer for capture.
>>
>> With this when both threads are in frozen state and no buffer
>> updates/captures will happen after frozen state.
>>
>> I think its not required to finish streaming of all frames completely to
>> let threads to enter frozen state as streaming can be continuous as well.
> Yes, only freezing in the middle of IO should be avoided.
>
> https://lwn.net/Articles/705269/
>
>>>> Hi Dmitry,
>>>>
>>>> Will update in v12 to not allow freeze in middle of a frame capture.
>>>>
>>>> Can you please confirm on above if you agree to allow freeze to
>>>> happen in b/w frame captures?
>>>>
>>>> Also as most feedback has been received from you by now, appreciate
>>>> if you can provide all in this v11 if you have anything else so we
>>>> will not have any new changes after v12.
> I'll take another look tomorrow / during weekend and let you know.