Date:   Thu, 30 Apr 2020 14:26:24 -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 v11 6/9] media: tegra: Add Tegra210 Video input driver


On 4/30/20 2:17 PM, Dmitry Osipenko wrote:
> 30.04.2020 23:02, Sowjanya Komatineni пишет:
>> On 4/30/20 12:53 PM, Sowjanya Komatineni wrote:
>>> On 4/30/20 12:46 PM, Sowjanya Komatineni wrote:
>>>> 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
>>> Correction, there will always be 1 outstanding buffer except beginning
>>> during beginning of stream.
>>>
>>> Except during beginning frames, done list will not be empty for all
>>> subsequent streaming process
>> Also to be clear, hardware sees next frame start event prior to previous
>> frame mw_ack event as mw_ack event happens after frame end. So once
>> initial buffer got queued to done list to finish processes, while
>> waiting for mw_ack next frame start happens and pushes next buffer to
>> done list.
> What about this variant:
>
> 1. "start" thread wakes up to start capture
>
> 2. system freezing activates
>
> 3. "finish" thread wakes up and freezes

finish thread will wake up only when done list is not 
empty/kthread_stop/wake even from capture start thread.

Also when I said will allow try_to_free when done list is empty I meant 
to have this at end of capture_done() in finish thread

>
> 4. "start" thread issues capture and freezes
>
> And again, I assume that system's freezing should wake
> wait_event_interruptible(), otherwise it won't be possible to freeze
> idling threads at all and freezing should fail (IIUC).

Based on kernel doc on freezing, looks like when we mark thread as 
freezable, freeze state happens when we explicitly call try_to_freeze.

I don't think its other way where freeze causes wait_event_interruptible 
to wake up.

> And in this case synchronization between start/finish threads should be
> needed in regards to freezing.

Was thinking to have counter to track outstanding frame w.r.t single 
shot issue b/w start and finish and allow to freeze only when no 
outstanding frames in process.

This will make sure freeze will not happen when any buffers are in progress

> Note that this could be a wrong assumption, I'm not closely familiar
> with how freezer works.

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