Date:   Thu, 30 Apr 2020 13:02:01 -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 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.


>
>>
>>>> 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.

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