linux-kernel - Re: [RFC PATCH v6 6/9] media: tegra: Add Tegra210 Video input driver

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Date:   Tue, 7 Apr 2020 16:38:07 -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 v6 6/9] media: tegra: Add Tegra210 Video input driver


On 4/7/20 4:36 PM, Sowjanya Komatineni wrote:
>
> On 4/7/20 4:12 PM, Dmitry Osipenko wrote:
>> External email: Use caution opening links or attachments
>>
>>
>> 08.04.2020 01:22, Sowjanya Komatineni пишет:
>>> On 4/7/20 3:08 PM, Dmitry Osipenko wrote:
>>>> External email: Use caution opening links or attachments
>>>>
>>>>
>>>> 08.04.2020 00:08, Sowjanya Komatineni пишет:
>>>> ...
>>>>>>> I think you need a semaphore with resource count = 2.
>>>>>> we hold on to issuing capture if more than 2 buffers are queued 
>>>>>> and it
>>>>>> continues only after fifo has min 1 slot empty
>>>>> Just want to close on this part of feedback. Hope above 
>>>>> explanation is
>>>>> clear regarding triggering/issuing at max 2 frame capture to VI HW 
>>>>> and
>>>>> also regarding capture threads where they use 
>>>>> wait_event_interruptible
>>>>> to prevent blocking waiting for buffers to be available for captures.
>>>>>
>>>>> So no changes related to this part are needed in v7.
>>>>   From what I see in the code, you "hold on" by making kthread to 
>>>> spin in
>>>> a busy-loop while caps_inflight >= SYNCPT_FIFO_DEPTH. So some change
>>>> should be needed to prevent this.
>>>>
>>>> The wait_event_interruptible seems should be okay.
>>> We don't want to prevent that as we already have buffers available for
>>> capture so as soon as VI HW issuing single shot is done and when min 1
>>> slot is empty we should continue with issuing for another capture.
>>>
>>> As long as buffers are available, we should continue to capture and
>>> should not hold
>>>
>> I suppose that taking a shot takes at least few milliseconds, which
>> should be unacceptable to waste.
> As long as buffers are in queue we have to keep processing each buffer 
> and between buffers obviously we have to wait for previous frames to 
> finish and this why we have separate thread for frame finish where we 
> can have next buffer capture ready and issue while previous frame 
> memory write happens