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

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Date:   Tue, 7 Apr 2020 16:57:42 -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 4:38 PM, Sowjanya Komatineni wrote:
>
> 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
Also we specified numbers buffers as 3 to vb2 queue. So this is rare 
case but to prevent issuing more than 2 at a time as VI HW is only 
double buffered and syncpt fifo max depth is 2 added this to be safer.