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

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Message-ID: <db7c7051-5674-cdb9-0aa4-ee94125b3024@gmail.com>
Date:   Wed, 8 Apr 2020 02:12:26 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Sowjanya Komatineni <skomatineni@...dia.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

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.