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Subject: Re: [RFC PATCH v6 6/9] media: tegra: Add Tegra210 Video input driver
From:   Sowjanya Komatineni <skomatineni@nvidia.com>
To:     Dmitry Osipenko <digetx@gmail.com>, <thierry.reding@gmail.com>,
        <jonathanh@nvidia.com>, <frankc@nvidia.com>, <hverkuil@xs4all.nl>,
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On 4/8/20 10:45 AM, Sowjanya Komatineni wrote:
>
> On 4/8/20 7:21 AM, Dmitry Osipenko wrote:
>> External email: Use caution opening links or attachments
>>
>>
>> 08.04.2020 03:00, Sowjanya Komatineni =D0=BF=D0=B8=D1=88=D0=B5=D1=82:
>> ...
>>>>>>> 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.
>>> To be more clear, when more buffers are enqueued from userspace always
>>> capture list will be full and thread will be busy in capture till=20
>>> either
>>> error or stop stream request happens.
>>>
>> If kthreads take more than 1% of CPU time during capture (video) with
>> more than 2 buffers in queue, then it's not good and I think you should
>> do something about it. If kthreads stay at ~0%, then it should be okay
>> as-is.
>
> VI outstanding requests max can only be 2=C2=A0 as syncpt fifo depth is 2=
=C2=A0=20
> and waiting to issue next capture when already 2 captures are inflight=20
> happens only during beginning of streaming where buffers allocated go=20
> thru capture for first time after queuing.
>
> same buffers are returned to userspace after capture and same=20
> allocated buffers will be queued back for subsequent captures.
>
> So this case of holding to issue single shot when already single shot=20
> is issue for 2 frames simultaneous happens only during beginning of=20
> start stream and also we set num_buffers to allocate for queue as 3=20
> although 2 is good enough where we will not hit this case even during=20
> streaming start with 2 buffers
>
As 2 buffers are good enough to be clear will update in v7 to use 2=20
buffers so we don't need to check for more than 2 outstanding buffers.