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Subject: Re: [RFC PATCH v6 6/9] media: tegra: Add Tegra210 Video input driver
To:     Dmitry Osipenko <digetx@gmail.com>, <thierry.reding@gmail.com>,
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From:   Sowjanya Komatineni <skomatineni@nvidia.com>
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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 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 allocated=20
buffers will be queued back for subsequent captures.

So this case of holding to issue single shot when already single shot is=20
issue for 2 frames simultaneous happens only during beginning of start=20
stream and also we set num_buffers to allocate for queue as 3 although 2=20
is good enough where we will not hit this case even during streaming=20
start with 2 buffers