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    SubjectRe: [RFC PATCH v6 6/9] media: tegra: Add Tegra210 Video input driver
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    On 4/8/20 11:58 AM, Sowjanya Komatineni wrote:
    >
    > 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 пишет:
    >>> ...
    >>>>>>>> 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  as syncpt fifo depth is 2 
    >> and waiting to issue next capture when already 2 captures are
    >> inflight happens only during beginning of streaming where buffers
    >> allocated go thru capture for first time after queuing.
    >>
    >> same buffers are returned to userspace after capture and same
    >> allocated buffers will be queued back for subsequent captures.
    >>
    >> So this case of holding to issue single shot when already single shot
    >> is issue for 2 frames simultaneous happens only during beginning of
    >> start stream and also we set num_buffers to allocate for queue as 3
    >> although 2 is good enough where we will not hit this case even during
    >> streaming start with 2 buffers
    >>
    > As 2 buffers are good enough to be clear will update in v7 to use 2
    > buffers so we don't need to check for more than 2 outstanding buffers.

    correction: With 3 buffers, as soon as buffer is available capture
    starts. So right most times I see it waiting for few ms before 3rd
    capture to get through.

    As only 2 frames single shot can be issued in sequence (inflight
    requests), instead of waiting for 1 of the request to finish, we can use
    2 buffers and avoid waiting as 2 buffers are good enough. Will change
    this in v7.



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