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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 12:38 PM, Sowjanya Komatineni wrote:
    >
    > 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.
    >
    >
    >
    Tested with 3 buffers and by checking outstanding buffers in process by
    VI hw and holding to start capture till one outstanding buffer in
    process by HW.
    Also tested with 2 buffers without checking for outstanding buffers.

    In both cases, I see same %CPU for the kthreads and is < 1%

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