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

Sowjanya Komatineni
2020-04-08T18:58:57Z

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.