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

[Sowjanya Komatineni]

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 ÐÐÑÐÑ:
> > ...
> > > > > > > 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 2Â as syncpt fifo depth is 2Â 
> 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.