[Nouveau] Video Hardware Decoding: Jittery Rectangles on Nvidia GT218 NVA8 VP4.

[Ralph Corderoy]

Hi Ilia,

A fortnight ago, you wrote:
> > The video plays, CPU load is less (my aim), but there's ‘tearing’ of
> > the picture as if small rectangles that are updates are appearing in
> > the wrong location, off by a little.  If I step through the frames
> > with mpv's ‘.’ and ‘,’ then I've found a pattern: one frame's
> > picture is good, followed by N bad ones where N is 3 or 7, i.e.
> > every 4th or 8th frame is okay.  Don't know if that's a clue or
> > helps someone here recognise a known problem.
>
> Unfortunately I've never tracked down the cause for this.
> https://nouveau.freedesktop.org/wiki/VideoAcceleration/ - see note #4.

That sounds like the problem, but it is all the way through TV programme
from the BBC iPlayer's service.

> I have, over time, collected some sample videos where this happens in
> the first few frames. The plan was to do mmt traces of the blob
> driver, and figure out what it was doing differently. If this is
> something you're interested in, I'd be happy to provide some guidance.

I've two mmio traces, nouveau and nvidia, each using mpv(1) to play the
same ten-second extract of an iPlayer MP4, with and without ‘--vo=vdpau
--hwdec=vdpau’.

    $ grep -n ^MARK nvidia-4.mmiotrace
    20:MARK 65645.302939 buffer_size_kb: 524288
    21:MARK 65645.385776 starting x
    1447554:MARK 65681.979892 x started
    1448560:MARK 65682.446747 mpv none start
    1868197:MARK 65701.153770 mpv none done
    1869462:MARK 65702.748005 mpv vo start
    2633729:MARK 65724.465154 mpv vo done
    2634956:MARK 65726.259829 mpv vo hwdec start
    5347520:MARK 65759.841432 mpv vo hwdec done
    5349019:MARK 65762.431137 killing cat
    $

hwdec on nvidia easily overflowed the default 1,408 KiB trace buffer,
whereas nouveau didn't.

Are these helpful?  Shall I submit them to the Gmail address given in
https://nouveau.freedesktop.org/wiki/MmioTrace/

-- 
Cheers, Ralph.