[vlc-devel] [PATCH 4/4] avcodec: va: get the picture to send to the vout during Extract() not Get()

[Rémi Denis-Courmont]

Le keskiviikkona 30. marraskuuta 2016, 9.03.34 EET Steve Lhomme a écrit :
> On Tue, Nov 29, 2016 at 5:48 PM, Rémi Denis-Courmont <remi at remlab.net> 
wrote:
> > Le tiistaina 29. marraskuuta 2016, 14.19.38 EET Steve Lhomme a écrit :
> >> In some cases the decoder pool is stuck on picture_pool_Wait() because
> >> the
> >> va keeps requesting referenced pictures without outputing anything (yet).
> > 
> > I have not seen this in recent times and the problem is in no way specific
> > to hardware acceleration in any case.
> 
> It it related to the fact the library we use (libavcodec) requires all
> decoding buffers to be allocated and provided at once for DVXA2 and
> D3D11VA.

No. That wouldn´t cause a deadlock; that would just make the pool empty or 
close to empty.

> And there's at least a good reason for that, the pool must
> provide the pictures in LIFO order.

The pool used to be FIFO order. Now it´s constant order, which reduces 
(demand-paged) memory usage if reference count is over-estimated, and at least 
theoretically improves locality.

This is irrelevant here. Neither LIFO nor FIFO order, nor any other order 
provide any useful warranty for anything since picture buffers can be released 
asynchronously by various components (decoder, filters, splitter and output). 
In other words, the design is purely down to statistical optimization.

> The surface copying in Direct3D is
> asynchronous and when a B frame is decoded it's released right away by
> the decoder and possibly reused for the next decoding, if the pool
> provides pictures in FIFO order.

You can influence the order by abusing either the picture context or the 
picture pool lock function. But I expect that to become intractable and be 
removed in VLC 4.0 as part of pool rework.

Regardless...

> That results in glitches or crashes.

...then your code is racy and broken regardless. And this patch does not fix 
that problem.

> Would the picture pool change be OK with you ?

No.

-- 
Rémi Denis-Courmont
https://www.remlab.net/