[FFmpeg-devel] [PATCH][RFC] avcodec: disallow hwaccel with frame threads

[Andreas Cadhalpun]

On 20.01.2016 10:11, wm4 wrote:
> On Tue, 19 Jan 2016 23:54:48 +0100
> Andreas Cadhalpun <andreas.cadhalpun at googlemail.com> wrote:
>> On 19.01.2016 21:46, wm4 wrote:
>>> It was discussed to death on #libav-devel too.  
>>
>> And the conclusion was to allow hwaccel with frame threads?
> 
> From what I remember, the VLC developer in question showed no
> interest in suggesting a proper solution.

:-/

>>> There was this idea that this could be fixed by having some sort of
>>> wrapper decoder, which could take care of the fallback transparently
>>> and without getting into these nasty threading issues. But no idea how
>>> much work that would be.  
>>
>> Can't the framework just ignore the threading setting when using a hwaccel?
> 
> The problem is software fallback. If you run decoding with hwaccel and
> 1 thread, and then fallback to software decoding, you can't just
> increase the number of threads of the open context.

Thanks for explaining.

> This is why other hwaccel users simply recreate the decoder context
> (and it works great). VLC could perhaps easily change their code to do
> this.

I thought so, too, until a VLC developer claimed it wouldn't be easy.

> I'm quite surprised the developer seems to prefer to play such
> games, instead of at least suggesting a solution or even just asking
> the ffmpeg project for a revert.

That was a surprise for me as well, and not a pleasant one.

> I think it was mentioned that some newer hwaccels (VP9?) make it even
> harder to somehow keep multithreaded hwaccels working, which is why
> running with threading was disabled now. Hendrik can provide a better
> explanation.

OK. But I think this has worked more or less well in the past, so adding
this error message and thus breaking this use is somewhat an API change.
As such a deprecation period seems appropriate.

Best regards,
Andreas