[vlc-devel] DXVA no CPU access design

[Julian Scheel]

Am 21.04.15 um 20:00 schrieb Rémi Denis-Courmont:
> Le 2015-04-21 19:24, Steve Lhomme a écrit :
>> It seems that we're turning around discussing solutions that never
>> work. So let's summarize the constraints we have to try to find a
>> working solution. Unless we code a DXVA2 decoder from scratch (that
>> may be a possibility since we have packtizers), we have to deal with
>> how avcodec works
>>
>> 1/ the avcodec decoder is opened, found to be capable of handling the
>> codec, and so is used.
>>
>> 2/ It then starts decoding the input stream.
>>
>> 3/ When it has enough data to write to an output frame, it first tries
>> to define the output format using the decoder->get_format() calback,
>> by providing some possible ones, including the hardware accelerated
>> variants. This is where we open our DXVA2 module.
>
> The libavcodec plugin should update ->fmt_out then call
> decoder_UpdateVideoFormat(). Currently this is postponed to
> ffmpeg_NewPictBuf(). IMU, that's what libavcodec get_format() is meant
> for. At that point, the vout (or more generally, the decoder output)
> will be initialized. Error checking is also possible.

Afaicr decoder_UpdateVideoFormat() is not required to be called 
explicitly by the codec. It's sufficient to just update the fmt_out of 
the codec and the next decoder_PictureNew() will call 
decoder_UpdateVideoFormat() so that the new format is populated down the 
vout chain.

> I am not sure that if all decoder owners implement
> ->pf_vout_format_update() correctly yet, so that might need to be fixed.
>
>> 4/ If we finds a va decoder, we set it up using vlc_va_Setup()
>
> Again, vlc_va_Setup() is only a work around for limitations of
> libavcodec. I am not sure the problem even still exists. Hardware
> initialization should be in vlc_va_New().
>
>> At this point the decoder is provided with a `hwaccel_context` that
>> contains the list of `surface` it can use to read the decoded frames
>> coming out of the va.

Where is this hwaccel_context coming from? And what prevents you from 
retrieving it in the vout and putting it into a picture_sys_t?

-Julian