[vlc-devel] [PATCH 04/31] display: don't store the dummy context in the display anymore

[Steve Lhomme]

On 2019-09-24 11:41, Steve Lhomme wrote:
> On 2019-09-23 19:11, Rémi Denis-Courmont wrote:
>> Le maanantaina 23. syyskuuta 2019, 18.01.09 EEST Steve Lhomme a écrit :
>>> Use the one from the vout thread if it exists.
>>>
>>> Later the video context will come from the decoder (if any).
>>
>> I still don't get why the VD should care/know about the decoder device 
>> at all.
>> If even VD knows about the decoder device, then what's the point of the
>> distinct video context?
> 
> The hint has to work both ways. If a VAAPI decoder uses a VADisplay and 
> the VD uses another one it may not work at all (not sure if the same 
> value would be used if the default display is used in both cases).
> 
> With D3D it's the same thing, with external rendering we want the VD to 
> use the D3D device provided by the host (otherwise it just cannot work). 
> That means the decoder should also use this device. And it knows about 
> this device/hint through the "decoder device". I agree that the VD may 
> read that external D3D device on itself without using the "decoder 
> device" at all. Given the "decoder device" may not be created at all 
> (it's created only on demand which won't happen for AV1 playback for 
> example) maybe I should not rely on it at all and get the host D3D 
> device by other means.
> 
> (there are also possibilities to use a different D3D device for decoding 
> and rendering which I have local support for, but that's besides the point)
> 
> So IMO it all comes down to VAAPI and whether the VADisplay value must 
> be created once and used in the decoder and the VD.

I forgot an important case. When using a GPU decoder, a CPU filter, then 
a GPU filter and then the display. If we don't store the "decoder 
device" (in the filter (chain) owner) the CPU>GPU filter will need to 
create a new "decoder device" when we could use a cached one, matching 
the one used to initialize the VD (via the video context). We will 
likely need to recreate a VD when we shouldn't have to.

> I think in all cases the VADisplay created for the decoder will be 
> pushed in the video context when creating the display. So in all cases 
> it should match on both sides.
> 
> I'll modify my patch (and working branches) accordingly.
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