[Spice-devel] [spice-gtk v1 3/4] channel-display-gst: improve check for decoder element

[Francois Gouget]

On Wed, 19 Oct 2016, Victor Toso wrote:
[...]
> > In a different context I have found that I can fnid dfbvideosink and
> > even instantiate it. But it will refuse to switch to the READY state and
> > is thus unusable. That makes sense since I was not using it in a Direct
> > FB context.
> 
> Interesting. Was it possible to set a dfb context later on? Otherwise it
> could be a bug, no?

I don't think that's a bug. The GStreamer documentation says:

* GST_STATE_NULL: this is the default state. No resources are allocated 
  in this state, so, transitioning to it will free all resources. 
  [...]

* GST_STATE_READY: in the ready state, an element has allocated all of 
  its global resources, that is, resources that can be kept within 
  streams. You can think about opening devices, allocating buffers and 
  so on. However, the stream is not opened in this state, so the stream 
  positions is automatically zero. If a stream was previously opened, it 
  should be closed in this state, and position, properties and such 
  should be reset. 

When the dfbvideosink is instantiated is is in the NULL state and since 
no resource is allocated in this state it does not matter whether the 
device is available or not. It's only when transitioning to the READY 
state that it would try to open the relevant device and so that's where 
it returns an error.
 

> > I expect hardware decoders will be even worse. For instance vaapidecode
> > is likely to be present but may not work if you don't have an Intel
> > card. Even if you can instantiate it, it may cause a VP8 pipeline caps
> > negotiation to fail if your graphics does not support VP8.
> 
> Yes, that's why I think we need a different approach for hardware
> decoders.

We will need code that can try the hardware decoder first and fallback 
to the software one if it's not usable. But I don't think this 
necessarily impacts gstvideo_has_codec() if implemented using 
create_gstreamer_decoder() whereas with your code it will necessarily be 
impacted.


-- 
Francois Gouget <fgouget at codeweavers.com>