I read today a short article about video acceleration in X. First of all I was already unhappy because I had to read. Personally I think all the news sites should come in a comic form. With as few captions as possible. Just like Perl I'm a write-only entity. Then I read that Gallium isn't an option when it comes to video acceleration because it exposes programmable pipeline and I got dizzy.
So I got off the carousel and I thought to myself: "Well, that's wrong", and apparently no one else got that. Clearly the whole "we're connected" thing is a lie because it doesn't matter how vividly I think about stuff, others still don't see what's in my head (although to the person who sent me cheese when I was in Norway - that was extremely well done). So cumbersome as it might be I'm doing the whole "writing my thoughts down" thing. Look at me! I'm writing! No hands! sdsdfewwwfr vnjbnhm nhn. Hands back on.
I think the confusion stems from the fact that the main interface in Gallium is the context interface, which does in fact model the programmable pipeline. Because of the incredibly flexible nature of the programmable pipeline a huge set of APIs is covered just by reusing the context interface. But in modern GPUs there are still some fixed function parts that are not easily addressable by the programmable pipeline interface. Video is a great example of that. To a lesser degree so is basic 2D acceleration (lesser because some of the modern GPUs don't have 2D engines at all anymore).
But, and it's a big but ("and I can not lie" <- song reference, pointing out to let everyone know that I'm all about music) nothing stops us from adding interfaces which deal exclusively with the fixed function parts of modern GPUs. In fact it has already been done as the work on a simple 2D interface has already started.
Basic idea is that the state trackers which need some specific functionality use the given interface. For example the Exa state tracker would use the Gallium 2d interface, instead of the main context interface. In this case the Gallium hardware driver will have a choice: it can either implement the given interface directly in hardware, or it can use the default implementation.
The default implementation is something Gallium will provide as part of the auxiliary libraries. The default implementation will use the main context interface to emulate the entire functionality of the other interface.
Video decoding framework would use the same semantics. So it would be an additional interface(s) with default implementation on top of the 3D pipeline. Obviously some parts of the video support are quite difficult to implement on top of the 3D pipeline but the whole point of this is that: for hardware that supports it you get the whole shabangabang, for hardware that doesn't you get a reasonable fallback. Plus in the latter case the driver authors don't have to write a single line of hardware specific code.
So a very nice project for someone would be to take VDPAU, VA-API or any video framework of your choice and implement a state tracker for that API on top of Gallium and design an interface(s) that could be added to Gallium to implement the API in a way that makes full usage of the fixed functionality video units found in the GPUs. I think this is the way our XvMC state tracker is heading.
This is the moment where we break into a song.