[Libva] Decoding and rendering to DRM

[Yann Dirson]

Hello,

I started to investigate how to render the decoded video to screen
without going through X11,
and DRM/KMS looks likes the best candidate, with hints in the libva
API doc that it is
supported.

Is there already some code somewhere doing that ?  gstreamer-vaapi,
which at first glance
looked the most promising candidate apparently in fact does not do that :)
Even a pretty complicated app could do the trick, I'm using a
dedicated LD_PRELOAD API
tracer to understand how various apps are using libva.


The approach I am trying is:

* start with vaGetDisplayDRM()
* use vaQuerySurfaceAttributes to identify available pixel_format and
mem_type (here NV12
 format only, and MEM_TYPE_KERNEL_DRM is advertized) - pitches and
offsets will have to
 match the underlying DRM framebuffer (it's not obvious to me yet if
the latter is/can be created
 automatically or if the caller has to provide it)
* create my vaSurface's using those parameters
* decode picture as usual, targetting those DRM-backed surfaces
* use vaAcquireBufferHandle to get a handle to the underlying DRM
frame buffer and from there
 get DRM to display it and eg. drmModePageFlip() it

The problem I have, is that vaAcquireBufferHandle is apparently
supposed to lock the *VABuffer*
used as backing store for the surface, but I don't see an API to get
that object from the surface.

I'm suspecting VASurfaceAttribExternalBuffers::buffers could be a way
for the app to provide them
on surface creation (did not find much hints in the API doc about this
field, not even if it's an in or
out parameter), and that those should be created at the DRM level, but
then I cannot create a
VABufferID for this before I get a VAContextID, and that one in turns
requires the surfaces to be
created already, so even if I create the DRM frame buffers at first
I'm still left with a missing link.

I'm also quite new to DRM as well, which does not help, and I'm wondering:

* are dumb buffers sufficient for this use, or should I rather turn to
libgbm, or should I simply get
 them created automatically ?
* declaring a frame buffer as NV12 seems to require use of
drmModeAddFB2(), but the latter
 seems to expect one *handle* per plane and return a *buf_id*, I guess
it is the latter which
 should be bassed as "buffer handles" through
VASurfaceAttribExternalBuffers::buffers, despite
 the different terminlology

Am I (at least to some extent) on the right track ?


Also the following is still obscure to me: what's the semantics of
VASurfaceAttrib.flags for
vaCreateSurfaces ?  It took me some time to figure out that my
original guess of setting them
to VA_SURFACE_ATTRIB_SETTABLE was what was causing an "invalid parameter" error

Best regards,
-- 
Yann Dirson <yann at blade-group.com>
Blade -- 90 avenue des Ternes, 75017 Paris