[Libva] Decoding and rendering to DRM

[Chen, Peng C]

Hi,
     Here is a example for using drm rendering with libva decoding. 
https://github.com/01org/libyami-utils/blob/master/examples/grid.cpp

-----Original Message-----
From: Libva [mailto:libva-bounces at lists.freedesktop.org] On Behalf Of Yann Dirson
Sent: Wednesday, July 6, 2016 6:52 AM
To: libva at lists.freedesktop.org
Cc: tech <tech at blade-group.com>
Subject: [Libva] Decoding and rendering to DRM

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 _______________________________________________
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