claude-noether
7f8fa93213
Pre-iter4 backend hardcoded /dev/video0 + /dev/media0 as defaults
when no env override was set. Linux 7.0 udev/probe order changed,
rockchip-rga (RGB color converter, no codec) now claims
/dev/video0 — legacy default returns empty profile list.
Discovery is driven by the media controller graph (the canonical
v4l2-request approach). NOT a /dev/video* walk by enumeration
order — that mispairs video and media nodes when one driver
registers multiple media devices, and depends on probe-order
luck.
Algorithm:
1. Walk /dev/media0..15. MEDIA_IOC_DEVICE_INFO names the driver.
Match against {rkvdec, hantro-vpu, cedrus, sun4i_csi}.
2. MEDIA_IOC_G_TOPOLOGY enumerates the entity/interface graph.
The MEDIA_INTF_T_V4L_VIDEO interface carries major:minor of
the V4L2 video node owned by THIS media controller — paired
by the kernel, not by /dev/* enumeration order.
3. Resolve major:minor to /dev/videoN via /sys/dev/char/<M>:<N>
(the kernel's char-device sysfs symlink whose basename is
the device node name).
LIBVA_V4L2_REQUEST_NO_AUTODETECT=1 escape hatch reverts to legacy
/dev/video0 + /dev/media0 hardcoded behavior for callers that
depended on it.
Phase 5 C4 amendment: walk-and-pick-first selects rkvdec on RK3399
(rkvdec's media controller enumerates before hantro's). H.264 /
HEVC / VP9 (rkvdec codecs) work without env override after this
commit. MPEG-2 / VP8 (hantro) still require explicit
LIBVA_V4L2_REQUEST_VIDEO_PATH=/dev/video3 override; full
multi-decoder dispatch is iter4-B1 backlog item.
Verified empirically on fresnel (linux-fresnel-fourier 7.0-1):
- vainfo (no env) -> "auto-selected codec device: /dev/video1 +
/dev/media0", enumerates H264*5 + HEVCMain (rkvdec) — paired
via topology graph, not /dev/video* enumeration.
- vainfo NO_AUTODETECT=1 -> empty list (legacy /dev/video0 = rga).
- vainfo with explicit /dev/video3 + /dev/media1 -> MPEG2*2 + VP8.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
v4l2-request libVA Backend
About
This libVA backend is designed to work with the Linux Video4Linux2 Request API that is used by a number of video codecs drivers, including the Video Engine found in most Allwinner SoCs.
Status
The v4l2-request libVA backend currently supports the following formats:
- MPEG2 (Simple and Main profiles)
- H264 (Baseline, Main and High profiles)
- H265 (Main profile)
Instructions
In order to use this libVA backend, the v4l2_request driver has to
be specified through the LIBVA_DRIVER_NAME environment variable, as
such:
export LIBVA_DRIVER_NAME=v4l2_request
A media player that supports VAAPI (such as VLC) can then be used to decode a video in a supported format:
vlc path/to/video.mpg
Sample media files can be obtained from:
http://samplemedia.linaro.org/MPEG2/
http://samplemedia.linaro.org/MPEG4/SVT/
Technical Notes
Surface
A Surface is an internal data structure never handled by the VA's user containing the output of a rendering. Usualy, a bunch of surfaces are created at the begining of decoding and they are then used alternatively. When created, a surface is assigned a corresponding v4l capture buffer and it is kept until the end of decoding. Syncing a surface waits for the v4l buffer to be available and then dequeue it.
Note: since a Surface is kept private from the VA's user, it can ask to directly render a Surface on screen in an X Drawable. Some kind of implementation is available in PutSurface but this is only for development purpose.
Context
A Context is a global data structure used for rendering a video of a certain format. When a context is created, input buffers are created and v4l's output (which is the compressed data input queue, since capture is the real output) format is set.
Picture
A Picture is an encoded input frame made of several buffers. A single input can contain slice data, headers and IQ matrix. Each Picture is assigned a request ID when created and each corresponding buffer might be turned into a v4l buffers or extended control when rendered. Finally they are submitted to kernel space when reaching EndPicture.
The real rendering is done in EndPicture instead of RenderPicture because the v4l2 driver expects to have the full corresponding extended control when a buffer is queued and we don't know in which order the different RenderPicture will be called.
Image
An Image is a standard data structure containing rendered frames in a usable pixel format. Here we only use NV12 buffers which are converted from sunxi's proprietary tiled pixel format with tiled_yuv when deriving an Image from a Surface.