claude-noether
b6a65fc692
Live probe of rpi-hevc-dec on higgs (Pi CM5, kernel 6.12.75-rpt-rpi-2712, Debian 13 trixie) answers Phase 0 open questions Q1, Q2, Q5, Q6 empirically; Q3 partial; Q4 still open. Q1 (EXT_SPS): NOT present. Only standard V4L2_CID_STATELESS_HEVC_*. Probe ctrl id 0xa97 returns EINVAL — same gate iter2's has_hevc_ext_sps_rps_rkvdec uses. iter31 alpha-29 plumbing applies. Q2 (hevc_start_code): default 0 "No Start Code"; matches our behaviour. Q3 (NC12 SAND tile layout): partial. CAPTURE S_FMT for 1280x720 NC12 returns sizeimage=1382400 (linear NV12 byte count) but bytesperline=1080 (suspect, encodes SAND col count not linear stride). Need kernel-doc / driver-source read before writing detile primitive. Q4 (DRM modifier round-trip): hwdownload rejects SAND-tiled drm_prime (-38 Function not implemented). Backend CPU-detile to NV12 is the safe path for Firefox. Q5 (submission ordering): empirical ioctl trace shows canonical V4L2 stateless flow. Two notes for the backend: kdirect uses V4L2_MEMORY_DMABUF for both queues (we use MMAP for CAPTURE on rkvdec); kdirect does NOT need the iter25 SPS pre-seed pattern - rpi-hevc-dec takes explicit NC12 + dims directly. Q6 (packaging): Debian 13 trixie. Phase 8 needs a debian/ tree, not just PKGBUILD. Decision in Phase 1. Other findings: ffmpeg 7.1.3 from stock Debian is built with --enable-v4l2-request. kdirect engagement line: Hwaccel V4L2 HEVC stateless V4; devices: /dev/media1,/dev/video19; buffers: src DMABuf, dst DMABuf; swfmt=rpi4_8 No libva ICD installed (only armada-drm_dri.so). mpv installable. Firefox 145 + rpi-firefox-mods present. Phase 0 closed. Phase 1 opens with goal: HEVC bit-exact libva-vs-kdirect on higgs for 1280x720 Main 8-bit via the new RPI_HEVC_DEC driver_kind slot + NC12 detile primitive. Co-Authored-By: Claude Opus 4.7 <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.