claude-noether
ab79ed5e4d
Phase 3 iteration on the av1_larger.ivf (352x288 film_grain-50) fixture.
208x208 test_av1.ivf remains bit-exact PASS at sha 029ee72c214b37c1
(libva == kdirect, post-reference_frame_ts plumbing).
Negative result this commit: setting V4L2_AV1_FILM_GRAIN_FLAG_UPDATE_GRAIN
unconditionally when apply_grain=1 triggered a userspace SIGSEGV in
ffmpeg on the av1_larger fixture (consistent across runs). Reverted to
implicit update_grain=0 — same behavior as before the experiment
(silent 0-byte output, no segfault).
Hypothesis ruled out: the 352x288 silent-decode-failure is NOT solved
by always-update_grain. A/B test earlier also confirmed that omitting
the FILM_GRAIN control entirely (AV1_NO_FG=1) still produces 0 bytes,
so film_grain is not the trigger.
Remaining Phase 3 investigation candidates:
- tile_info field shape — single-tile av1_larger may stress mi_col/
row_starts sentinel differently than single-tile test_av1
- segmentation / quantization fields — different streams use
different combinations
- order_hints[] still zero (VAAPI doesn't expose per-ref)
- kernel-side dev_dbg in vpu981 driver would expose which
control field validation rejects
- strace -e ioctl on the failing decode reveals MEDIA_REQUEST_IOC_QUEUE
return value
Sibling-iteration parallel: ampere-kernel-decoders iter2-5 took
multiple iterations to localize the HEVC OOPS to kernel-side
ext_sps NULL init + slice_params; AV1 likely needs the same depth
of kernel-side instrumentation for the 352x288 case.
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.