test0r
9de1be34ef
The load-bearing fix from diff_against_ffmpeg.md (campaign repo).
Adds src/h264_slice_header.{c,h} — a minimal H.264 slice_header()
bit-parser per ITU-T H.264 (08/2024) §7.3.3. Parses just enough of
the slice header to populate the V4L2 DECODE_PARAMS fields VAAPI
doesn't carry and that hantro G1 hardware reads directly out of
DECODE_PARAMS into MMIO registers:
dec_param->dec_ref_pic_marking_bit_size -> G1_REG_DEC_CTRL5_REFPIC_MK_LEN
dec_param->idr_pic_id -> G1_REG_DEC_CTRL5_IDR_PIC_ID
dec_param->pic_order_cnt_bit_size -> G1_REG_DEC_CTRL6_POC_LENGTH
dec_param->pic_order_cnt_lsb -> hantro reflist builder (poc_type=0)
dec_param->delta_pic_order_cnt_bottom -> same
dec_param->delta_pic_order_cnt0/1 -> hantro reflist builder (poc_type=1)
Without these set correctly, hantro's hardware bitstream parser
walks past zero bits in the slice header, lands on garbage, decodes
zero pixels — the all-zero CAPTURE output observed across both mpv
and Firefox during 2026-05-04 Phase 0 (see libva-multiplanar campaign
phase0_evidence/2026-05-04-kernel-trace/findings.md).
Implementation:
- Minimal RBSP bit reader (br_read_u/_ue/_se), MSB-first, fault-flag
on overrun.
- Emulation-prevention unescape (strips 0x03 after 0x00 0x00) on
the first 64 bytes of the slice — slice headers fit comfortably.
- Walks slice_header() up to and including dec_ref_pic_marking(),
measuring bit positions for the *_bit_size fields.
- Skips ref_pic_list_modification() and pred_weight_table() —
needed only to advance the bit position to dec_ref_pic_marking().
- Returns a struct with the V4L2 fields plus diagnostics
(first_mb_in_slice, slice_type, pps_id, frame_num).
Wired into h264_va_picture_to_v4l2 (src/h264.c) right after the
nal_ref_idc/nal_unit_type extraction. SPS/PPS context is built from
VAPicture's seq_fields and pic_fields; num_ref_idx_l0/l1_active
defaults come from VASlice (best available substitute for the
parsed PPS values). On parse success, populates decode_params with
the recovered values + emits a request_log with the decoded fields
for cross-validation against VAAPI's pre-parsed values.
src/meson.build: adds h264_slice_header.{c,h} to sources.
Cross-references:
- FFmpeg libavcodec/h264_slice.c (Kwiboo v4l2-request-n8.1) — populates
H264SliceContext::ref_pic_marking_bit_size / pic_order_cnt_bit_size
by the same bit-precise parse, then v4l2_request_h264.c forwards
to V4L2.
- Linux drivers/media/platform/verisilicon/hantro_g1_h264_dec.c
set_params() — the register-write code that reads these fields.
MVC nal_unit_type 20/21 unhandled (this fork strips MVC alongside
HEVC). Multi-slice non-IDR streams parse the first slice's header
only; for FRAME_BASED mode that's fine — kernel sees the whole
bitstream and parses subsequent slices itself.
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.