2651e4cfdf8ea7c56ed7800008f11ec108598dd4
1 Commits
| Author | SHA1 | Message | Date | |
|---|---|---|---|---|
 claude-noether
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2651e4cfdf |
iter4 Phase 2: situation analysis — VP9 backend gaps + compressed-
header parser requirement
Source-read of every file the iter4 patch series will touch, plus
kernel UAPI + VAAPI + downstream FFmpeg + kernel rkvdec reference
sources. Conducted on noether against fork tip e1aca9c (iter3 close).
Critical scope-shaping finding: rkvdec on RK3399 REQUIRES
V4L2_CID_STATELESS_VP9_COMPRESSED_HDR (not optional). Per
drivers/staging/media/rkvdec/rkvdec-vp9.c::rkvdec_vp9_run_preamble
lines 752-754:
ctrl = v4l2_ctrl_find(&ctx->ctrl_hdl,
V4L2_CID_STATELESS_VP9_COMPRESSED_HDR);
if (WARN_ON(!ctrl))
return -EINVAL;
VAAPI does NOT expose compressed-header probability updates
(va_dec_vp9.h:50-192 — only frame parameters + segmentation;
vendor VAAPI drivers parse compressed header in firmware/GPU).
Therefore the libva backend MUST parse the compressed header
itself via a VPX boolean decoder + inv_map_table[]. ~150-200 LOC
of bitstream parsing logic (port from FFmpeg
v4l2_request_vp9.c::fill_compressed_hdr).
Bug enumeration (12 sites):
B1 config.c::RequestQueryConfigProfiles enum block missing
B2 config.c::RequestCreateConfig VP9 case missing
B3 config.c::RequestQueryConfigEntrypoints VP9 case missing
B4 src/vp9.c new file ~500-600 LOC
B5 src/vp9.h new file ~35-45 LOC
B6 src/vp9_rac.h NEW or inline (Phase 4
plan locks Option A:
inline in vp9.c)
B7 picture.c::codec_set_controls VP9 dispatch missing
B8 picture.c::codec_store_buffer 2 buffer-type cases
(Picture + Slice;
NOT 4 like VP8)
B9 picture.c::RequestBeginPicture predicted no reset
needed (no flag-state
like VP8 iqmatrix_set)
B10 surface.h::object_surface::params union vp9 member missing
B11 meson.build vp9.c/vp9.h not in lists
B12 buffer.c predicted no change
needed (VP9 uses
Picture/Slice/SliceData
— all whitelisted)
Non-bugs (intentionally untouched): context.c (no DECODE_MODE/
START_CODE menus per FFmpeg ref), video.c (CAPTURE-side format
list), v4l2.c (fourcc-agnostic), include/hevc-ctrls.h (already
includes <linux/v4l2-controls.h>).
Contract surface cited verbatim:
V4L2_CID_STATELESS_VP9_FRAME = 0xa40b2c (~144 bytes — much
smaller than VP8's 1232 bytes because VP9_FRAME carries no
entropy table; that's in COMPRESSED_HDR)
V4L2_CID_STATELESS_VP9_COMPRESSED_HDR = 0xa40b2d (~1947 bytes
— coef[4][2][2][6][6][3] alone is 1728 bytes)
Per-frame submission: 2 controls batched in single S_EXT_CTRLS
v4l2_request_vp9.c references confirmed: 2-control shape,
runtime-probed COMPRESSED_HDR availability (rkvdec advertises
it; we MUST provide)
VAAPI buffer types: 2 per frame (Picture + Slice) vs iter3 VP8's
4. NO Probability buffer (VP9 keeps probs in compressed header).
NO IQMatrix (VP9 keeps quant in slice's per-segment seg_param[8]).
VAAPI → V4L2 mapping table: 30+ fields enumerated. Several gap
candidates identified for Phase 3 empirical resolution:
Q1 lf.ref_deltas/mode_deltas/flags — not in VAAPI; FFmpeg reads
from VP9Context internal. BBB likely zero.
Q2 quant.base_q_idx + deltas — VAAPI exposes only effective
per-segment scales. Inverse-derive needed.
Q3 reference_mode — not in VAAPI. Default to SELECT?
Q4 interpolation_filter mapping (FFmpeg ^ remap)
Q5 reset_frame_context off-by-one (FFmpeg > 0 ? - 1 : 0)
Q6 Per-segment feature_data[8][4] derivation from VAAPI's
effective scales is non-trivial
Q7 mpv 0.41.0 VP9 hwdec engagement (per memory feedback_hw_
decode_engagement_check.md — known gap from iter3 VP8)
Q8 rkvdec dma_resv issue? (predicted NO based on iter1+iter2
successful mpv-DMA-BUF-GL on rkvdec)
Patch-shape prediction: ~580-690 LOC across 5 modified + 2 new
files (closer to iter2 HEVC's 470 than iter3 VP8's 370). Compressed-
header parser is the dominant cost.
Phase 3 baseline targets queued: cross-validator strace verbatim
S_EXT_CTRLS payloads (both controls), VAAPI consumer trace, mpv-
VP9-vaapi engagement check, rkvdec readback non-zero check.
Phase 4 plan structure anticipated: 10-clause template per
iter2/iter3, with new Clause 8 dedicated to compressed-header
parser.
Refs:
phase0_findings_iter4.md (Phase 1 lock)
phase8_iteration3_close.md (predecessor)
references/ffmpeg-kwiboo/libavcodec/v4l2_request_vp9.c (V4L2 ref)
references/ffmpeg-kwiboo/libavcodec/vaapi_vp9.c (VAAPI ref)
/home/mfritsche/src/linux-rfc/drivers/staging/media/rkvdec/
rkvdec-vp9.c (kernel driver — confirms COMPRESSED_HDR
requirement at lines 752-754)
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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