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Mirrors phase6/step1/ from the ohm_gl_fix campaign. Contract-correct
hantro multi-planar / chromium-149-era stateless H.264 port of
bootlin's libva-v4l2-request, patches 0001..0018 + fourier-local.
Honest characterisation in README:
- Builds cleanly on chromium-builder LXC (boltzmann)
- vainfo enumerates H.264 profiles cleanly with LIBVA_DRIVER_NAME=v4l2_request
- NOT on Brave's decode path on ohm_gl_fix stack — Brave uses
Chromium's own V4L2VideoDecoder in media/gpu/v4l2/.
- Most likely useful for a future Firefox-via-libavcodec-vaapi
campaign, modulo a separate Mesa-panfrost WSI pitch issue.
- DEBUG patches (0010, 0011, 0014) intentionally kept in series
for development; remove for cleaner production runs.
Audit trail in the source repo at ohm_gl_fix:
phase6/step1/audit_0008_decode_params_2026-05-01.md
phase6/step1/api_contract_findings_2026-05-01.md
phase3_remeasure_2026-05-02/B3_decoder_discovery.md (why this
isn't on Brave's path)
83 lines
3.3 KiB
Diff
83 lines
3.3 KiB
Diff
From: Markus Fritsche <fritsche.markus@gmail.com>
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Date: 2026-05-02
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Subject: [PATCH] h264: derive PFRAME / BFRAME flags from VASlice slice_type
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v4l2_ctrl_h264_decode_params.flags has PFRAME and BFRAME bits per
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ext-ctrls-codec-stateless.rst. fourier never set them; libva-v4l2-
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request relied on each backing driver tolerating frame-class
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ambiguity.
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Kernel survey (linux 6.19.x):
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- tegra-vde/h264.c (lines 783-799) consumes both flags to select
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the inter-frame decode kernel. Without them the I-frame kernel
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runs on P/B content.
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- visl-trace-h264.h uses them for decode tracing.
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- hantro / rkvdec / cedrus / mediatek / qcom-iris-stateless do
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not consume the flags.
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Hantro on ohm decoded bbb cleanly without these flags set (see
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phase6/step1/ohm_smoke_2026-05-02T060255Z_post_0015/), so this is
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an upstreamability fix for cross-driver portability rather than a
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correctness fix for hantro.
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VAAPI's VASliceParameterBufferH264.slice_type maps directly to the
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H.264 slice_header() slice_type field. Per spec 7.4.3:
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0=P 1=B 2=I 3=SP 4=SI; 5..9 = "all slices in the picture have
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this slice_type." `slice_type % 5` recovers the underlying type
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in either encoding form.
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In FRAME_BASED mode we only see surface->params.h264.slice from the
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most-recent VASliceParameterBuffer — that's fine: a single coded
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picture has a uniform slice_type for the purposes of the PFRAME /
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BFRAME flag (multi-slice frames may mix slice types in some streams,
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but the flag's semantic is "this is an inter-coded frame," which
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holds if any slice is P or B; using the last-seen slice's type is
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a reasonable approximation).
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Cross-reference: ext-ctrls-codec-stateless.rst Decode Parameters
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Flags table.
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Signed-off-by: Markus Fritsche <fritsche.markus@gmail.com>
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---
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--- a/src/h264.c
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+++ b/src/h264.c
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@@ -587,6 +587,38 @@
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&surface->params.h264.slice,
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&surface->params.h264.picture, &slice, &weights);
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+ /*
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+ * Derive PFRAME / BFRAME flags in v4l2_ctrl_h264_decode_params.flags
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+ * from VASliceParameterBufferH264.slice_type. VAAPI's slice_type
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+ * matches the H.264 spec slice_type semantic: 0=P, 1=B, 2=I, 3=SP,
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+ * 4=SI; values 5..9 mean "all slices in the picture have this
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+ * slice_type" (mod 5 yields the underlying type). VAAPI consumers
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+ * (ffmpeg, mpv) populate this for every slice; in FRAME_BASED mode
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+ * we only see the most-recent slice's params, but slice_type is
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+ * uniform across a single coded picture for our purposes.
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+ *
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+ * Kernel consumers that read these flags: tegra-vde
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+ * (drivers/media/platform/nvidia/tegra-vde/h264.c lines 783-799 of
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+ * 6.19.x) selects the inter-frame decode kernel. Hantro / rkvdec /
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+ * cedrus / mediatek / qcom-iris-stateless do not consume them.
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+ * Setting them keeps the libva-v4l2-request fork upstreamable
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+ * across drivers without affecting hantro behaviour.
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+ *
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+ * Cross-reference: ext-ctrls-codec-stateless.rst Decode Parameters
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+ * Flags — V4L2_H264_DECODE_PARAM_FLAG_PFRAME / _BFRAME.
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+ */
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+ switch (surface->params.h264.slice.slice_type % 5) {
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+ case H264_SLICE_P:
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+ decode.flags |= V4L2_H264_DECODE_PARAM_FLAG_PFRAME;
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+ break;
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+ case H264_SLICE_B:
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+ decode.flags |= V4L2_H264_DECODE_PARAM_FLAG_BFRAME;
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+ break;
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+ default:
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+ /* I / SP / SI: no extra flag. */
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+ break;
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+ }
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+
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sps.profile_idc = h264_profile_to_idc(profile);
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/*
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