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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)
129 lines
5.2 KiB
Diff
129 lines
5.2 KiB
Diff
From: Markus Fritsche <fritsche.markus@gmail.com>
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Date: 2026-05-01
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Subject: [PATCH] h264: omit per-slice controls in FRAME_BASED mode
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Identified by cross-reference against GStreamer's
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gst-plugins-bad/sys/v4l2codecs/gstv4l2codech264dec.c (upstream commit
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9e3e775). At lines 1263-1304, GStreamer gates SLICE_PARAMS and
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PRED_WEIGHTS submission on is_slice_based(self):
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if (is_slice_based (self)) {
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control[num_controls].id = V4L2_CID_STATELESS_H264_SLICE_PARAMS;
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...
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control[num_controls].id = V4L2_CID_STATELESS_H264_PRED_WEIGHTS;
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...
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}
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In V4L2_STATELESS_H264_DECODE_MODE_FRAME_BASED, the kernel parses the
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bitstream itself from the OUTPUT-queue payload; per-slice controls in
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the request trigger cluster-validation EINVAL at error_idx=count
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(observed on RK3568 hantro-vpu, kernel 6.19.10).
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This patch:
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- Reorders controls[] so FRAME_BASED-required entries come first
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(SPS, PPS, SCALING_MATRIX, DECODE_PARAMS at indices 0..3) and the
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SLICE_BASED-only entries come last (SLICE_PARAMS, PRED_WEIGHTS at
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indices 4..5).
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- Defaults num_controls=4 (FRAME_BASED), expanding to 5 for
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SLICE_BASED and 6 when V4L2_H264_CTRL_PRED_WEIGHTS_REQUIRED.
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- Hardcodes slice_based=false for now since patch 0002 sets the
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device to FRAME_BASED unconditionally. A TODO marks the spot for
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the planned probe-then-set commit, which will populate
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context->decode_mode at CreateContext via VIDIOC_QUERYCTRL/
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G_EXT_CTRLS and replace the hardcoded false with a runtime check.
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Diagnosis chain:
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- patch 0005 reduced one EINVAL per frame on PRED_WEIGHTS
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submission, but cluster-level rejection persisted at error_idx=5
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(count) — meaning kernel walked all 5 controls cleanly but
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rejected the request as a whole.
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- dmesg silent → rejection in V4L2 core (v4l2-ctrls-request.c /
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v4l2-h264.c), not in hantro driver where it could log.
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- GStreamer reference confirmed FRAME_BASED contract: only 4
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sequence-and-frame-level controls go in the per-request batch.
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After this patch the kernel should accept the per-request controls
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and actually decode the bitstream into the CAPTURE buffer.
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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 2026-05-01 20:30:02.632190563 +0000
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+++ b/src/h264.c 2026-05-01 20:49:46.937497317 +0000
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@@ -531,6 +531,21 @@
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sps.profile_idc = h264_profile_to_idc(profile);
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+ /*
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+ * Per-request control batch, ordered so the controls REQUIRED in
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+ * V4L2_STATELESS_H264_DECODE_MODE_FRAME_BASED come first
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+ * (indices 0..3) and the SLICE_BASED-only controls come last
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+ * (indices 4..5).
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+ *
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+ * Cross-reference: GStreamer gst-plugins-bad
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+ * sys/v4l2codecs/gstv4l2codech264dec.c (commit 9e3e775,
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+ * lines 1263-1304) gates SLICE_PARAMS and PRED_WEIGHTS on
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+ * is_slice_based(self); under FRAME_BASED only SPS/PPS/
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+ * SCALING_MATRIX/DECODE_PARAMS are submitted. The kernel
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+ * parses the bitstream itself in FRAME_BASED mode; submitting
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+ * per-slice controls in that mode triggers cluster-validation
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+ * EINVAL at error_idx=count.
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+ */
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struct v4l2_ext_control controls[6] = {
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{
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.id = V4L2_CID_STATELESS_H264_SPS,
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@@ -545,14 +560,14 @@
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.p_h264_scaling_matrix = &matrix,
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.size = sizeof(matrix),
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}, {
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- .id = V4L2_CID_STATELESS_H264_SLICE_PARAMS,
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- .p_h264_slice_params = &slice,
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- .size = sizeof(slice),
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- }, {
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.id = V4L2_CID_STATELESS_H264_DECODE_PARAMS,
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.p_h264_decode_params = &decode,
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.size = sizeof(decode),
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}, {
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+ .id = V4L2_CID_STATELESS_H264_SLICE_PARAMS,
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+ .p_h264_slice_params = &slice,
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+ .size = sizeof(slice),
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+ }, {
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.id = V4L2_CID_STATELESS_H264_PRED_WEIGHTS,
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.ptr = &weights,
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.size = sizeof(weights),
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@@ -560,20 +575,24 @@
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};
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/*
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- * PRED_WEIGHTS is conditionally required per kernel UAPI:
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- * V4L2_H264_CTRL_PRED_WEIGHTS_REQUIRED(pps, slice) is only
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- * true when explicit weighted prediction applies (P/SP slice
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- * with WEIGHTED_PRED flag, or B slice with weighted_bipred_idc
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- * == 1). Submitting it unconditionally on a frame that does
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- * not need it triggers EINVAL at error_idx=5 on hantro and
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- * other drivers that strictly enforce the spec.
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+ * Decode-mode dispatch. Patch 0002 unconditionally sets the
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+ * device to FRAME_BASED, so we hardcode that here. When the
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+ * planned probe-then-set commit lands, slice_based becomes
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+ * context->decode_mode == V4L2_STATELESS_H264_DECODE_MODE_SLICE_BASED
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+ * with context->decode_mode populated at CreateContext via
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+ * VIDIOC_QUERYCTRL/G_EXT_CTRLS.
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*
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- * controls[5] is PRED_WEIGHTS (last in array); narrow the
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- * submission count to exclude it when not required.
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+ * FRAME_BASED: 4 controls (SPS, PPS, SCALING_MATRIX, DECODE_PARAMS).
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+ * SLICE_BASED: +SLICE_PARAMS (always), +PRED_WEIGHTS (when
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+ * V4L2_H264_CTRL_PRED_WEIGHTS_REQUIRED).
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*/
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- unsigned int num_controls = 6;
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- if (!V4L2_H264_CTRL_PRED_WEIGHTS_REQUIRED(&pps, &slice))
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+ const bool slice_based = false; /* TODO: probe via context->decode_mode */
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+ unsigned int num_controls = 4;
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+ if (slice_based) {
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num_controls = 5;
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+ if (V4L2_H264_CTRL_PRED_WEIGHTS_REQUIRED(&pps, &slice))
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+ num_controls = 6;
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+ }
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rc = v4l2_set_controls(driver_data->video_fd, surface->request_fd,
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controls, num_controls);
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