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context: enable ANNEX_B start-code emission to match device

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Patch 0002 sets V4L2_CID_STATELESS_H264_START_CODE to ANNEX_B on the
device, telling the kernel that OUTPUT-buffer payloads will contain
0x00 0x00 0x01 NAL start codes. picture.c::codec_store_buffer has
the prepend logic guarded by `if (context->h264_start_code)`, but
that boolean is set ONLY inside h264_get_controls() — a function
that exists but is never called.

Result: device expects ANNEX_B, libva-v4l2-request feeds raw NAL
payloads with no start codes, kernel cannot find slice boundaries,
hantro emits a zeroed CAPTURE buffer. mpv reports successful decode
because the V4L2 round-trip succeeds (no EINVAL); the visual output
is a flat dark-green frame (NV12 zero through BT.709).

Identified via:
  - Patch 0006 cleared the EINVAL cluster-rejection (128 → 0 on
    bbb_1080p30) but visual output remained flat green.
  - GStreamer reference (gstv4l2codech264dec.c:1363-1377) confirms
    start codes are required when ANNEX_B is selected.
  - Source-archaeology of fourier's picture.c:67-74 showed the gate
    on context->h264_start_code.

Fix: in context.c::RequestCreateContext, immediately after patch
0002's device-control block, set context_object->h264_start_code =
true to match the ANNEX_B mode we just programmed. Hardcoded for
now (matches 0002's hardcoded set); replaced with a runtime probe
in the planned probe-then-set commit.

Signed-off-by: Markus Fritsche <fritsche.markus@gmail.com>
This commit is contained in:
Markus Fritsche
2026-05-01 12:00:00 +00:00
parent 4246d5d537
commit 4078368104
1 changed files with 17 additions and 0 deletions
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src/context.c
+17
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@@ -146,6 +146,23 @@ VAStatus RequestCreateContext(VADriverContextP context, VAConfigID config_id,
dev_ctrls, 2);
}
/*
* Mirror the ANNEX_B start-code mode set on the device above
* into context_object->h264_start_code so picture.c::
* codec_store_buffer prepends 0x00 0x00 0x01 to each slice
* payload it copies into the OUTPUT buffer. Without this, the
* kernel — which we just told to expect ANNEX_B — sees a raw
* NAL stream with no start codes, fails to find slice
* boundaries, and emits a zeroed CAPTURE buffer (visually a
* flat dark-green frame).
*
* h264_get_controls() exists for this purpose but is never
* called in the current code path; the planned probe-then-set
* commit will replace this hardcoded assignment with a runtime
* read of the kernel's accepted START_CODE value.
*/
context_object->h264_start_code = true;
rc = v4l2_set_stream(driver_data->video_fd, output_type, true);
if (rc < 0) {
status = VA_STATUS_ERROR_OPERATION_FAILED;