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daemon: filter tiny pause-time bitstreams (closes #17)

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libva-v4l2-request-fourier flushes a stub packet into the V4L2
OUTPUT_MPLANE queue at playback-pause boundaries.  The payload is
shorter than any parseable H.264 NAL (3-byte start code + 1-byte
NAL header = 4 bytes minimum); avcodec_send_packet returns
AVERROR_INVALIDDATA (-1094995529), which propagated to the kernel
as a decode failure.  Firefox then marked H.264-via-VAAPI as
broken for the session and routed every subsequent frame to
libmozavcodec SW — pause never recovered to HW.

At the REQ_DECODE entry in chardev_client.c::handle_req_decode,
short-circuit any bitstream below the minimum-parseable threshold:
log INFO, skip daedalus_decoder_run_request, and reply RESP_FRAME
with status=DAEDALUS_DECODE_NO_FRAME so libva's V4L2 surface pool
stays healthy and Firefox doesn't see a failure.

Repro: Pi CM5 trixie, daedalus-v4l2 0.1.0+r41 + ffmpeg-v4l2-
request-fourier 2:8.1+rfourier+gb57fbbe-9, Firefox YouTube avc1.
Play → daemon decodes at ~46 fps.  Pause ≥ 1s.  Resume → daemon
silent; sudo journalctl -u daedalus-v4l2 --since '10s' | grep -c
'decoder: OK' = 0.  Last entry before silence:

    REQ_DECODE cookie=N codec=3 bitstream=3 bytes ...
    [h264 @ ...] no frame!
    [ERR] decoder: avcodec_send_packet failed: -1094995529

After this fix the 3-byte sentinel logs as 'tiny bitstream 3
bytes — dropping as no-op' and the libavcodec context is
untouched; the next real REQ_DECODE proceeds normally.

Scope NOT covered (intentionally deferred):
- A more general "tolerate AVERROR_INVALIDDATA mid-stream" path.
  Worth doing later but masks unrelated bugs.
- Investigating WHY libva sends the 3-byte sentinel on pause.
  Likely an upstream libva-v4l2-request-fourier issue; tracked
  separately if this filter is not enough.

Wire protocol unchanged.  No DAEDALUS_PROTO_VERSION bump.
This commit is contained in:
Markus Fritsche
2026-05-22 17:26:25 +02:00
parent 6e6dfa144d
commit 3e4e6e8eae
1 changed files with 23 additions and 0 deletions
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daemon/src/chardev_client.c
+23
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@@ -181,6 +181,29 @@ static int handle_req_decode(struct chardev_client *cli,
req.capture_width, req.capture_height,
req.capture_num_planes);
/*
* Degenerate-bitstream filter (issue #17): libva-v4l2-request-
* fourier flushes a stub packet into the OUTPUT_MPLANE queue at
* playback-pause boundaries. The payload is shorter than any
* parseable H.264 NAL (3-byte start code + 1-byte NAL header =
* 4 bytes minimum); avcodec_send_packet returns
* AVERROR_INVALIDDATA, which we used to propagate to the kernel
* as a decode failure. Firefox then marks H.264-via-VAAPI as
* broken for the session and routes every subsequent frame to
* libmozavcodec SW — pause never recovers to HW.
*
* Drop the request as a no-op decode and reply RESP_FRAME OK so
* libva's V4L2 state machine keeps its surface pool alive.
*/
if (req.bitstream_len < 4) {
log_info("REQ_DECODE cookie=%u: tiny bitstream %u bytes — dropping as no-op (pause-time sentinel)",
hdr->cookie, req.bitstream_len);
memset(&resp, 0, sizeof(resp));
resp.status = DAEDALUS_DECODE_NO_FRAME;
return send_response(cli, DAEDALUS_MSG_RESP_FRAME,
hdr->cookie, &resp, sizeof(resp));
}
/*
* Open dmabuf-fds for every CAPTURE plane and mmap them.
* If this fails we still attempt the decode (so the kernel