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daedalus-v4l2/docs/roadmap.md
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marfrit 1ae9528e76 Phase 8.8: throughput baseline + multi-codec streams + HDR 
Per the correctness-before-speed principle: measure before
optimising. Roadmap going in said "QPU dispatch substitution
to hit 30fps@1080p". Measurement on hertz shows the FFmpeg
software path already hits 65-88 fps@1080p across all three
codecs — QPU substitution would be premature optimisation.

So 8.8 ships what's actually useful:
1. Per-frame timing in test_m2m_stream.
2. Multi-frame AV1 + H.264 streams verified byte-exact at
   1080p (closes the "VP9-only stream tests" gap from 8.7).
3. HDR / 10-bit via V4L2_PIX_FMT_P010 + daemon
   pack_p010_to_plane.

Test harness (tools/test_m2m_stream.c):
- Per-frame µs timing via CLOCK_MONOTONIC; reports mean/p50/
  p99/min/max + wall ms + fps.
- Annex-B H.264 parser: split on 3-/4-byte start codes,
  accumulate NALs into access units (push on VCL NAL types
  1 or 5). Without AU grouping FFmpeg rejects SPS/PPS-only
  buffers as "no frame!".
- Format auto-detect (DKIF magic → IVF; else Annex-B).
- Optional 6th arg `[capture]`: nv12m | p010.
- CAPTURE mmap path generalised for num_planes==1 (P010).

Kernel (kernel/daedalus_v4l2_main.c):
- CAPTURE formats array {NV12M, P010}; enum_fmt walks it.
- daedalus_fill_capture_fmt takes a fourcc:
    NV12M: 2 planes, W*H + W*H/2 bytes, bpl=W
    P010:  1 plane,  W*H*2 + W*H bytes, bpl=W*2
- try_fmt preserves caller fourcc when supported.
- daedalus_complete_resp_frame's dmabuf path now sets each
  plane's payload to vb2_plane_size(vb,p) — generalises
  cleanly across 1-plane (P010) and 2-plane (NV12M) layouts;
  the daemon fully populates the plane so payload =
  sizeimage.

Daemon (daemon/src/decoder.c):
- pack_p010_to_plane: YUV420P10LE → P010 single-plane.
  10-bit samples shifted left by 6 to MSB-align in 16-bit
  words per V4L2 ABI. Y at base+0, interleaved CbCr right
  after Y plane (per format spec for single-plane P010).
  Strips source stride padding; respects destination stride.
- daedalus_decoder_run_request dispatches on
  req->capture_pix_fmt (NV12M → pack_nv12_to_planes; P010
  → pack_p010_to_plane; else warn + skip).
- Includes <linux/videodev2.h> for fourcc constants.

Verification on hertz (Pi 5, 6.12.75+rpt-rpi-2712):

1080p throughput baseline (30 frames testsrc, dmabuf path):

  VP9   1080p:  mean 12.0 ms,  p99 15.9 ms,  fps **83.1**, byte-exact ✓
  AV1   1080p:  mean 15.4 ms,  p99 41.0 ms,  fps **65.0**, byte-exact ✓
  H.264 1080p:  mean 11.3 ms,  p99 21.5 ms,  fps **88.3**, byte-exact ✓

All 2-3× over the 30fps-floor-is-fine criterion.

HDR / 10-bit 1080p P010:
  10 frames, 62 MB output, fps **48.8**, byte-exact vs
  `ffmpeg -pix_fmt p010le -f rawvideo`.

Small-frame P010 (320×240): fps 966 — fixed daemon overhead
dominates at low resolutions.

v4l2-compliance unchanged from 8.7: 49/49 passing.
Format enumeration confirms NM12 + P010 on CAPTURE.

Clean SIGTERM + rmmod; no kernel oops/WARN.

Roadmap update (docs/roadmap.md):
- 8.8 marked closed with closure-doc reference, including
  the explicit "QPU substitution not needed" rationale.
- 8.9 reshaped: libva-v4l2-request consumer integration
  (per project_consumer_target memory) — the actual
  user-facing endpoint.

Per correctness-before-speed:
- Measured first; QPU work explicitly justified-out via data.
- Byte-exact pixel comparison for every codec/format combo
  (NV12: VP9, AV1, H.264; P010: VP9 10-bit at 320×240 and
  1080p).
- AU grouping in the Annex-B parser is the correct
  semantic boundary, not just a workaround.
- vb2_plane_size for payload generalises to any plane
  count, not hardcoded to 2.

Phase 8.9 next: libva-v4l2-request integration — close
the loop from YouTube/Firefox to /dev/video0 + daemon
playback.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
2026-05-18 16:34:05 +00:00

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daedalus-v4l2 — roadmap

Sub-phases

Phase 8.1 — kernel module skeleton

Out-of-tree kernel module that:

  • Registers /dev/videoNN with VFL_TYPE_VIDEO + a no-op V4L2 stateless dispatch table.
  • Accepts open/close, S_FMT, REQBUFS ioctls without doing anything (yet).
  • Builds against /lib/modules/$(uname -r)/build.

Deliverable: modprobe daedalus_v4l2 works, v4l2-ctl --list-devices shows the new device.

Phase 8.2 — kernel ↔ daemon chardev bridge

  • Kernel module creates /dev/daedalus-v4l2 chardev.
  • Defines a simple req/resp protocol in include/daedalus_v4l2_proto.h.
  • Daemon connects, exchanges echo requests.

Deliverable: ping-pong test passes.

Phase 8.3 — daemon FFmpeg dlopen + parse

  • Daemon links libdaedalus_core.a from sibling.
  • Daemon dlopens FFmpeg.
  • Test program: feed a VP9 IVF file to FFmpeg parsers, extract block-level metadata, validate against expected.

Deliverable: daemon can parse a VP9 frame and walk the block-level info.

Phase 8.4 — daemon ↔ kernel decode round-trip (closed 2026-05-18)

Shipped as a debugfs-triggered chardev round-trip rather than the original V4L2-ioctl plan (which moved to Phase 8.5).

  • REQ_DECODE / RESP_FRAME wire protocol
  • Daemon decodes VP9 via FFmpeg dlopen, returns FNV-1a digest
  • Verified content-dependent + deterministic; structured error handling for bad bitstreams

See docs/phase_8_4_closure.md.

Phase 8.5 — full V4L2 m2m driver (closed 2026-05-18)

Real V4L2 m2m driver — userspace clients drive S_FMT/REQBUFS/QBUF/DQBUF the standard way. Bitstream flows kernel→daemon as inline REQ_DECODE payload; decoded NV12 pixels flow daemon→kernel as inline RESP_FRAME payload. Works end-to-end for small frames (≤ ~64 KiB NV12).

Deliverable hit: kernel m2m driver passes most v4l2-compliance checks; tools/test_m2m_decode produces a NV12 frame that's byte-for-byte identical to ffmpeg -pix_fmt nv12 reference.

See docs/phase_8_5_closure.md.

Phase 8.6 — dmabuf + AV1 + H.264 + stateless controls (closed 2026-05-18)

  • CAPTURE (and OUTPUT) on vb2_dma_contig_memops.
  • New DAEDALUS_IOC_GET_DMABUF chardev ioctl — daemon mmaps the in-flight CAPTURE buffer, decodes pixels in place, sends RESP_FRAME metadata-only.
  • 64 KiB frame-size cap removed. 1080p VP9 + 128×96 AV1
    • 128×96 H.264 all byte-exact against reference FFmpeg decode.
  • V4L2 stateless controls registered for VP9 / AV1 / H.264 (11 controls visible to userspace).
  • Colorspace round-trip fix (TRY_FMT preserve, S_FMT OUTPUT→CAPTURE propagation).
  • Cookie unified across V4L2 + debugfs paths.
  • v4l2-compliance: 47/48 (only DECODER_CMD remains, needs media controller — moved to 8.7).

See docs/phase_8_6_closure.md.

Phase 8.7 — media controller + multi-frame streaming (closed 2026-05-18)

  • Media controller bound via v4l2_m2m_register_media_controller + media_device_register; /dev/mediaN published.
  • tools/test_m2m_stream parses IVF and pushes frames sequentially through a 4-deep buffer ring; daemon AVCodecContext preserves reference frames across calls.
  • 30-frame VP9 320×240 stream byte-exact (3.46 MB across 1 keyframe + 29 P-frames).
  • 10-frame VP9 1080p stream byte-exact (31 MB across 10 frames at full HD).
  • v4l2-compliance: 49/49 passing (was 47/48 in 8.6; media controller added a 49th test and closed DECODER_CMD).

See docs/phase_8_7_closure.md.

Phase 8.8 — throughput baseline + multi-codec streams + HDR (closed 2026-05-18)

  • Per-frame µs timing in test_m2m_stream; multi-codec baseline:
    • VP9 1080p: 83.1 fps
    • AV1 1080p: 65.0 fps
    • H.264 1080p: 88.3 fps All byte-exact vs ffmpeg reference; all 2-3× over the 30fps-floor-is-fine criterion.
  • QPU dispatch substitution explicitly not needed — measurement shows the FFmpeg software path already clears the target on Pi 5's Cortex-A76. Substitution moves to the optimisation roadmap.
  • Annex-B H.264 access-unit splitter in the test harness (NALs grouped by VCL boundary).
  • HDR / 10-bit: V4L2_PIX_FMT_P010 added as CAPTURE format; daemon pack_p010_to_plane handles YUV420P10LE → P010 with MSB-aligned 10-bit data. 10-bit 1080p byte-exact at 48.8 fps.

See docs/phase_8_8_closure.md.

Phase 8.9 — libva-v4l2-request integration (the actual consumer)

  1. Patch libva-v4l2-request to recognise our driver via the media controller graph (the project_consumer_target memory's libva-v4l2-request-fourier target).
  2. End-to-end test: Firefox / mpv → libva → /dev/video0 → daemon → on-screen frame.
  3. Long-form (60s+) playback stress with buffer recycling.
  4. Multi-frame HDR tests for AV1 + H.264.

After 8.9 the project's user-facing loop is closed. Optimisation phases (QPU dispatch, 4K, encoders) ship when motivated.

Effort estimate

Each phase: ~1 week of focused work (~40 hours). Total: 7 weeks for v1.

Could be split across multiple sessions / contributors.

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