Phase 8.5: full V4L2 m2m driver, VP9 decode via QBUF/DQBUF
Replaces the Phase 8.4 debugfs-triggered chardev path with a
real V4L2 m2m driver. Userspace clients now drive decoding the
standard way — S_FMT / REQBUFS / QBUF on the OUTPUT (bitstream)
queue, DQBUF on the CAPTURE (NV12M) queue. Kernel device_run
packs the bitstream into REQ_DECODE; daemon decodes via FFmpeg;
RESP_FRAME's inline NV12 pixel payload lands in the CAPTURE
buffer. Phase 8.6 swaps the inline payload for dmabuf so big
frames stop being capped at 64 KiB.
Kernel (daedalus_v4l2_main.c, rewritten + main.h added):
- Per-open struct daedalus_ctx: v4l2_fh, m2m_ctx, ctrl_handler,
per-queue v4l2_pix_format_mplane.
- Two vb2_queues (vb2_vmalloc_memops for both — no DMA needed
yet; 8.6 switches CAPTURE to dma_contig for dmabuf-export):
OUTPUT = V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE, VP9_FRAME
CAPTURE = V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE, NV12M
- Full v4l2_ioctl_ops table: querycap, enum_fmt, g/s/try_fmt
for both queues, reqbufs/querybuf/qbuf/dqbuf/create_bufs/
prepare_buf/expbuf/streamon/streamoff via v4l2_m2m_ioctl_*
helpers.
- v4l2_m2m_ops.device_run: peeks next OUTPUT buf, builds
REQ_DECODE inline with the bitstream bytes, enqueues with an
auto-incrementing cookie, stores {ctx, src_buf, dst_buf} in
a per-device inflight list. Job stays open until RESP_FRAME.
- daedalus_complete_resp_frame(): pops the inflight entry,
memcpys inline NV12 pixels into the CAPTURE buffer (Y plane
+ interleaved CbCr), finishes via
v4l2_m2m_buf_done_and_job_finish — NOT plain buf_done +
job_finish, which leaves the src buf on the m2m queue and
causes device_run to immediately re-run on the same input
(caught on first run; second REQ_DECODE for same bitstream +
eventual oops in stop_streaming on teardown).
Kernel (daedalus_v4l2_chardev.c):
- RESP_FRAME handler now hands inline pixel payload to
daedalus_complete_resp_frame so it lands in the CAPTURE
vb2 buffer. Existing PONG and debugfs test_decode paths still
work; the latter produces a harmless ratelimited "unknown
cookie" since it bypasses V4L2 m2m.
Daemon (decoder.c, decoder.h):
- daedalus_decoder_run_request signature extended with
(nv12_out, nv12_cap, nv12_used). After the FNV-1a digest the
decoder packs YUV420P into NV12 in the caller's buffer: Y
plane line-by-line stripped of stride padding; Cb/Cr
interleaved into a single chroma plane. Truncation silent —
kernel only memcpys what fits in the CAPTURE plane.
Daemon (chardev_client.c):
- handle_req_decode allocates a response buffer sized for the
full chardev payload, lets decoder fill the pixel area
after the resp_frame struct, sends the full payload via the
existing send_response.
Test client (tools/test_m2m_decode.c, new):
- Minimal V4L2 m2m client: S_FMT both queues, REQBUFS 1 each,
mmap+fill OUTPUT, QBUF both, STREAMON, poll, DQBUF, dump
CAPTURE planes to a raw NV12 file. ~250 LOC; verifies the
whole flow without needing v4l2-ctl framing.
Roadmap update (docs/roadmap.md):
- Phase 8.4 retitled "daemon ↔ kernel decode round-trip"
to reflect what actually shipped (vs. the original V4L2-
ioctl-driven plan which moved here).
- Phase 8.5 retitled "full V4L2 m2m driver" with closure
status.
- Phase 8.6 reshaped to two tracks: dmabuf + AV1/H.264/
stateless controls + media controller. Adds the punch list
of v4l2-compliance failures (DECODER_CMD, S_FMT colorspace)
that 8.6 will fix.
Verification on hertz (Pi 5, 6.12.75+rpt-rpi-2712):
Kernel + daemon build clean (-Wall -Wextra clean both sides).
Test harness drives one VP9 keyframe end-to-end:
OUTPUT REQBUFS -> 2
CAPTURE REQBUFS -> 2
QBUF OUTPUT[0] bytesused=1566
QBUF CAPTURE[0]; STREAMON both
poll revents=0x5
DQBUF OUTPUT[0] flags=0x4001 (DONE)
DQBUF CAPTURE[0] flags=0x4000 payloads=[12288, 6144]
wrote 12288 Y + 6144 UV bytes to /tmp/out_m2m.nv12
Pixel correctness vs reference:
ffmpeg -i vp9_small.ivf -pix_fmt nv12 -f rawvideo -y ref.nv12
cmp /tmp/out_m2m.nv12 /tmp/ref.nv12 → match ✓
Byte-for-byte identical to FFmpeg's stock CPU decode.
v4l2-compliance: detected as Stateless Decoder; most ioctls
pass; two expected fails documented in closure doc
(DECODER_CMD/media controller, S_FMT colorspace).
Clean teardown: SIGTERM the daemon, rmmod the module, no
oops/WARN in dmesg.
Per correctness-before-speed:
- Real V4L2 ioctl table (not stubs); uses v4l2-core helpers
where they exist instead of reinventing.
- v4l2_m2m_buf_done_and_job_finish (not the manual sequence)
to keep scheduler state consistent.
- Bit-exact reference comparison, not just "looks right."
- Documented every compliance failure with the planned fix.
- All resource paths (kmalloc/kfree, inflight list cleanup,
src/dst buf removal in stop_streaming) handled on every
error branch.
Phase 8.6 next: dmabuf-export for CAPTURE (removes 64 KiB
frame-size cap), add AV1+H.264 codecs, add V4L2 stateless
controls + media controller binding, fix the colorspace +
cookie-namespace compliance issues.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
This commit is contained in:
+78
-3
@@ -132,10 +132,67 @@ static int decoder_open_codec(struct daedalus_decoder *dec, uint32_t codec_id,
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return 0;
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}
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/*
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* Pack the decoded YUV planes into NV12 (Y followed by interleaved
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* CbCr) in @out, advancing @out and tracking total bytes written
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* in @used. Truncates silently at @cap. Returns 0 on success,
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* -EINVAL if the source format isn't planar YUV 4:2:0.
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*/
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static int pack_nv12(struct AVFrame *fr, const AVPixFmtDescriptor *desc,
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uint8_t *out, size_t cap, size_t *used)
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{
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int y, cw, ch, h, w;
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size_t y_size, uv_size;
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*used = 0;
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if (!desc || !out || !cap)
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return -EINVAL;
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if (desc->nb_components < 3)
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return -EINVAL;
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w = fr->width;
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h = fr->height;
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cw = AV_CEIL_RSHIFT(w, desc->log2_chroma_w);
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ch = AV_CEIL_RSHIFT(h, desc->log2_chroma_h);
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y_size = (size_t) w * (size_t) h;
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uv_size = (size_t) cw * (size_t) ch * 2u;
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/* Y plane: w bytes per line stripped of stride padding. */
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if (*used + y_size > cap)
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return -EINVAL;
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for (y = 0; y < h; y++)
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memcpy(out + *used + (size_t) y * (size_t) w,
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fr->data[0] + (size_t) y * (size_t) fr->linesize[0],
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(size_t) w);
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*used += y_size;
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/* Interleave Cb and Cr into a single NV12 chroma plane. */
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if (*used + uv_size > cap)
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return -EINVAL;
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{
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uint8_t *uv = out + *used;
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int x;
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for (y = 0; y < ch; y++) {
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const uint8_t *u = fr->data[1] +
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(size_t) y * (size_t) fr->linesize[1];
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const uint8_t *v = fr->data[2] +
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(size_t) y * (size_t) fr->linesize[2];
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for (x = 0; x < cw; x++) {
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uv[(y * cw + x) * 2 + 0] = u[x];
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uv[(y * cw + x) * 2 + 1] = v[x];
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}
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}
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*used += uv_size;
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}
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return 0;
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}
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int daedalus_decoder_run_request(struct daedalus_decoder *dec,
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const struct daedalus_req_decode *req,
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const uint8_t *bitstream,
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struct daedalus_resp_frame *resp)
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struct daedalus_resp_frame *resp,
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uint8_t *nv12_out, size_t nv12_cap,
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size_t *nv12_used)
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{
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struct ffmpeg_loader *fm = dec->loader;
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struct AVCodecContext *ctx = NULL;
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@@ -143,6 +200,8 @@ int daedalus_decoder_run_request(struct daedalus_decoder *dec,
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memset(resp, 0, sizeof(*resp));
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resp->codec_id = req->codec_id;
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if (nv12_used)
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*nv12_used = 0;
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rc = decoder_open_codec(dec, req->codec_id, &ctx);
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if (rc == -ENOSYS) {
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@@ -255,10 +314,26 @@ int daedalus_decoder_run_request(struct daedalus_decoder *dec,
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resp->chroma_len = chroma_len;
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resp->fnv1a_yuv = h;
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log_info("decoder: OK %dx%d fmt=%d (%s) fnv1a=0x%08x luma=%u chroma=%u",
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/*
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* Pack pixels as NV12 into the caller's buffer if
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* provided and big enough. Truncation is silent —
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* the kernel will only copy as much as fits in the
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* CAPTURE plane, so partial fills are fine for Phase
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* 8.5. Phase 8.6 (dmabuf) eliminates the truncation.
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*/
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if (nv12_out && nv12_cap) {
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int prc = pack_nv12(fr, desc, nv12_out, nv12_cap,
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nv12_used);
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if (prc < 0)
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log_warn("decoder: NV12 pack failed (cap=%zu pix_fmt=%d) — kernel will see metadata only",
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nv12_cap, fr->format);
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}
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log_info("decoder: OK %dx%d fmt=%d (%s) fnv1a=0x%08x luma=%u chroma=%u nv12=%zu",
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fr->width, fr->height, fr->format,
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desc ? desc->name : "?",
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h, luma_len, chroma_len);
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h, luma_len, chroma_len,
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nv12_used ? *nv12_used : 0u);
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}
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fm->av_frame_unref(dec->frame);
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