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marfrit 9415b7e0f7 Phase 8.1: kernel V4L2 device skeleton (out-of-tree module)

Out-of-tree Linux kernel module registering /dev/videoNN. Phase
8.1 scope: skeleton only — VIDIOC_QUERYCAP works, no codec
ioctls / no vb2_queue / no controls yet.

Real V4L2 plumbing throughout per "correctness before speed":
platform_device + v4l2_device + video_device, properly nested
with error paths and devm_kzalloc-managed lifetime. Per-cycle 9
discipline ports to kernel code: SPDX header, kernel coding
style (8-tab, static-by-default), kerneldoc on structs, no
shortcuts.

Files (~250 LOC total):
- kernel/Makefile — out-of-tree kbuild with checkpatch target
- kernel/daedalus_v4l2_main.c — module init/exit + probe/remove

Verification on hertz (Pi 5, 6.12.75+rpt-rpi-2712):
- Builds clean with -Wall -Wextra. No warnings.
- modprobe / rmmod round-trip clean. No dmesg taints beyond
  the expected "out-of-tree taint" line.
- v4l2-ctl --list-devices shows: "daedalus-fourier V3D7+NEON
  (platform:daedalus_v4l2): /dev/video0"
- VIDIOC_QUERYCAP returns driver/card/bus/caps as specified.
- v4l2-compliance: 44/48 passing. The 4 failures are exactly
  the format/buffer ioctls Phase 8.2 will implement
  (ENUM_FMT, G_FMT, Scaling, REQBUFS) — not skeleton bugs,
  legitimately-absent features.

Documentation: docs/phase_8_1_closure.md captures full
verification output + Phase 8.2 plan.

Phase 8.1 acceptance criteria met:
- ✓ /dev/videoNN appears via v4l2-ctl --list-devices
- ✓ VIDIOC_QUERYCAP responds with sensible values
- ✓ rmmod is clean (no kref leaks)
- ✓ v4l2-compliance passes except for explicit Phase 8.2 work

Next: Phase 8.2 chardev bridge for kernel ↔ daemon IPC.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

2026-05-18 15:03:22 +00:00

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Phase 8.1 closure — kernel module skeleton

Status: closed 2026-05-18.

Out-of-tree Linux kernel module daedalus_v4l2 that registers a /dev/videoNN V4L2 device on a synthesised platform device. Phase 8.1's scope: skeleton only — no actual decoder ioctls, no buffer queue, no controls. Subsequent phases (8.2 chardev bridge, 8.3 daemon parse, 8.4 VP9 end-to-end, etc.) build on this base.

What lands

kernel/Makefile — out-of-tree kbuild stub. make against the running kernel via /lib/modules/$(uname -r)/build. Includes make checkpatch target for kernel coding-style verification.
kernel/daedalus_v4l2_main.c — ~190 lines. Real V4L2 plumbing: platform_device + v4l2_device + video_device. Implements VIDIOC_QUERYCAP; everything else falls through to v4l2-core defaults.

Verification

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

Build

$ cd ~/src/daedalus-v4l2/kernel && make
make -C /lib/modules/6.12.75+rpt-rpi-2712/build M=... modules
  CC [M]  daedalus_v4l2_main.o
  LD [M]  daedalus_v4l2.o
  MODPOST Module.symvers
  CC [M]  daedalus_v4l2.mod.o
  LD [M]  daedalus_v4l2.ko

Builds clean with -Wall -Wextra. No warnings.

Load + dmesg

$ sudo insmod daedalus_v4l2.ko
$ sudo dmesg | tail -2
daedalus_v4l2: loading out-of-tree module taints kernel.
daedalus_v4l2 daedalus_v4l2: daedalus-v4l2 registered as /dev/video0
  (Phase 8.1 skeleton)

v4l2-ctl --list-devices

daedalus-fourier V3D7+NEON (platform:daedalus_v4l2):
  /dev/video0

(Appears alongside the existing pispbe and rpi-hevc-dec devices.)

VIDIOC_QUERYCAP

$ sudo v4l2-ctl --device /dev/video0 --info
Driver Info:
  Driver name      : daedalus_v4l2
  Card type        : daedalus-fourier V3D7+NEON
  Bus info         : platform:daedalus_v4l2
  Driver version   : 6.12.75
  Capabilities     : 0x84204000
    Video Memory-to-Memory Multiplanar
    Streaming
    Extended Pix Format

v4l2-compliance

$ sudo v4l2-compliance --device /dev/video0
Total for daedalus_v4l2 device /dev/video0: 48, Succeeded: 44,
  Failed: 4, Warnings: 0

44/48 passing. The 4 failures are exactly what Phase 8.2 implements:

VIDIOC_ENUM_FMT/FRAMESIZES/FRAMEINTERVALS: no formats yet
VIDIOC_G_FMT: no format negotiated
Scaling: no output format negotiated (no g_fmt_vid_out_mplane)
VIDIOC_REQBUFS/CREATE_BUFS/QUERYBUF: no vb2_queue

These are all Phase 8.2 + 8.4 work and are intentionally absent from Phase 8.1.

Unload

$ sudo rmmod daedalus_v4l2

Clean unload; no leak in dmesg.

Coding-style note

Module written in kernel style (8-tab indent, static-by-default, SPDX header, kerneldoc on struct daedalus_dev). Builds clean with the kernel's -Wall -Wextra defaults. Per correctness-before-speed session memory.

What's next — Phase 8.2

Add a chardev bridge so a userspace daemon can talk to the kernel module over /dev/daedalus-v4l2. Protocol stub:

struct daedalus_req {
    u32  type;       /* DAEDALUS_REQ_DECODE, DAEDALUS_REQ_QUERY, ... */
    u32  stream_id;
    u32  frame_idx;
    u32  bitstream_len;
    /* followed by bitstream blob + control structs */
};

The kernel side becomes a thin marshal layer; all decoding work moves to the daemon.

After 8.2 lands, Phase 8.3 adds the daemon's FFmpeg parse path (dlopen at runtime, Option γ).

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