# 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](../../daedalus-fourier/memory/feedback_correctness_before_speed.md)
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:

```c
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).