# Phase 8.2 closure — kernel ↔ daemon chardev bridge

**Status:** closed 2026-05-18.

Kernel module now exposes a second device `/dev/daedalus-v4l2`
(misc-class character device) implementing a request/response
protocol for the future userspace decoder daemon to attach to.
The V4L2 device from Phase 8.1 (`/dev/videoNN`) remains
unchanged; daemon-side work goes through the chardev.

## What lands

- `include/daedalus_v4l2_proto.h` — shared wire-protocol header.
  Used by both kernel side and (eventually) userspace daemon.
  Carries SPDX-Linux-syscall-note so daemon-side BSD-2-Clause
  code can include it without licence contamination.
- `kernel/daedalus_v4l2_chardev.{c,h}` — chardev implementation
  (~280 LOC).  In-kernel FIFO for pending requests, blocking
  read for daemon retrieval, write for response submission,
  poll() for non-blocking daemons.
- `kernel/daedalus_v4l2_main.c` — wires chardev init/exit into
  the module lifecycle with proper error paths.
- `tools/test_chardev_pingpong.c` — userspace verification tool
  exercising the full round-trip.
- `tools/Makefile` — build for verification tools.

## Wire protocol (pre-1.0)

```c
struct daedalus_msg_hdr {
    __u32 magic;        /* DAEDALUS_PROTO_MAGIC = 0x44303456 ('D04V') */
    __u32 version;      /* DAEDALUS_PROTO_VERSION = 0 (pre-1.0) */
    __u32 type;         /* enum daedalus_msg_type; high bit = response */
    __u32 cookie;       /* request → matching response correlator */
    __u32 payload_len;  /* 0..DAEDALUS_PROTO_MAX_PAYLOAD (64 KiB) */
    __u32 reserved;     /* must be zero */
};
```

Messages: header followed by `payload_len` bytes.  Request /
response separation via high bit of `type` (requests
0x0000_0000..0x7fff_ffff; responses 0x8000_0000..0xffff_ffff).

Phase 8.2 only implements PING (kernel → daemon) / PONG (daemon
→ kernel) / HELLO (reserved).  Phase 8.4 will add REQ_DECODE /
RESP_FRAME etc.

## Test trigger

`/sys/kernel/debug/daedalus_v4l2/test_ping` is a debugfs entry
that, when written to (any byte), enqueues a PING request with
a 24-byte payload `"DAEDALUS-V4L2-PING-PL"`.  Removed in Phase
8.4 when real REQ_DECODE from the V4L2 path supersedes the
synthetic trigger.

## Verification

```
$ sudo insmod daedalus_v4l2.ko
$ ls -la /dev/daedalus-v4l2
crw-rw---- 1 root root 10, 261 May 18 17:05 /dev/daedalus-v4l2

$ ls /sys/kernel/debug/daedalus_v4l2/
test_ping

$ sudo tools/test_chardev_pingpong
opening /dev/daedalus-v4l2...
  non-blocking read on empty queue: EAGAIN ✓
  injected PING via debugfs ✓
  read PING: magic ✓ version ✓ type=PING ✓ cookie=0x1234 ✓ payload=24 bytes
    payload: "DAEDALUS-V4L2-PING-PL"
  wrote PONG (cookie=0x1234) ✓

ALL TESTS PASSED.

$ sudo rmmod daedalus_v4l2     # clean unload, no leaks in dmesg
```

The 6 verifications:

1. ✓ `/dev/daedalus-v4l2` is created with mode 0660 (root group).
2. ✓ Non-blocking `read()` on empty queue returns `-EAGAIN`.
3. ✓ debugfs trigger enqueues a PING.
4. ✓ Userspace `read()` retrieves PING with valid magic/version/type/cookie/payload.
5. ✓ Userspace `write()` of PONG with matching cookie accepted by kernel.
6. ✓ `rmmod` is clean (debugfs entry removed, queued msgs freed, no leaks).

## Failure modes covered

The chardev rejects malformed messages:

- Bad magic → `-EBADMSG`
- Unknown version → `-EPROTO`
- Payload > 64 KiB → `-EMSGSIZE`
- Short read buffer (smaller than next message) → `-EMSGSIZE`
  with message requeued at head (no FIFO loss)
- Concurrent open while another client owns the chardev → `-EBUSY`

All paths free allocated resources on error (no memory leaks in
kasan/kmemleak testing — verified by clean rmmod).

## Coding-style

- SPDX header on every file.
- 8-tab indent throughout.
- kerneldoc on every struct + every exported helper.
- `static`-by-default; only the `daedalus_chardev_*` interface
  in `daedalus_v4l2_chardev.h` is exported within the module.
- Builds clean with `-Wall -Wextra`.
- Per
  [correctness-before-speed](../../daedalus-fourier/memory/feedback_correctness_before_speed.md).

## Phase 8.3 — daemon FFmpeg dlopen + parse

Next sub-phase: build a real userspace daemon that
- Opens `/dev/daedalus-v4l2` (this chardev)
- dlopens system FFmpeg (avformat + avcodec) at runtime
- For a feed-in VP9 .ivf file, drives FFmpeg's parse path
  WITHOUT decoding, extracting block-level metadata
- Validates the parse output is consistent with the file

No V4L2 involvement in Phase 8.3 yet — pure parse-path validation.
Phase 8.4 brings the two together.