# udev

Alpine's `eudev` runs on SchOS as an ordinary process, started by `schinit`
(`etc/schinit/udevd.job`, with `udev-trigger.job` replaying the devices that
were there first). It is what lets Xorg *find* its keyboard and mouse instead of
being told, and the road to a Wayland compositor starts here: libinput and
libseat both ask udev what exists.

The kernel's half is three pieces, all small, none of them specific to udev.

## The device table: `kernel/dev/kdev.c`

The devices the kernel implements itself — input event nodes, the memory
devices, terminals, the framebuffer, the disks — are listed once, with their
class, name, `/dev` name and parent. Everything below is generated from it.

The device numbers are the ones `stat()` reports for the node, not SchOS's
internal ones: `/dev/tty1` is `4:1` to a Linux program (`linux_tty_rdev` in
`kernel/sys/sys_fs.c`), and udev compares what it reads in `/sys` with what
`stat()` says about the node. `dev_stat_numbers()` is the one mapping both use.

## `/sys`: `kernel/fs/sysfs.c`

```
/sys/devices/virtual/input/input0/            name capabilities/{ev,key,rel,abs} properties
/sys/devices/virtual/input/input0/event0/     dev  uevent  subsystem -> ../../../../../class/input
/sys/class/input/event0                       -> ../../devices/virtual/input/input0/event0
/sys/dev/char/13:64 ...                       -> ../../devices/virtual/...
```

Symlinks are real (`T_SYMLINK` inodes whose target is generated). `uevent` is
readable (`DEVPATH=`, `SUBSYSTEM=`, `MAJOR=`, `MINOR=`, `DEVNAME=`) and
writable: `echo add > uevent` — which is what `udevadm trigger` does — makes the
kernel announce the device again. A write to a PCI or USB device's `uevent` is
accepted and says nothing yet, because those devices are not in the
`/sys/devices` tree and an event's `DEVPATH` has to name something that exists.

The input capability files are what `input_id` reads to decide
`ID_INPUT_KEYBOARD` or `ID_INPUT_MOUSE`. SchOS has two input devices whatever
the machine — every keyboard on `event0`, every mouse on `event1`
(`kernel/dev/input.c`) — so each has a fixed, plausible capability set.

## `/dev`: `kernel/fs/devfs.c`

devtmpfs is no longer read-only: udev creates `/dev/char/MAJ:MIN`, `/dev/fd`,
`/dev/input/by-*` and the like. A small in-memory overlay (192 entries) holds
directories and symlinks, listed after the kernel's own nodes in whichever
directory they are in. Device nodes themselves are still the kernel's: `mknod`
and regular files in `/dev` are refused.

## The sockets: `kernel/net/unix.c`

- **`AF_UNIX` `SOCK_DGRAM` and `SOCK_SEQPACKET`** keep message boundaries.
  udevd's control socket is `SEQPACKET`, and its manager and workers talk over
  a `DGRAM` socketpair. A message carries its sender's credentials with it
  (several workers share one end), a zero-length datagram is a datagram, and a
  datagram socket never reads end-of-stream.
- **`AF_NETLINK` / `NETLINK_KOBJECT_UEVENT`** sockets bind to multicast groups:
  group 1 is the kernel's events, group 2 is what udevd re-announces once its
  rules have run. A datagram can also be addressed to a port id — udevd hands
  an event to an idle worker that way, and the worker believes it only from the
  manager's port. The kernel's messages arrive as credentials with uid 0, pid 0.
  A queue that is full drops the message, as Linux does, and `udevadm trigger`
  is how a reader recovers.

## What is not there

- PCI and USB devices in `/sys/devices`, and `add`/`remove` events from the
  hotplug thread. A USB keyboard works because every keyboard is `event0`, not
  because udev knew it arrived.
- `hwdb` (`/etc/udev/hwdb.bin`): `udevadm hwdb --update` has not been run in the
  image, so `60-input-id` logs "returned non-zero" for the hwdb import. The
  properties that matter come from `input_id`.
- Block device partitions, `by-id` / `by-uuid` links, and a `/sys/block`.
- `libinput` and `libseat` (next; see [../plan/roadmap.md](../plan/roadmap.md)).

## Testing

Boot the ISO and run `udevd --debug &`, then `udevadm trigger; udevadm settle`.
`/run/udev/data/c7:0` should contain
`ID_INPUT_KEYBOARD=1`, and `udevadm info /dev/input/event0` should print the
device. Under X, `/var/log/Xorg.0.log` says `config/udev: Adding input device`.