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finit/doc/keventd.md
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Joachim Wiberg 5a3972714a keventd: defer pidfile until coldplug queue is drained 
When started with -c (the default when keventd is the device manager),
gate the pidfile on the kernel's uevent_seqnum having been stable for
200ms.  Up to now ready signaling with the pidfile was done right after
coldplug() triggered the kernel to re-emit events, but before uev_run()
had drained any of them, so <pid/keventd> really only meant "listening
on netlink".

With the gate, services that depend on <pid/keventd> can now assume /dev
is populated and persistent symlinks are live.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>
2026-08-16 22:03:34 +02:00

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Bundled Device Manager
======================
The kernel event daemon `keventd` is a built-in device manager bundled
with Finit. It replaces the need for external device managers like
mdev, mdevd, or udevd on systems where a lighter-weight solution is
preferred, particularly on embedded systems.
It is enabled by default since Finit v5, and needs libblkid from
util-linux (the `libblkid-dev` package, or whatever your distribution
calls it) to read the filesystem UUID and label behind the
`/dev/disk/by-uuid/` and `/dev/disk/by-label/` symlinks. To disable it
and use an external device manager instead:
./configure --without-keventd
Features
--------
When started, keventd listens on a `NETLINK_KOBJECT_UEVENT` socket for
kernel events and handles:
- **Device node creation**: creates and removes `/dev` nodes with
correct permissions on device add/remove events
- **udev rules**: matches events against rules read from
`/lib/udev/rules.d/`, `/run/udev/rules.d/`, and `/etc/udev/rules.d/`,
with a curated ruleset derived from eudev installed by default
- **Persistent symlinks**: creates `by-id` and `by-path` symlinks under
`/dev/disk/`, `/dev/input/`, and elsewhere, for stable device naming
- **Firmware loading**: responds to kernel firmware requests by searching
`/lib/firmware/` and writing firmware data to sysfs
- **Module loading**: parses `MODALIAS` from uevents and spawns `modprobe`
to load the appropriate kernel module
- **Coldplug**: with the `-c` flag, walks `/sys/devices` and triggers
add events for all devices present at boot
- **Power supply monitoring**: tracks AC power status and provides the
`sys/pwr/ac` condition
- **Device conditions**: sets `dev/*` conditions in the Finit condition
system when device nodes appear or disappear
- **Netlink rebroadcast**: rebroadcasts processed uevents to netlink
group 0x4 for [libudev-zero][] consumers (enabled by default)
Device Nodes
------------
On receiving an `add` event with `MAJOR`, `MINOR`, and `DEVNAME`
fields, keventd creates the corresponding device node in `/dev` using
`mknod()`. Parent directories are created automatically (e.g.,
`/dev/input/` for `/dev/input/event0`).
On `remove` events, the device node and its associated symlinks and
conditions are cleaned up.
### Default Permissions
keventd applies permissions based on built-in rules that match on
device subsystem and name:
| **Subsystem** | **Pattern** | **Mode** | **Owner:Group** |
|---------------|--------------------------------------------|----------|-----------------|
| block | sd*, vd*, nvme*, mmcblk*, loop*, dm-*, md* | 0660 | root:disk |
| tty | tty[0-9]* | 0620 | root:tty |
| tty | ttyS*, ttyUSB*, ttyACM* | 0660 | root:dialout |
| input | event*, mouse*, mice | 0660 | root:root |
| sound | * | 0660 | root:audio |
| video4linux | * | 0660 | root:video |
| drm | card*, render* | 0660 | root:video |
| (any) | null, zero, full, random, urandom | 0666 | root:root |
| (any) | console | 0600 | root:root |
| (default) | | 0660 | root:root |
Persistent Symlinks
-------------------
Symlinks come from two places. A handful are built into keventd and are
created for every device in the subsystem, whatever rules are loaded.
For block devices, under `/dev/disk/`:
- **by-id**: based on the device serial number and model, read from
sysfs attributes (`/sys/.../device/vendor`, `model`, `serial`)
- **by-path**: based on the device topology path
For input devices, under `/dev/input/`:
- **by-id**: based on the device name from sysfs
- **by-path**: based on the physical device path
The rest come from `SYMLINK+=` in the udev rules, so what you get
depends on the ruleset installed. The curated rules Finit ships add,
among others:
| **Directory** | **Links** |
|----------------|--------------------------------------------------------|
| `/dev/disk/` | `by-uuid`, `by-label`, `by-partuuid`, `by-partlabel`, `by-diskseq`, and further `by-id` and `by-path` names for NVMe, virtio, MMC, WWN, and FireWire |
| `/dev/input/` | longer `by-id` and `by-path` names than the built-in ones, down to the USB interface number |
| `/dev/serial/` | `by-id`, `by-path` |
| `/dev/snd/` | `by-id`, `by-path` |
| `/dev/v4l/` | `by-id`, `by-path` |
| `/dev/tape/` | `by-id`, `by-path` |
| `/dev/dri/` | `by-path` |
They also set up a few fixed names: `/dev/rtc`, `/dev/cdrom`, and
`/dev/virtio-ports/<name>`.
The `by-uuid` and `by-label` links are why keventd needs libblkid. The
rules read the filesystem metadata off the device with
`IMPORT{builtin}="blkid"`, and libblkid is what that builtin calls.
Built-in and rule-provided symlinks alike are tracked internally and
removed when the corresponding device is unplugged.
Firmware Loading
----------------
When a kernel driver requests firmware (via `request_firmware()`), the
kernel sends a uevent with a `FIRMWARE=` field. keventd handles this
by:
1. Searching for the firmware file in order:
- `/lib/firmware/updates/<kernel-version>/<name>`
- `/lib/firmware/updates/<name>`
- `/lib/firmware/<kernel-version>/<name>`
- `/lib/firmware/<name>`
2. Writing `1` to `/sys/<devpath>/loading` to signal start
3. Copying the firmware data to `/sys/<devpath>/data`
4. Writing `0` to `/sys/<devpath>/loading` on success (or `-1` on failure)
This is particularly important early in boot when drivers for graphics
cards, network adapters, and other hardware need firmware before they
can operate.
Module Loading
--------------
When a device add event includes a `MODALIAS` field, keventd spawns
`modprobe -bq <modalias>` to load the matching kernel module. Module
loading is done asynchronously (keventd does not wait for modprobe to
complete) to avoid blocking other event processing.
Coldplug
--------
To handle devices that were present before keventd started, it supports
a coldplug mode activated with the `-c` flag. This walks the entire
`/sys/devices` tree and writes `add` to each `uevent` file, causing the
kernel to re-emit add events for all existing devices.
This replaces the separate `coldplug` script previously used with mdev.
When `-c` is used, keventd defers its pidfile (and the `pid/keventd`
condition Finit asserts from it) until the coldplug event queue has
been fully drained. Services that depend on `<pid/keventd>` can
therefore assume `/dev` is populated and persistent symlinks are live,
without needing a separate `settle` step.
Netlink Rebroadcast
-------------------
The Linux kernel sends uevents to netlink multicast group 1 (bit 0) of
`NETLINK_KOBJECT_UEVENT`. Only the device manager listens on this raw
kernel group. Userspace consumers — applications using libudev —
expect to receive processed events on a separate netlink group.
systemd/udevd established the convention of rebroadcasting processed
events to a separate group, and [libudev-zero][], a daemonless drop-in
replacement for libudev, listens on group `0x4` for these events.
Without a device manager rebroadcasting, graphical applications,
Wayland/X11 compositors, libinput, and anything else using libudev to
monitor device hotplug will never see any events.
keventd rebroadcasts by default to netlink group 4 (`0x4`). A second
netlink socket is created at startup, and after each uevent has been
fully processed (device nodes created, symlinks set up, modules loaded),
the original raw event is sent to the configured group(s).
Rebroadcasting after processing ensures that device nodes and symlinks
already exist by the time consumers receive the notification.
Use `-g GROUP` to override the default group mask, or `-G` to disable
rebroadcast entirely. Bit 0 of the group mask is always forced off to
prevent a feedback loop with the kernel's own multicast group.
### Background
The netlink uevent architecture uses separate multicast groups to
isolate the kernel-to-device-manager channel from the device-manager-to-
application channel:
| **Group** | **Bit** | **Purpose** |
|-----------|---------|---------------------------------------------|
| 1 | 0 | Kernel events (device manager listens here) |
| 4 | 2 | Processed events (libudev consumers listen) |
This two-group design was established by systemd/udevd and is the de
facto standard. [mdevd][] implements the same mechanism via its `-O`
flag, and keventd follows the same convention.
For more details, see:
- [libudev-zero][] — daemonless replacement for libudev
- [mdevd][] — mdev-compatible device manager with rebroadcast support
Conditions
----------
keventd provides conditions in two namespaces:
### Device Conditions (`dev/`)
When a device node is created, keventd asserts a corresponding condition
in `/run/finit/cond/dev/`. This allows services to wait for specific
devices:
service mdadm {
description = "RAID monitor"
runlevel = "2345"
conditions = { "dev/sda" }
command = "/usr/sbin/mdadm --monitor /dev/md0"
}
service gps-daemon {
description = "GPS daemon"
runlevel = "2345"
conditions = { "dev/ttyUSB0" }
command = "/usr/sbin/gps-daemon"
}
Network interfaces are not device nodes and do not live in the `dev/`
namespace -- a `/dev/wan` node created by a user must not be confused
with a `wan` interface. To wait for an interface, use the `netlink`
plugin's `net/<iface>/exist` condition, plus `net/<iface>/up` (admin
up) and `net/<iface>/running` (carrier present) to gate on link state:
service dhcpcd {
description = "DHCP client"
runlevel = "2345"
conditions = { "net/wan/exist" }
command = "/usr/sbin/dhcpcd"
}
keventd provides the parallel `class/net/<iface>` condition, like for
any other sysfs class device.
When the device is removed, the condition is cleared and Finit stops
the dependent services.
### Class Conditions (`class/`)
Many devices live in sysfs without a `/dev/` node -- DSA switch ports,
IIO sensors, LEDs, backlight, PHYs, regulators. For those, keventd
asserts `class/<subsystem>/<sysname>` on every `add` event so services
can still wait for them:
service blink-blue {
description = "LED driver"
runlevel = "2345"
conditions = { "class/leds/blue" }
command = "/usr/sbin/blink-blue"
}
The condition is cleared on `remove`.
### Driver Conditions (`driver/`)
`driver/<name>` is asserted while the driver `<name>` is bound to at
least one device, from the kernel's `bind`/`unbind` uevents. Use this
to gate on slow-probing hardware whose readiness isn't marked by a
class device or `/dev` node, such as a switch core or complex PHY:
service dsa-probe {
description = "DSA topology probe"
runlevel = "2345"
conditions = { "driver/mt7530" }
command = "/usr/sbin/dsa-probe"
}
A driver bound to several devices keeps the condition asserted until
the last device is unbound. To wait for one specific device instance,
gate on what its probe creates instead: the `class/` condition or
`/dev` node of the child device.
### Power Supply Conditions (`sys/pwr/`)
keventd monitors the `power_supply` subsystem and provides:
- `sys/pwr/ac` — asserted when AC power is connected
This is useful for preventing power-hungry services from running on
battery:
service cron {
description = "Cron daemon"
runlevel = "2345"
conditions = { "sys/pwr/ac", "pid/syslogd" }
command = "cron -f"
}
Usage
-----
keventd [-cdGhnpSv] [-g GROUP] [-r DIR] [-t SECONDS]
Options:
-c Run coldplug at startup
-d Enable debug mode (foreground, verbose)
-g GROUP Override netlink rebroadcast group (default: 4)
-G Disable netlink rebroadcast entirely
-h Show help text
-n Run in foreground (no daemon)
-p Passive mode: power supply events only
-r DIR Extra rules directory
-S Settle mode: wait for kernel uevent queue to quiet, then exit
-t SEC Settle timeout (default 30s, used with -S)
-v Show version
In normal operation, Finit starts keventd automatically via its system
configuration. The `-d` flag is useful for debugging device issues --
it runs keventd in the foreground and logs all received uevents.
`keventd -S` is a one-shot command, not a flag to the running daemon.
It is the `udevadm settle` equivalent for migration scenarios:
keventd -S -t 10 && start-graphical-session
It does not talk to the running keventd (or any device manager) -- it
simply opens `/sys/kernel/uevent_seqnum` and polls until the kernel's
sequence counter has been stable for 200ms, then exits zero. After
the `-t SECONDS` timeout (default 30) it exits non-zero instead.
Because `uevent_seqnum` is maintained by the kernel itself, `-S` works
regardless of which device manager is active, or even if none is.
Prefer the condition-based model (`<dev/X>`, `<class/...>`,
`<driver/...>`) over settle when you control the service definition --
settle is racy with slow probes that fire after the queue appears
quiet. It is provided for legacy boot scripts and init transitions
where condition wiring isn't feasible.
Debug logging can also be toggled at runtime by sending `SIGUSR1`:
kill -USR1 $(pidof keventd)
Integration with Finit
----------------------
keventd is a standalone daemon started by Finit as an internal service.
It communicates with Finit exclusively through the filesystem-based
condition system — creating and removing symlinks in `/run/finit/cond/`.
This means keventd can also be tested independently:
# Run in debug mode to see all kernel events
keventd -d
# Run with coldplug to populate /dev from scratch
keventd -c -n
# Run without rebroadcast (e.g., headless embedded system)
keventd -c -G
Only one device manager should be active at a time. This is settled
at build time: with the hotplug plugin enabled, Finit starts keventd
in passive mode (`-p`), leaving device management to the udevd, mdevd,
or mdev service from `system/10-hotplug.conf` and monitoring only
power supply events. Without the hotplug plugin, keventd runs as the
system device manager (`keventd -c`).
[libudev-zero]: https://github.com/illiliti/libudev-zero
[mdevd]: https://skarnet.org/software/mdevd/
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