Ueventd manages /dev
, sets permissions for /sys
, and handles firmware uevents. It has default behavior described below, along with a scripting language that allows customizing this behavior, built on the same parser as init.
Ueventd has one generic customization parameter, the size of rcvbuf_size for the ueventd socket. It is customized by the uevent_socket_rcvbuf_size
parameter, which takes the format of
uevent_socket_rcvbuf_size <size>
For example
uevent_socket_rcvbuf_size 16M
Sets the uevent socket rcvbuf_size to 16 megabytes.
Ueventd listens to the kernel uevent sockets and creates/deletes nodes in /dev
based on the incoming add/remove uevents. It defaults to using 0600
mode and root
user/group. It always creates the nodes with the SELabel from the current loaded SEPolicy. It has three default behaviors for the node path:
/dev/block/<basename uevent DEVPATH>
. There are symlinks created to this node at /dev/block/<type>/<parent device>/<basename uevent DEVPATH>
, /dev/block/<type>/<parent device>/by-name/<uevent PARTNAME>
, and /dev/block/by-name/<uevent PARTNAME>
if the device is a boot device./dev/<uevent DEVNAME>
if DEVNAME
was specified for the uevent, otherwise as /dev/bus/usb/<bus_id>/<device_id>
where bus_id
is uevent MINOR / 128 + 1
and device_id
is uevent MINOR % 128 + 1
./dev/<basename uevent DEVPATH>
The permissions can be modified using a ueventd.rc script and a line that beings with /dev
. These lines take the format of
devname mode uid gid
For example
/dev/null 0666 root root
When /dev/null
is created, its mode will be set to 0666
, its user to root
and its group to root
.
The path can be modified using a ueventd.rc script and a subsystem
section. There are three to set for a subsystem: the subsystem name, which device name to use, and which directory to place the device in. The section takes the below format of
subsystem <subsystem_name> devname uevent_devname|uevent_devpath [dirname <directory>]
subsystem_name
is used to match uevent SUBSYSTEM
value
devname
takes one of two options
uevent_devname
specifies that the name of the node will be the uevent DEVNAME
uevent_devpath
specified that the name of the node will be basename uevent DEVPATH
dirname
is an optional parameter that specifies a directory within /dev
where the node will be created.
For example
subsystem sound devname uevent_devpath dirname /dev/snd
Indicates that all uevents with SUBSYSTEM=sound
will create nodes as /dev/snd/<basename uevent DEVPATH>
.
Ueventd by default takes no action for /sys
, however it can be instructed to set permissions for certain files in /sys
when matching uevents are generated. This is done using a ueventd.rc script and a line that begins with /sys
. These lines take the format of
nodename attr mode uid gid
For example
/sys/devices/system/cpu/cpu* cpufreq/scaling_max_freq 0664 system system
When a uevent that matches the pattern /sys/devices/system/cpu/cpu*
is sent, the matching sysfs attribute, cpufreq/scaling_max_freq
, will have its mode set to 0664
, its user to to system
and its group set to system
.
Note that *
matches as a wildcard and can be used anywhere in a path.
Ueventd automatically serves firmware requests by searching through a list of firmware directories for a file matching the uevent FIRMWARE
. It then forks a process to serve this firmware to the kernel.
The list of firmware directories is customized by a firmware_directories
line in a ueventd.rc file. This line takes the format of
firmware_directories <firmware_directory> [ <firmware_directory> ]*
For example
firmware_directories /etc/firmware/ /odm/firmware/ /vendor/firmware/ /firmware/image/
Adds those 4 directories, in that order to the list of firmware directories that will be tried by ueventd. Note that this option always accumulates to the list; it is not possible to remove previous entries.
Ueventd will wait until after post-fs
in init, to keep retrying before believing the firmwares are not present.
Ueventd must create devices in /dev
for all devices that have already sent their uevents before ueventd has started. To do so, when ueventd is started it does what it calls a ‘coldboot’ on /sys
, in which it writes ‘add’ to every ‘uevent’ file that it finds in /sys/class
, /sys/block
, and /sys/devices
. This causes the kernel to regenerate the uevents for these paths, and thus for ueventd to create the nodes.
For boot time purposes, this is done in parallel across a set of child processes. ueventd.cpp
in this directory contains documentation on how the parallelization is done.