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platform_system_core/init/README.ueventd.md
Tom Cherry aefb141c67 Actually add README contents for ueventd 
A long time coming

Test: n/a
Change-Id: I6050e74d7497d5a7760f615fe2ad7d5c78a66ab9
2019-08-12 13:13:57 -07:00

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Ueventd

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.

/dev

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:

  1. Block devices are created as /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.
  2. USB devices are created as /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.
  3. All other devices are created as /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

  1. uevent_devname specifies that the name of the node will be the uevent DEVNAME
  2. 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>.

/sys

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.

Firmware loading

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.

Coldboot

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.