Merge "Actually add README contents for ueventd" am: 53620ca1 am: 000e1fc4

# 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.