Merge "Actually add README contents for ueventd"
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Change-Id: Ie3a2f1cda99b483e67932b261ec6bc21db6176dd
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Tom Cherry
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# Ueventd
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-------
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Ueventd manages `/dev`, sets permissions for `/sys`, and handles firmware uevents. It has default
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behavior described below, along with a scripting language that allows customizing this behavior,
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built on the same parser as init.
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Ueventd has one generic customization parameter, the size of rcvbuf_size for the ueventd socket. It
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is customized by the `uevent_socket_rcvbuf_size` parameter, which takes the format of
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uevent_socket_rcvbuf_size <size>
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For example
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uevent_socket_rcvbuf_size 16M
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Sets the uevent socket rcvbuf_size to 16 megabytes.
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## /dev
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----
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Ueventd listens to the kernel uevent sockets and creates/deletes nodes in `/dev` based on the
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incoming add/remove uevents. It defaults to using `0600` mode and `root` user/group. It always
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creates the nodes with the SELabel from the current loaded SEPolicy. It has three default behaviors
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for the node path:
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1. Block devices are created as `/dev/block/<basename uevent DEVPATH>`. There are symlinks created
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to this node at `/dev/block/<type>/<parent device>/<basename uevent DEVPATH>`,
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`/dev/block/<type>/<parent device>/by-name/<uevent PARTNAME>`, and `/dev/block/by-name/<uevent
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PARTNAME>` if the device is a boot device.
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2. USB devices are created as `/dev/<uevent DEVNAME>` if `DEVNAME` was specified for the uevent,
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otherwise as `/dev/bus/usb/<bus_id>/<device_id>` where `bus_id` is `uevent MINOR / 128 + 1` and
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`device_id` is `uevent MINOR % 128 + 1`.
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3. All other devices are created as `/dev/<basename uevent DEVPATH>`
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The permissions can be modified using a ueventd.rc script and a line that beings with `/dev`. These
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lines take the format of
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devname mode uid gid
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For example
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/dev/null 0666 root root
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When `/dev/null` is created, its mode will be set to `0666`, its user to `root` and its group to
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`root`.
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The path can be modified using a ueventd.rc script and a `subsystem` section. There are three to set
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for a subsystem: the subsystem name, which device name to use, and which directory to place the
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device in. The section takes the below format of
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subsystem <subsystem_name>
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devname uevent_devname|uevent_devpath
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[dirname <directory>]
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`subsystem_name` is used to match uevent `SUBSYSTEM` value
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`devname` takes one of two options
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1. `uevent_devname` specifies that the name of the node will be the uevent `DEVNAME`
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2. `uevent_devpath` specified that the name of the node will be basename uevent `DEVPATH`
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`dirname` is an optional parameter that specifies a directory within `/dev` where the node will be
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created.
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For example
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subsystem sound
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devname uevent_devpath
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dirname /dev/snd
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Indicates that all uevents with `SUBSYSTEM=sound` will create nodes as `/dev/snd/<basename uevent
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DEVPATH>`.
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## /sys
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----
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Ueventd by default takes no action for `/sys`, however it can be instructed to set permissions for
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certain files in `/sys` when matching uevents are generated. This is done using a ueventd.rc script
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and a line that begins with `/sys`. These lines take the format of
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nodename attr mode uid gid
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For example
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/sys/devices/system/cpu/cpu* cpufreq/scaling_max_freq 0664 system system
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When a uevent that matches the pattern `/sys/devices/system/cpu/cpu*` is sent, the matching sysfs
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attribute, `cpufreq/scaling_max_freq`, will have its mode set to `0664`, its user to to `system` and
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its group set to `system`.
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Note that `*` matches as a wildcard and can be used anywhere in a path.
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## Firmware loading
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----------------
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Ueventd automatically serves firmware requests by searching through a list of firmware directories
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for a file matching the uevent `FIRMWARE`. It then forks a process to serve this firmware to the
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kernel.
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The list of firmware directories is customized by a `firmware_directories` line in a ueventd.rc
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file. This line takes the format of
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firmware_directories <firmware_directory> [ <firmware_directory> ]*
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For example
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firmware_directories /etc/firmware/ /odm/firmware/ /vendor/firmware/ /firmware/image/
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Adds those 4 directories, in that order to the list of firmware directories that will be tried by
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ueventd. Note that this option always accumulates to the list; it is not possible to remove previous
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entries.
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Ueventd will wait until after `post-fs` in init, to keep retrying before believing the firmwares are
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not present.
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## Coldboot
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--------
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Ueventd must create devices in `/dev` for all devices that have already sent their uevents before
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ueventd has started. To do so, when ueventd is started it does what it calls a 'coldboot' on `/sys`,
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in which it writes 'add' to every 'uevent' file that it finds in `/sys/class`, `/sys/block`, and
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`/sys/devices`. This causes the kernel to regenerate the uevents for these paths, and thus for
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ueventd to create the nodes.
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For boot time purposes, this is done in parallel across a set of child processes. `ueventd.cpp` in
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this directory contains documentation on how the parallelization is done.
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