Booting And Initial Device Setup; Monitoring The Running Udev Daemon - Novell LINUX ENTERPRISE SERVER 11 - ADMINISTRATION Administration Manual

calling modprobe for every event that carries a MODALIAS key. If modprobe
$MODALIAS is called, it matches the device alias composed for the device with the
aliases provided by the modules. If a matching entry is found, that module is loaded.
All this is automatically triggered by udev.

11.4 Booting and Initial Device Setup

All device events happening during the boot process before the udev daemon is running
are lost, because the infrastructure to handle these events resides on the root file system
and is not available at that time. To cover that loss, the kernel provides a uevent file
located in the device directory of every device in the sysfs file system. By writing add
to that file, the kernel resends the same event as the one lost during boot. A simple loop
over all uevent files in /sys triggers all events again to create the device nodes and
perform device setup.
As an example, a USB mouse present during boot may not be initialized by the early
boot logic, because the driver is not available at that time. The event for the device
discovery was lost and failed to find a kernel module for the device. Instead of manually
searching for possibly connected devices, udev just requests all device events from the
kernel after the root file system is available, so the event for the USB mouse device
just runs again. Now it finds the kernel module on the mounted root file system and the
USB mouse can be initialized.
From userspace, there is no visible difference between a device coldplug sequence and
a device discovery during runtime. In both cases, the same rules are used to match and
the same configured programs are run.
11.5 Monitoring the Running udev
Daemon
The program udevadm monitor can be used to visualize the driver core events and
the timing of the udev event processes.
UEVENT[1185238505.276660] add
(usb)
UDEV [1185238505.279198] add
(usb)
/devices/pci0000:00/0000:00:1d.2/usb3/3-1
/devices/pci0000:00/0000:00:1d.2/usb3/3-1
Dynamic Kernel Device Management with udev 133