Booting And Initial Device Setup; Debugging Udev Events - Novell LINUX ENTERPRISE DESKTOP 10 SP2 - DEPLOYMENT GUIDE 08-05-2008 Deployment Manual

aliases provided by the modules. If a matching entry is found, that module is loaded.
All this is triggered by udev and happens automatically.

21.4 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 lives on the root file system
and is not available at that time. To cover that loss, the kernel provides a uevent file
for 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 that time. The event for the device discov-
ery 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.

21.5 Debugging udev Events

The program udevmonitor can be used to visualize the driver core events and the
timing of the udev event processes.
UEVENT[1132632714.285362] add@/devices/pci0000:00/0000:00:1d.1/usb2/2-2
UEVENT[1132632714.288166] add@/devices/pci0000:00/0000:00:1d.1/usb2/2-2/2-2:1.0
UEVENT[1132632714.309485] add@/class/input/input6
UEVENT[1132632714.309511] add@/class/input/input6/mouse2
UEVENT[1132632714.309524] add@/class/usb_device/usbdev2.12
UDEV [1132632714.348966] add@/devices/pci0000:00/0000:00:1d.1/usb2/2-2
UDEV [1132632714.420947] add@/devices/pci0000:00/0000:00:1d.1/usb2/2-2/2-2:1.0
UDEV [1132632714.427298] add@/class/input/input6
UDEV [1132632714.434223] add@/class/usb_device/usbdev2.12
UDEV [1132632714.439934] add@/class/input/input6/mouse2
Dynamic Kernel Device Management with udev 465