[LFS Trac] #1785: Udev Rules need verified for accuracy, completeness, and bloat

LFS Trac trac at linuxfromscratch.org
Tue May 9 15:07:18 PDT 2006

#1785: Udev Rules need verified for accuracy, completeness, and bloat
 Reporter:  archaic at linuxfromscratch.org  |        Owner:  lfs-book at linuxfromscratch.org
     Type:  task                          |       Status:  new                          
 Priority:  high                          |    Milestone:  6.2                          
Component:  Book                          |      Version:  SVN                          
 Severity:  normal                        |   Resolution:                               
 Keywords:                                |  
Comment (by archaic at linuxfromscratch.org):

 Okay, here's round one. Please help correct this for accuracy, grammar,
 readability. I don't know how pedantic and technical I should be for the

 All discussion of devices below refers to devices which can be properly
 exported by the kernel via sysfs (as discussed on xref="the page that

 Uevent: An event generated by the kernel whenever a device is added or
 In this context, added doesn't necessarily mean the device was just
 plugged in.
 It means the kernel now recognizes it. All devices are "added" when the
 is first booting up.

 ACTION=="add", DEVPATH=="/devices/*", ENV{PHYSDEVBUS}=="?*",

 The files, directories, and symlinks in /sys do not all appear at once.
 appear as the kernel identifies and enumerates them. However, the uevent
 be generated as soon as the device is known to the kernel. For some
 devices, it
 may take the kernel an extra few milliseconds to export all the
 information to
 /sys/devices while Udev may already be processing the uevent. To avoid
 conditions, this rule matches those devices (which have an ACTION of add,
 DEVPATH of /devices/*, and a non-empty environment variable named
 and causes Udev to wait until the kernel has created the needed "bus" file
 this case it will be a symlink) for that particular device before it
 to match any other rule. This gives the kernel time to enumerate the
 devices and export them via sysfs.

 ACTION=="add", SUBSYSTEM=="scsi", WAIT_FOR_SYSFS="ioerr_cnt"

 Very similar to the previous rule, any uevent for devices handled by the
 kernel's SCSI subsystem where the ACTION to take is "add" the device, Udev
 wait for the existence of the file /sys/bus/scsi/devices/<device


 The majority of rules exist to create device nodes. In the above example
 uevent generated by the kernel with the name "mice" will cause a device
 node to
 be created at /dev/mice with a default ownership of root:root and default
 permissions of 0660. To change the permissions or ownership, one would
 the above rule as such:

 KERNEL=="mice", MODE="0644", GROUP="some group"

 Further additions to the rule allow modifying the location where the
 device node
 is created. For example, /dev/mice is not the standard location for the
 device node, /dev/input/mice is. We can acheive this with:

 KERNEL=="mice", MODE="0644", GROUP="some group", NAME="input/%k"

 Finally, if this device commonly has a symlink associated with it -- for
 example, /dev/mouse often is a symlink to /dev/input/mice -- you can
 this with the following:

 KERNEL=="mice", MODE="0644", GROUP="some group", NAME="input/%k",

 A final word of caution: Any particular rule must be written on one line
 and a
 comma must separate each part of the rule.

Ticket URL: <http://wiki.linuxfromscratch.org/lfs/ticket/1785>
LFS Trac <http://wiki.linuxfromscratch.org/lfs/>
Linux From Scratch: Your Distro, Your Rules.

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