#1790: udev rules need documented
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Reporter: [EMAIL PROTECTED] | Owner: [email protected]
Type: task | Status: new
Priority: normal | Milestone: 6.2
Component: Book | Version: SVN
Severity: normal | Resolution:
Keywords: |
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Comment (by [EMAIL PROTECTED]):
Okay, here's round one. Please help correct this for accuracy, grammar,
and readability. I don't know how pedantic and technical I should be for
the book.
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
dicusses this").
Uevent: An event generated by the kernel whenever a device is added or
removed. 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 kernel is first booting up.
ACTION=="add", DEVPATH=="/devices/*", ENV{PHYSDEVBUS}=="?*",
WAIT_FOR_SYSFS="bus"
The files, directories, and symlinks in /sys do not all appear at once.
They appear as the kernel identifies and enumerates them. However, the
uevent will 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 race conditions, this rule matches those devices
(which have an ACTION of add, DEVPATH of /devices/*, and a non-empty
environment variable named PHYSDEVBUS) and causes Udev to wait until the
kernel has created the needed "bus" file (in this case it will be a
symlink) for that particular device before it attempts to match any other
rule. This gives the kernel time to enumerate the various 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
will wait for the existence of the file /sys/bus/scsi/devices/<device
number>/ioerr_cnt.
KERNEL=="mice"
The majority of rules exist to create device nodes. In the above example
any 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 fine-tune 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 mice 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
acheive this with the following:
KERNEL=="mice", MODE="0644", GROUP="some group", NAME="input/%k",
SYMLINK="mouse"
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/1790>
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