Here's a typical code snippet from the init script:
[0] mount -t proc proc /proc
[1] mount -t sysfs sysfs /sys
-[2] echo /bin/mdev > /proc/sys/kernel/hotplug
+[2] echo /sbin/mdev > /proc/sys/kernel/hotplug
[3] mdev -s
Alternatively, without procfs the above becomes:
[1] mount -t sysfs sysfs /sys
-[2] sysctl -w kernel.hotplug=/bin/mdev
+[2] sysctl -w kernel.hotplug=/sbin/mdev
[3] mdev -s
[6] mount -t devpts devpts /dev/pts
The simple explanation here is that [1] you need to have /sys mounted before
-executing mdev. Then you [2] instruct the kernel to execute /bin/mdev whenever
+executing mdev. Then you [2] instruct the kernel to execute /sbin/mdev whenever
a device is added or removed so that the device node can be created or
destroyed. Then you [3] seed /dev with all the device nodes that were created
while the system was booting.
660 permissions.
The file has the format:
- <device regex> <uid>:<gid> <octal permissions>
+ [-][envmatch]<device regex> <uid>:<gid> <permissions>
+or
+ [envmatch]@<maj[,min1[-min2]]> <uid>:<gid> <permissions>
+or
+ $envvar=<regex> <uid>:<gid> <permissions>
+
For example:
hd[a-z][0-9]* 0:3 660
The config file parsing stops at the first matching line. If no line is
matched, then the default of 0:0 660 is used. To set your own default, simply
create your own total match like so:
+
.* 1:1 777
-You can rename/relocate device nodes by using the next optional field.
- <device regex> <uid>:<gid> <octal permissions> [>path]
+You can rename/move device nodes by using the next optional field.
+
+ <device regex> <uid>:<gid> <permissions> [=path]
+
So if you want to place the device node into a subdirectory, make sure the path
has a trailing /. If you want to rename the device node, just place the name.
- hda 0:3 660 >drives/
-This will relocate "hda" into the drives/ subdirectory.
- hdb 0:3 660 >cdrom
+ hda 0:3 660 =drives/
+This will move "hda" into the drives/ subdirectory.
+ hdb 0:3 660 =cdrom
This will rename "hdb" to "cdrom".
+Similarly, ">path" renames/moves the device but it also creates
+a direct symlink /dev/DEVNAME to the renamed/moved device.
+
+You can also prevent creation of device nodes with the 4th field as "!":
+ tty[a-z]. 0:0 660 !
+ pty[a-z]. 0:0 660 !
+
If you also enable support for executing your own commands, then the file has
the format:
- <device regex> <uid>:<gid> <octal permissions> [<@|$|*> <command>]
+ <device regex> <uid>:<gid> <permissions> [=path] [@|$|*<command>]
+ or
+ <device regex> <uid>:<gid> <permissions> [>path] [@|$|*<command>]
+ or
+ <device regex> <uid>:<gid> <permissions> [!] [@|$|*<command>]
+
+For example:
+---8<---
+# block devices
+([hs]d[a-z]) root:disk 660 >disk/%1/0
+([hs]d[a-z])([0-9]+) root:disk 660 >disk/%1/%2
+mmcblk([0-9]+) root:disk 660 >disk/mmc/%1/0
+mmcblk([0-9]+)p([0-9]+) root:disk 660 >disk/mmc/%1/%2
+# network devices
+(tun|tap) root:network 660 >net/%1
+---8<---
+
The special characters have the meaning:
@ Run after creating the device.
$ Run before removing the device.
filename of the firmware which mdev will load out of /lib/firmware/ and into
the kernel via the sysfs interface. The exact filename is hardcoded in the
kernel, so look there if you need to know how to name the file in userspace.
+
+------------
+ SEQUENCING
+------------
+
+Kernel does not serialize hotplug events. It increments SEQNUM environmental
+variable for each successive hotplug invocation. Normally, mdev doesn't care.
+This may reorder hotplug and hot-unplug events, with typical symptoms of
+device nodes sometimes not created as expected.
+
+However, if /dev/mdev.seq file is found, mdev will compare its
+contents with SEQNUM. It will retry up to two seconds, waiting for them
+to match. If they match exactly (not even trailing '\n' is allowed),
+or if two seconds pass, mdev runs as usual, then it rewrites /dev/mdev.seq
+with SEQNUM+1.
+
+IOW: this will serialize concurrent mdev invocations.
+
+If you want to activate this feature, execute "echo >/dev/mdev.seq" prior to
+setting mdev to be the hotplug handler. This writes single '\n' to the file.
+NB: mdev recognizes /dev/mdev.seq consisting of single '\n' character
+as a special case. IOW: this will not make your first hotplug event
+to stall for two seconds.
projects / oweals / busybox.git / blobdiff