updates, you also need to have hotplugging enabled in your kernel.
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=/sbin/mdev
+[3] mdev -s
+
+
Of course, a more "full" setup would entail executing this before the previous
code snippet:
-[4] mount -t tmpfs mdev /dev
+[4] mount -t tmpfs -o size=64k,mode=0755 tmpfs /dev
[5] mkdir /dev/pts
[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
+ $envvar=<regex> <uid>:<gid> <permissions>
+
For example:
hd[a-z][0-9]* 0:3 660
The config file parsing stops at the first matching line. If no line is
matched, then the default of 0:0 660 is used. To set your own default, simply
create your own total match like so:
+
.* 1:1 777
+You can rename/move device nodes by using the next optional field.
+
+ <device regex> <uid>:<gid> <permissions> [=path]
+
+So if you want to place the device node into a subdirectory, make sure the path
+has a trailing /. If you want to rename the device node, just place the name.
+ hda 0:3 660 =drives/
+This will move "hda" into the drives/ subdirectory.
+ hdb 0:3 660 =cdrom
+This will rename "hdb" to "cdrom".
+
+Similarly, ">path" renames/moves the device but it also creates
+a direct symlink /dev/DEVNAME to the renamed/moved device.
+
+You can also prevent creation of device nodes with the 4th field as "!":
+ tty[a-z]. 0:0 660 !
+ pty[a-z]. 0:0 660 !
+
If you also enable support for executing your own commands, then the file has
the format:
- <device regex> <uid>:<gid> <octal permissions> [<@|$|*> <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.
* Run both after creating and before removing the device.
The command is executed via the system() function (which means you're giving a
-command to the shell), so make sure you have a shell installed at /bin/sh.
+command to the shell), so make sure you have a shell installed at /bin/sh. You
+should also keep in mind that the kernel executes hotplug helpers with stdin,
+stdout, and stderr connected to /dev/null.
For your convenience, the shell env var $MDEV is set to the device name. So if
-the device 'hdc' was matched, MDEV would be set to "hdc".
+the device "hdc" was matched, MDEV would be set to "hdc".
----------
FIRMWARE
/lib/firmware/ directory. At runtime, the kernel will invoke mdev with the
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 want to know what to name the file in
-userspace.
+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 / platform / upstream / busybox.git / blobdiff