linux mdev hotplug (en)

------------- MDEV Primer-------------For those of us who know how to use mdev, a primer might seem lame.  Foreveryone else, mdev is a weird black box that they hear is awesome, but can'tseem to get their head around how it works.  Thus, a primer.----------- Basic Use-----------Mdev has two primary uses: initial population and dynamic updates.  Bothrequire sysfs support in the kernel and have it mounted at /sys.  For dynamicupdates, 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 /sbin/mdev > /proc/sys/kernel/hotplug[3] mdev -sAlternatively, without procfs the above becomes:[1] mount -t sysfs sysfs /sys[2] sysctl -w kernel.hotplug=/sbin/mdev[3] mdev -sOf course, a more "full" setup would entail executing this before the previouscode snippet:[4] mount -t tmpfs -o size=64k,mode=0755 tmpfs /dev[5] mkdir /dev/pts[6] mount -t devpts devpts /dev/ptsThe simple explanation here is that [1] you need to have /sys mounted beforeexecuting mdev.  Then you [2] instruct the kernel to execute /sbin/mdev whenevera device is added or removed so that the device node can be created ordestroyed.  Then you [3] seed /dev with all the device nodes that were createdwhile the system was booting.For the "full" setup, you want to [4] make sure /dev is a tmpfs filesystem(assuming you're running out of flash).  Then you want to [5] create the/dev/pts mount point and finally [6] mount the devpts filesystem on it.------------- MDEV Config   (/etc/mdev.conf)-------------Mdev has an optional config file for controlling ownership/permissions ofdevice nodes if your system needs something more than the default root/root660 permissions.The file has the format:[-]<device regex><uid>:<gid> <permissions>or@<maj[,min1[-min2]]><uid>:<gid> <permissions>or$envvar=<regex><uid>:<gid> <permissions>For example:hd[a-z][0-9]* 0:3 660The config file parsing stops at the first matching line.  If no line ismatched, then the default of 0:0 660 is used.  To set your own default, simplycreate your own total match like so:.* 1:1 777You can rename/move device nodes by using the next optional field.<device regex> <uid>:<gid> <permissions> [=path]So if you want to place the device node into a subdirectory, make sure the pathhas a trailing /.  If you want to rename the device node, just place the name.hda 0:3 660 =drives/This will move "hda" into the drives/ subdirectory.hdb 0:3 660 =cdromThis will rename "hdb" to "cdrom".Similarly, ">path" renames/moves the device but it also createsa direct symlink /dev/DEVNAME to the renamed/moved device.You can also prevent creation of device nodes with the 4th field as "!":tty[a-z]. 0:0 660 !pty[a-z]. 0:0 660 !If you also enable support for executing your own commands, then the file hasthe format:<device regex> <uid>:<gid> <permissions> [=path] [@|$|*<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:disk660>disk/%1/0([hs]d[a-z])([0-9]+)root:disk660>disk/%1/%2mmcblk([0-9]+)root:disk660>disk/mmc/%1/0mmcblk([0-9]+)p([0-9]+)root:disk660>disk/mmc/%1/%2# network devices(tun|tap)root:network660>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 acommand to the shell), so make sure you have a shell installed at /bin/sh.  Youshould 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 ifthe device "hdc" was matched, MDEV would be set to "hdc".---------- FIRMWARE----------Some kernel device drivers need to request firmware at runtime in order toproperly initialize a device.  Place all such firmware files into the/lib/firmware/ directory.  At runtime, the kernel will invoke mdev with thefilename of the firmware which mdev will load out of /lib/firmware/ and intothe kernel via the sysfs interface.  The exact filename is hardcoded in thekernel, 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 environmentalvariable for each successive hotplug invocation. Normally, mdev doesn't care.This may reorder hotplug and hot-unplug events, with typical symptoms ofdevice nodes sometimes not created as expected.However, if /dev/mdev.seq file is found, mdev will compare itscontents with SEQNUM. It will retry up to two seconds, waiting for themto 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.seqwith SEQNUM+1.IOW: this will serialize concurrent mdev invocations.If you want to activate this feature, execute "echo >/dev/mdev.seq" prior tosetting mdev to be the hotplug handler. This writes single '\n' to the file.NB: mdev recognizes /dev/mdev.seq consisting of single '\n' characteras a special case. IOW: this will not make your first hotplug eventto stall for two seconds.