busybox/docs/mdev.txt
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   1-------------
   2 MDEV Primer
   3-------------
   4
   5For those of us who know how to use mdev, a primer might seem lame.  For
   6everyone else, mdev is a weird black box that they hear is awesome, but can't
   7seem to get their head around how it works.  Thus, a primer.
   8
   9-----------
  10 Basic Use
  11-----------
  12
  13Mdev has two primary uses: initial population and dynamic updates.  Both
  14require sysfs support in the kernel and have it mounted at /sys.  For dynamic
  15updates, you also need to have hotplugging enabled in your kernel.
  16
  17Here's a typical code snippet from the init script:
  18[0] mount -t proc proc /proc
  19[1] mount -t sysfs sysfs /sys
  20[2] echo /sbin/mdev > /proc/sys/kernel/hotplug
  21[3] mdev -s
  22
  23Alternatively, without procfs the above becomes:
  24[1] mount -t sysfs sysfs /sys
  25[2] sysctl -w kernel.hotplug=/sbin/mdev
  26[3] mdev -s
  27
  28
  29Of course, a more "full" setup would entail executing this before the previous
  30code snippet:
  31[4] mount -t tmpfs -o size=64k,mode=0755 tmpfs /dev
  32[5] mkdir /dev/pts
  33[6] mount -t devpts devpts /dev/pts
  34
  35The simple explanation here is that [1] you need to have /sys mounted before
  36executing mdev.  Then you [2] instruct the kernel to execute /sbin/mdev whenever
  37a device is added or removed so that the device node can be created or
  38destroyed.  Then you [3] seed /dev with all the device nodes that were created
  39while the system was booting.
  40
  41For the "full" setup, you want to [4] make sure /dev is a tmpfs filesystem
  42(assuming you're running out of flash).  Then you want to [5] create the
  43/dev/pts mount point and finally [6] mount the devpts filesystem on it.
  44
  45-------------
  46 MDEV Config   (/etc/mdev.conf)
  47-------------
  48
  49Mdev has an optional config file for controlling ownership/permissions of
  50device nodes if your system needs something more than the default root/root
  51660 permissions.
  52
  53The file has the format:
  54        [-][envmatch]<device regex>     <uid>:<gid> <permissions>
  55or
  56        [envmatch]@<maj[,min1[-min2]]>  <uid>:<gid> <permissions>
  57or
  58        $envvar=<regex>         <uid>:<gid> <permissions>
  59
  60For example:
  61        hd[a-z][0-9]* 0:3 660
  62
  63The config file parsing stops at the first matching line unless this line
  64starts with "-". If no line is matched, then the default of 0:0 660 is used.
  65To set your own default, simply create your own total match like so:
  66
  67        .* 1:1 777
  68
  69You can rename/move device nodes by using the next optional field.
  70
  71        <device regex> <uid>:<gid> <permissions> [=path]
  72
  73So if you want to place the device node into a subdirectory, make sure the path
  74has a trailing /.  If you want to rename the device node, just place the name.
  75        hda 0:3 660 =drives/
  76This will move "hda" into the drives/ subdirectory.
  77        hdb 0:3 660 =cdrom
  78This will rename "hdb" to "cdrom".
  79
  80Similarly, ">path" renames/moves the device but it also creates
  81a direct symlink /dev/DEVNAME to the renamed/moved device.
  82
  83You can also prevent creation of device nodes with the 4th field as "!":
  84        tty[a-z]. 0:0 660 !
  85        pty[a-z]. 0:0 660 !
  86
  87If you also enable support for executing your own commands, then the file has
  88the format:
  89        <device regex> <uid>:<gid> <permissions> [=path] [@|$|*<command>]
  90    or
  91        <device regex> <uid>:<gid> <permissions> [>path] [@|$|*<command>]
  92    or
  93        <device regex> <uid>:<gid> <permissions> [!] [@|$|*<command>]
  94
  95For example:
  96---8<---
  97# block devices
  98([hs]d[a-z])            root:disk       660     >disk/%1/0
  99([hs]d[a-z])([0-9]+)    root:disk       660     >disk/%1/%2
 100mmcblk([0-9]+)          root:disk       660     >disk/mmc/%1/0
 101mmcblk([0-9]+)p([0-9]+) root:disk       660     >disk/mmc/%1/%2
 102# network devices
 103(tun|tap)               root:network    660     >net/%1
 104---8<---
 105
 106The special characters have the meaning:
 107        @ Run after creating the device.
 108        $ Run before removing the device.
 109        * Run both after creating and before removing the device.
 110
 111The command is executed via the system() function (which means you're giving a
 112command to the shell), so make sure you have a shell installed at /bin/sh.  You
 113should also keep in mind that the kernel executes hotplug helpers with stdin,
 114stdout, and stderr connected to /dev/null.
 115
 116For your convenience, the shell env var $MDEV is set to the device name.  So if
 117the device "hdc" was matched, MDEV would be set to "hdc".
 118
 119----------
 120 FIRMWARE
 121----------
 122
 123Some kernel device drivers need to request firmware at runtime in order to
 124properly initialize a device.  Place all such firmware files into the
 125/lib/firmware/ directory.  At runtime, the kernel will invoke mdev with the
 126filename of the firmware which mdev will load out of /lib/firmware/ and into
 127the kernel via the sysfs interface.  The exact filename is hardcoded in the
 128kernel, so look there if you need to know how to name the file in userspace.
 129
 130------------
 131 SEQUENCING
 132------------
 133
 134Kernel does not serialize hotplug events. It increments SEQNUM environmental
 135variable for each successive hotplug invocation. Normally, mdev doesn't care.
 136This may reorder hotplug and hot-unplug events, with typical symptoms of
 137device nodes sometimes not created as expected.
 138
 139However, if /dev/mdev.seq file is found, mdev will compare its
 140contents with SEQNUM. It will retry up to two seconds, waiting for them
 141to match. If they match exactly (not even trailing '\n' is allowed),
 142or if two seconds pass, mdev runs as usual, then it rewrites /dev/mdev.seq
 143with SEQNUM+1.
 144
 145IOW: this will serialize concurrent mdev invocations.
 146
 147If you want to activate this feature, execute "echo >/dev/mdev.seq" prior to
 148setting mdev to be the hotplug handler. This writes single '\n' to the file.
 149NB: mdev recognizes /dev/mdev.seq consisting of single '\n' character
 150as a special case. IOW: this will not make your first hotplug event
 151to stall for two seconds.
 152