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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    [-]<device regex>   <uid>:<gid> <permissions>
55or
56    @<maj[,min1[-min2]]>    <uid>:<gid> <permissions>
57or
58    $envvar=<regex>     <uid>:<gid> <permissions>
59
60For example:
61    hd[a-z][0-9]* 0:3 660
62
63The config file parsing stops at the first matching line.  If no line is
64matched, then the default of 0:0 660 is used.  To set your own default, simply
65create your own total match like so:
66
67    .* 1:1 777
68
69You can rename/move device nodes by using the next optional field.
70
71    <device regex> <uid>:<gid> <permissions> [=path]
72
73So if you want to place the device node into a subdirectory, make sure the path
74has a trailing /.  If you want to rename the device node, just place the name.
75    hda 0:3 660 =drives/
76This will move "hda" into the drives/ subdirectory.
77    hdb 0:3 660 =cdrom
78This will rename "hdb" to "cdrom".
79
80Similarly, ">path" renames/moves the device but it also creates
81a direct symlink /dev/DEVNAME to the renamed/moved device.
82
83You can also prevent creation of device nodes with the 4th field as "!":
84    tty[a-z]. 0:0 660 !
85    pty[a-z]. 0:0 660 !
86
87If you also enable support for executing your own commands, then the file has
88the format:
89    <device regex> <uid>:<gid> <permissions> [=path] [@|$|*<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.
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