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