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    Wed Aug 18 21:56:57 2010 UTC (13 years, 9 months ago) by niro
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    -updated to busybox-1.17.1
    1 niro 816 -------------
    2     MDEV Primer
    3     -------------
    4    
    5     For those of us who know how to use mdev, a primer might seem lame. For
    6     everyone else, mdev is a weird black box that they hear is awesome, but can't
    7     seem to get their head around how it works. Thus, a primer.
    8    
    9     -----------
    10     Basic Use
    11     -----------
    12    
    13     Mdev has two primary uses: initial population and dynamic updates. Both
    14     require sysfs support in the kernel and have it mounted at /sys. For dynamic
    15     updates, you also need to have hotplugging enabled in your kernel.
    16    
    17     Here's a typical code snippet from the init script:
    18     [0] mount -t proc proc /proc
    19     [1] mount -t sysfs sysfs /sys
    20 niro 984 [2] echo /sbin/mdev > /proc/sys/kernel/hotplug
    21 niro 816 [3] mdev -s
    22    
    23     Alternatively, without procfs the above becomes:
    24     [1] mount -t sysfs sysfs /sys
    25 niro 984 [2] sysctl -w kernel.hotplug=/sbin/mdev
    26 niro 816 [3] mdev -s
    27    
    28    
    29     Of course, a more "full" setup would entail executing this before the previous
    30     code 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    
    35     The simple explanation here is that [1] you need to have /sys mounted before
    36 niro 984 executing mdev. Then you [2] instruct the kernel to execute /sbin/mdev whenever
    37 niro 816 a device is added or removed so that the device node can be created or
    38     destroyed. Then you [3] seed /dev with all the device nodes that were created
    39     while the system was booting.
    40    
    41     For 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    
    49     Mdev has an optional config file for controlling ownership/permissions of
    50     device nodes if your system needs something more than the default root/root
    51     660 permissions.
    52    
    53     The file has the format:
    54 niro 1123 <device regex> <uid>:<gid> <permissions>
    55     or @<maj[,min1[-min2]]> <uid>:<gid> <permissions>
    56 niro 816
    57     For example:
    58     hd[a-z][0-9]* 0:3 660
    59    
    60     The config file parsing stops at the first matching line. If no line is
    61     matched, then the default of 0:0 660 is used. To set your own default, simply
    62     create your own total match like so:
    63     .* 1:1 777
    64    
    65     You can rename/move device nodes by using the next optional field.
    66 niro 1123 <device regex> <uid>:<gid> <permissions> [=path]
    67 niro 816 So if you want to place the device node into a subdirectory, make sure the path
    68     has a trailing /. If you want to rename the device node, just place the name.
    69     hda 0:3 660 =drives/
    70     This will move "hda" into the drives/ subdirectory.
    71     hdb 0:3 660 =cdrom
    72     This will rename "hdb" to "cdrom".
    73    
    74     Similarly, ">path" renames/moves the device but it also creates
    75     a direct symlink /dev/DEVNAME to the renamed/moved device.
    76    
    77 niro 1123 You can also prevent creation of device nodes with the 4th field as "!":
    78     tty[a-z]. 0:0 660 !
    79     pty[a-z]. 0:0 660 !
    80    
    81 niro 816 If you also enable support for executing your own commands, then the file has
    82     the format:
    83 niro 1123 <device regex> <uid>:<gid> <permissions> [=path] [@|$|*<command>]
    84 niro 816 or
    85 niro 1123 <device regex> <uid>:<gid> <permissions> [>path] [@|$|*<command>]
    86     or
    87     <device regex> <uid>:<gid> <permissions> [!] [@|$|*<command>]
    88 niro 984
    89     For example:
    90     ---8<---
    91     # block devices
    92     ([hs]d[a-z]) root:disk 660 >disk/%1/0
    93     ([hs]d[a-z])([0-9]+) root:disk 660 >disk/%1/%2
    94     mmcblk([0-9]+) root:disk 660 >disk/mmc/%1/0
    95     mmcblk([0-9]+)p([0-9]+) root:disk 660 >disk/mmc/%1/%2
    96     # network devices
    97     (tun|tap) root:network 660 >net/%1
    98     ---8<---
    99    
    100 niro 816 The special characters have the meaning:
    101     @ Run after creating the device.
    102     $ Run before removing the device.
    103     * Run both after creating and before removing the device.
    104    
    105     The command is executed via the system() function (which means you're giving a
    106     command to the shell), so make sure you have a shell installed at /bin/sh. You
    107     should also keep in mind that the kernel executes hotplug helpers with stdin,
    108     stdout, and stderr connected to /dev/null.
    109    
    110     For your convenience, the shell env var $MDEV is set to the device name. So if
    111     the device "hdc" was matched, MDEV would be set to "hdc".
    112    
    113     ----------
    114     FIRMWARE
    115     ----------
    116    
    117     Some kernel device drivers need to request firmware at runtime in order to
    118     properly initialize a device. Place all such firmware files into the
    119     /lib/firmware/ directory. At runtime, the kernel will invoke mdev with the
    120     filename of the firmware which mdev will load out of /lib/firmware/ and into
    121     the kernel via the sysfs interface. The exact filename is hardcoded in the
    122     kernel, so look there if you need to know how to name the file in userspace.
    123    
    124     ------------
    125     SEQUENCING
    126     ------------
    127    
    128     Kernel does not serialize hotplug events. It increments SEQNUM environmental
    129     variable for each successive hotplug invocation. Normally, mdev doesn't care.
    130     This may reorder hotplug and hot-unplug events, with typical symptoms of
    131     device nodes sometimes not created as expected.
    132    
    133     However, if /dev/mdev.seq file is found, mdev will compare its
    134     contents with SEQNUM. It will retry up to two seconds, waiting for them
    135     to match. If they match exactly (not even trailing '\n' is allowed),
    136     or if two seconds pass, mdev runs as usual, then it rewrites /dev/mdev.seq
    137     with SEQNUM+1.
    138    
    139     IOW: this will serialize concurrent mdev invocations.
    140    
    141     If you want to activate this feature, execute "echo >/dev/mdev.seq" prior to
    142     setting mdev to be the hotplug handler. This writes single '\n' to the file.
    143 niro 984 NB: mdev recognizes /dev/mdev.seq consisting of single '\n' character
    144 niro 816 as a special case. IOW: this will not make your first hotplug event
    145     to stall for two seconds.