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mdev.txt 5.4 KB

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