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diff --git a/release/src/router/busybox/docs/mdev.txt b/release/src/router/busybox/docs/mdev.txt new file mode 100644 index 00000000..a8a816ce --- /dev/null +++ b/release/src/router/busybox/docs/mdev.txt @@ -0,0 +1,127 @@ +------------- + MDEV Primer +------------- + +For those of us who know how to use mdev, a primer might seem lame. For +everyone else, mdev is a weird black box that they hear is awesome, but can't +seem to get their head around how it works. Thus, a primer. + +----------- + Basic Use +----------- + +Mdev has two primary uses: initial population and dynamic updates. Both +require sysfs support in the kernel and have it mounted at /sys. For dynamic +updates, you also need to have hotplugging enabled in your kernel. + +Here's a typical code snippet from the init script: +[0] mount -t proc proc /proc +[1] mount -t sysfs sysfs /sys +[2] echo /bin/mdev > /proc/sys/kernel/hotplug +[3] mdev -s + +Alternatively, without procfs the above becomes: +[1] mount -t sysfs sysfs /sys +[2] sysctl -w kernel.hotplug=/bin/mdev +[3] mdev -s + + +Of course, a more "full" setup would entail executing this before the previous +code snippet: +[4] mount -t tmpfs -o size=64k,mode=0755 tmpfs /dev +[5] mkdir /dev/pts +[6] mount -t devpts devpts /dev/pts + +The simple explanation here is that [1] you need to have /sys mounted before +executing mdev. Then you [2] instruct the kernel to execute /bin/mdev whenever +a device is added or removed so that the device node can be created or +destroyed. Then you [3] seed /dev with all the device nodes that were created +while the system was booting. + +For the "full" setup, you want to [4] make sure /dev is a tmpfs filesystem +(assuming you're running out of flash). Then you want to [5] create the +/dev/pts mount point and finally [6] mount the devpts filesystem on it. + +------------- + MDEV Config (/etc/mdev.conf) +------------- + +Mdev has an optional config file for controlling ownership/permissions of +device nodes if your system needs something more than the default root/root +660 permissions. + +The file has the format: + <device regex> <uid>:<gid> <octal permissions> + or @<maj[,min1[-min2]]> <uid>:<gid> <octal permissions> + +For example: + hd[a-z][0-9]* 0:3 660 + +The config file parsing stops at the first matching line. If no line is +matched, then the default of 0:0 660 is used. To set your own default, simply +create your own total match like so: + .* 1:1 777 + +You can rename/move device nodes by using the next optional field. + <device regex> <uid>:<gid> <octal permissions> [=path] +So if you want to place the device node into a subdirectory, make sure the path +has a trailing /. If you want to rename the device node, just place the name. + hda 0:3 660 =drives/ +This will move "hda" into the drives/ subdirectory. + hdb 0:3 660 =cdrom +This will rename "hdb" to "cdrom". + +Similarly, ">path" renames/moves the device but it also creates +a direct symlink /dev/DEVNAME to the renamed/moved device. + +If you also enable support for executing your own commands, then the file has +the format: + <device regex> <uid>:<gid> <octal permissions> [=path] [@|$|*<command>] + or + <device regex> <uid>:<gid> <octal permissions> [>path] [@|$|*<command>] +The special characters have the meaning: + @ Run after creating the device. + $ Run before removing the device. + * Run both after creating and before removing the device. + +The command is executed via the system() function (which means you're giving a +command to the shell), so make sure you have a shell installed at /bin/sh. You +should also keep in mind that the kernel executes hotplug helpers with stdin, +stdout, and stderr connected to /dev/null. + +For your convenience, the shell env var $MDEV is set to the device name. So if +the device "hdc" was matched, MDEV would be set to "hdc". + +---------- + FIRMWARE +---------- + +Some kernel device drivers need to request firmware at runtime in order to +properly initialize a device. Place all such firmware files into the +/lib/firmware/ directory. At runtime, the kernel will invoke mdev with the +filename of the firmware which mdev will load out of /lib/firmware/ and into +the kernel via the sysfs interface. The exact filename is hardcoded in the +kernel, so look there if you need to know how to name the file in userspace. + +------------ + SEQUENCING +------------ + +Kernel does not serialize hotplug events. It increments SEQNUM environmental +variable for each successive hotplug invocation. Normally, mdev doesn't care. +This may reorder hotplug and hot-unplug events, with typical symptoms of +device nodes sometimes not created as expected. + +However, if /dev/mdev.seq file is found, mdev will compare its +contents with SEQNUM. It will retry up to two seconds, waiting for them +to match. If they match exactly (not even trailing '\n' is allowed), +or if two seconds pass, mdev runs as usual, then it rewrites /dev/mdev.seq +with SEQNUM+1. + +IOW: this will serialize concurrent mdev invocations. + +If you want to activate this feature, execute "echo >/dev/mdev.seq" prior to +setting mdev to be the hotplug handler. This writes single '\n' to the file. +NB: mdev recognizes /dev/mdev.seq consisting of single '\n' characher +as a special case. IOW: this will not make your first hotplug event +to stall for two seconds. |