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diff --git a/docs/mdev.txt b/docs/mdev.txt new file mode 100644 index 0000000..51c3f0e --- /dev/null +++ b/docs/mdev.txt @@ -0,0 +1,68 @@ +------------- + 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: +[1] mount -t sysfs sysfs /sys +[2] echo /bin/mdev > /proc/sys/kernel/hotplug +[3] mdev -s + +Of course, a more "full" setup would entail executing this before the previous +code snippet: +[4] mount -t tmpfs mdev /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> +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 + +If you also enable support for executing your own commands, then the file has +the format: + <device regex> <uid>:<gid> <octal permissions> [<@|$|*> <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. + +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". |