2007-02-14 18:50:29 +05:30
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MDEV Primer
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For those of us who know how to use mdev, a primer might seem lame. For
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everyone else, mdev is a weird black box that they hear is awesome, but can't
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seem to get their head around how it works. Thus, a primer.
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Basic Use
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Mdev has two primary uses: initial population and dynamic updates. Both
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require sysfs support in the kernel and have it mounted at /sys. For dynamic
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updates, you also need to have hotplugging enabled in your kernel.
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Here's a typical code snippet from the init script:
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[1] mount -t sysfs sysfs /sys
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[2] echo /bin/mdev > /proc/sys/kernel/hotplug
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[3] mdev -s
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Of course, a more "full" setup would entail executing this before the previous
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code snippet:
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[4] mount -t tmpfs mdev /dev
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[5] mkdir /dev/pts
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[6] mount -t devpts devpts /dev/pts
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The simple explanation here is that [1] you need to have /sys mounted before
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executing mdev. Then you [2] instruct the kernel to execute /bin/mdev whenever
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a device is added or removed so that the device node can be created or
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destroyed. Then you [3] seed /dev with all the device nodes that were created
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while the system was booting.
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For the "full" setup, you want to [4] make sure /dev is a tmpfs filesystem
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(assuming you're running out of flash). Then you want to [5] create the
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/dev/pts mount point and finally [6] mount the devpts filesystem on it.
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MDEV Config (/etc/mdev.conf)
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Mdev has an optional config file for controlling ownership/permissions of
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device nodes if your system needs something more than the default root/root
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660 permissions.
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The file has the format:
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<device regex> <uid>:<gid> <octal permissions>
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For example:
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hd[a-z][0-9]* 0:3 660
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The config file parsing stops at the first matching line. If no line is
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matched, then the default of 0:0 660 is used. To set your own default, simply
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create your own total match like so:
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.* 1:1 777
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If you also enable support for executing your own commands, then the file has
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the format:
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<device regex> <uid>:<gid> <octal permissions> [<@|$|*> <command>]
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The special characters have the meaning:
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@ Run after creating the device.
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$ Run before removing the device.
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* Run both after creating and before removing the device.
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The command is executed via the system() function (which means you're giving a
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command to the shell), so make sure you have a shell installed at /bin/sh.
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For your convenience, the shell env var $MDEV is set to the device name. So if
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the device 'hdc' was matched, MDEV would be set to "hdc".
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2007-06-28 22:43:51 +05:30
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----------
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FIRMWARE
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Some kernel device drivers need to request firmware at runtime in order to
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properly initialize a device. Place all such firmware files into the
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/lib/firmware/ directory. At runtime, the kernel will invoke mdev with the
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filename of the firmware which mdev will load out of /lib/firmware/ and into
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the kernel via the sysfs interface. The exact filename is hardcoded in the
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kernel, so look there if you need to want to know what to name the file in
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userspace.
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