Difference between revisions of "Automatically mount removable media"

Revision as of 19:47, 16 February 2012

When using USB key or MMC/SD in a final application, it could be useful if these media were automatically mount when inserted by the user. We will explain you how to implement this behaviour in this tutorial.

Default case

• If you are using default armadeus rootfs, hotplugging is handled by an application called mdev. This can be easily checked with:

# cat /proc/sys/kernel/hotplug
/sbin/mdev

• It means that each time Linux kernel detect a new device it will call /sbin/mdev program with specific parameters.
• You can configure mdev with its config file: /etc/mdev.conf. To add automatic mount of USB and SD/MMC devices you can apply the following changes (- -> remove line, + -> add line) in the configuration file:

# Block devices
-sd[a-z].*       0:6     660
-mmcblk[0-9].*   0:6     660
+sd[a-z]          0:6     660
+mmcblk[0-9]     0:6     660
+sd[a-z][0-9]      0:6     660 */etc/hotplug/automounter.sh
+mmcblk[0-9]p[0-9] 0:6     660 */etc/hotplug/automounter.sh

• this way mdev will call a script named /etc/hotplug/automount.sh each time a USB drive or an MMC/SD, with a recognized partition table, is plugged in.
• create directory /etc/hotplug

# mkdir /etc/hotplug

• then we will have to write what this script is going to do. So take your favorite editor and fill /etc/hotplug/automounter.sh with:

# vi /etc/hotplug/automounter.sh

#!/bin/sh

destdir=/media

my_umount()
{
	if grep -qs "^/dev/$1 " /proc/mounts ; then
		umount "${destdir}/$1";
	fi

	[ -d "${destdir}/$1" ] && rmdir "${destdir}/$1"
}

my_mount()
{
	mkdir -p "${destdir}/$1" || exit 1

	if ! mount -t auto -o sync "/dev/$1" "${destdir}/$1"; then
		# failed to mount, clean up mountpoint
		rmdir "${destdir}/$1"
		exit 1
	fi
}

case "${ACTION}" in
add|"")
	my_umount ${MDEV}
	my_mount ${MDEV}
	;;
remove)
	my_umount ${MDEV}
	;;
esac

# chmod a+x /etc/hotplug/automounter.sh

• As we now use /media to create dynamic mount points we will move it from FLASH to RAM by using a tmpfs mount. Take your favorite editro again and edit /etc/fstab file to add it the following line at the end:

...
usbfs           /proc/bus/usb  usbfs    defaults          0      0
+tmpfs           /media         tmpfs    defaults          0      0

• then reboot and everything should now work :-). For example when plugging a MMC/SD you will get:

# mmc0: host does not support reading read-only switch. assuming write-enable.
mmc0: new SD card at address 0002
mmcblk0: mmc0:0002 SD    1.87 GiB 
 mmcblk0: p1

# ls -al /media/
total 32
drwxrwxrwt    3 root     root            60 Jan  1 00:27 .
drwxrwxr-x   18 root     root          1248 Feb  6  2012 ..
drwxrwxrwx    7 root     root         16384 Jan  1 00:00 mmcblk0p1

# mount
...
/dev/mmcblk0p1 on /media/mmcblk0p1 type vfat (rw,sync,fmask=0000,dmask=0000,allow_utime=0022,codepage=cp437,iocharset=iso8859-1)

• Don't forget that if you want to remove your device, you will have to manually umount it before:

# umount /media/mmcblk0p1

Other cases

• well... in that cases I suppose you know what you're doing !:-)
• if using udev instead of mdev for example, then you will have to activate corresponding Buildroot packages (eg usbmount)

Links

• http://lists.busybox.net/pipermail/buildroot/2011-August/045187.html
• http://linux.die.net/man/8/hotplug