Tuesday, February 07, 2012

PandaBoard: persistant MAC address via initrd

I worked out a solution to replace the random MAC address set by the smsc95xx kernel module with the MAC address generated by u-boot on the PandaBoard(ES). It should also work on the BeagleBoard.

As first you need this kernel patch to fix isses with exporting the assign type (PERM, RANDOM, STOLEN) to /sys/class/net/*/addr_assign_type correctly if the smsc95xx driver generate a random MAC address. Build your kernel, if you use openSUSE:Factory:ARM you can get RPMs here, until it's integrated into the official openSUSE kernel.

Now you need a mkinitrd with this patch or a you install these RPMs for openSUSE. This adds a initrd boot script to set the MAC address from kernel cmdline parameters. These are the available parameters and some example values:
  • setmac.set_mac_addr=01:23:45:67:89:ab
  • setmac.set_iface=eth0
  • setmac.set_module=smsc95xx
The setmac.set_mac_addr= is mandatory to change the MAC and you have to choose one of the other two parameters. Either you specify the network interface or the kernel driver/module. In case you use setmac.set_module the first network device provided by this module which has a random MAC gets the new address assigned. Please note: this script change the MAC of a interface only if addr_assign_type=1 , otherwise nothing will change.

If you have installed the new kernel and mkinitrd you may need to follow these steps:
# mount the boot partition which contains the uImage (and may also the MLO file) 
mount /dev/mmcblk0p1 /mnt
# make sure this partition contains the new uImage
# build a new initrd
mkinitrd -B
# create a uInitrd
mkimage -A arm -O linux -T ramdisk -C none -a 0 -e 0 -n initramfs -d /boot/initrd /mnt/uInitrd
Now you need to tell u-boot to boot from the uInitrd and to set the needed cmdline parameters. I prefer to use a uEnv.txt file instead of a boot.scr since you simply can change it without call mkimage. You can download my currently used uEnv.txt here. The content depends on your setup you may need to adapt yours:
bootargs=root=/dev/mmcblk0p2 rw rootwait rootfstype=ext3 console=ttyO2,115200n8 vram=16M omapfb.vram=0:16M
bootcmd=mmc rescan ; setenv bootargs ${bootargs} setmac.set_mac_addr="${usbethaddr}" setmac.set_iface=eth0; fatload mmc 0:1 0x80000000 uImage; fatload mmc 0:1 0x81600000 uInitrd; bootm 0x80000000 0x81600000
Now umount /mnt and reboot your system. If you already worked around the random MAC address problem: don't forget to remove/disable these hacks.