Raspberry Pi Flipflop

This is a replacement for the old kernel_emergency.img that old Raspberry Pi firmwares used, see http://elinux.org/RPI_safe_mode .

It lets you toggle booting two different partitions (kernels/distros/images) depending on the state of GPIO pin 3.

You specify a "normal partition" and a "safe partition". Usually the Raspberry Pi will boot the normal partion. If GPIO pin 3 (pin 5 on the physical header) is held to ground it will boot the safe partition.

I am using this in conjunction with a hardware watchdog that toggles the "safe boot" GPIO pin every few hours https://github.com/mkj/pihelp . That ensures that I can access the device even if I break one of the partitions - I have a fallback rescue partition.

At the time of writing (April 2015) this has been tested on a 512MB Raspberry Pi B (not +) with Linux 3.18.7

Matt Johnston matt@ucc.asn.au

Installing

• Get flipflop.initramfs. There's a prebuilt binary at https://matt.ucc.asn.au/rpi-flipflop/flipflop.initramfs or build flipflop by running make. I've been cross-compiling from a x86 Ubuntu box, I assume building on the Raspberry Pi itself should work too.

• Partition and set up a SD card

• mmcblk0p1 - vfat, perhaps 40MB. This is only used by flipflop. I'll call this the "first boot partition", it has a Linux kernel and the flipflop initramfs (and other boot files).

• "normal boot" partition, has all the boot files for the normal distro (eg /boot from raspbian)

• "safe" partition, also has all the boot files but for the safe distro. Its cmdline.txt probably points at a different root partition to normal.

• The root partitions for each of the normal/safe partitions

• Anything else (perhaps config file partition, see below)

• Set up a vfat mmcblk0p1 partition with the contents of https://github.com/raspberrypi/firmware/tree/master/boot or /boot from recent Raspbian.

COPYING.linux           cmdline.txt         issue.txt 
LICENCE.broadcom        config.txt          kernel.img 
LICENSE.oracle          fixup.dat           kernel7.img 
bcm2708-rpi-b-plus.dtb  fixup_cd.dat        overlays/
bcm2708-rpi-b.dtb       fixup_x.dat         start.elf 
bcm2709-rpi-2-b.dtb     flipflop.initramfs  start_cd.elf 
bootcode.bin            start_x.elf         flipflop.txt

• Copy flipflop.initramfs to the first boot partition. Edit config.txt and add a line

initramfs flipflop.initramfs followkernel

• Create a file flipflop.txt on the first boot partition to customise the config. An example to boot mmcblk0p5 in normal mode or mmcblk0p8 in safe mode

normal_bootpart 5
safe_bootpart 8

You can also point it at further config files to parse - this is useful if you want an editable config file on a different partition to avoid ever editing your mmcblk0p1 for safety.

normal_nextconf mmcblk0p2:vfat:nextconf.txt
safe_nextconf mmcblk0p2:vfat:flipflop.txt

The default configuration is hardcoded near the top of flipflop.c

• Install normal distros to your normal and safe partitions. It should now work.

How it works

Flipflop boots a Linux kernel that runs flipflop as /init in an initramfs. It looks at the GPIO pins then sets the Raspberry Pi's reboot parameter /sys/module/bcm2708/parameters/reboot_part as appropriate. Then it reboots and the device boots whichever partition you chose. It adds a few seconds to the boot time.

This works the same as NOOBS - it uses the same parameter to automatically boot the installed distro if the "safe mode" pin isn't held. The "separate config partition" idea came from NOOBS too.