GRUB boot selector switch

Firmware for the Hardware Boot Selection Switch project. This acts as a USB mass storage device containing a dynamic GRUB script, which sets a variable to indicate the position of a physical switch.

Building

On Linux, you'll need gcc-arm-none-eabi installed to build and openocd to flash.

# Pull in libopencm3
git submodule init
git submodule update

# Build libopencm3 (only needed once)
make -C libopencm3 -j8

# Build the firmware
make

# Flash with a J-Link programmer connected by SWD
make flash

# Flash with an ST-Link programmer connected by SWD
make flash PROGRAMMER=stlink

This project supports building and flashing multiple different parts. The following parts have been tested:

stm32f103c8 (default): found on the "blue pill" and "maple" STM32 dev boards
stm32f070f6

To build for your specific part, pass the DEVICE parameter to make on the command line (or export DEVICE as an environment variable):

make DEVICE=stm32f070f6
make flash DEVICE=stm32f070f6

Valid patterns for the DEVICE parameter can be found in libopencm3/ld/devices.data.

Wiring

The switch position is read from pin A6, which is pulled high internally. Connect a toggle switch between this pin and GND to control the reported state.

GRUB config

You'll need to modify your system's GRUB config to look for and read from this device.

On Debian and Arch based systems, this involves editing a script in /etc/grub.d/, then running update-grub to generate and validate the /boot/grub/grub.cfg file used when booting. Check your distribution's docs for how to do this on your system.

Here's an example of how the switch position can be used in a GRUB script. This replaces the existing set default= line in your config:

# Look for hardware switch device by its hard-coded filesystem ID
search --no-floppy --fs-uuid --set hdswitch 55AA-6922

# If found, read dynamic config file and select appropriate entry for each position
if [ "${hdswitch}" ] ; then
  source ($hdswitch)/switch_position_grub.cfg

  if [ "${os_hw_switch}" == 0 ] ; then
    # Boot Linux
    set default="0"
  elif [ "${os_hw_switch}" == 1 ] ; then
    # Boot Windows
    set default="2"
  else
    # Fallback to default
    set default="${GRUB_DEFAULT}"
  fi

else
  set default="${GRUB_DEFAULT}"
fi

Troubleshooting

The device is not accessible in GRUB

Ensure your BIOS is configured to enumerate USB storage devices. On some motherboards, this requires changing from "fast boot" to "normal boot". If you can boot from a flash drive, you should be able to access this device from GRUB.

Reading from an operating system

The dynamic files presented by this device cannot be reliably used when mounted in Windows or Linux. Operating systems (quite rightly) assume that the files on an underlying device won't change without their knowledge, so they cache reads heavily.

If you want to access the switch state from an operating system, you'll need to implement a second USB interface on the device, or work with the unmounted block device.