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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 varible to indicate the position of the physical switch.

Building

You'll need gcc-arm-none-eabi to build and openocd to flash.

# Pull in libopencm3
git submodule init
git submodule update

# Build opencm3 (only needed once)
cd libopencm3
make -j8

# Build firmware
cd src
make

# Flash (assumes a J-Link programmer connected to SWD on the device)
make flash

GRUB config

You'll need to modify your system's GRUB config to look for and read from this device. Here's an example of how this can be done in GRUB script:

# 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