1.29. Reboot Mode

To simplify debugging, many BootROMs sample registers that survive a warm reset to customize the boot. These registers can e.g. indicate that boot should happen from a different boot medium.

Likewise, many bootloaders reuse such registers, or if unavailable, non-volatile memory to determine whether the OS requested a special reboot mode, e.g. rebooting into an USB recovery mode. This is common on Android systems.

barebox implements the upstream device tree bindings for reboot-modes to act upon reboot mode protocols specified in the device tree.

The device tree nodes list a number of reboot modes along with a magic value for each. On reboot, an OS implementing the binding would take the reboot command’s argument and match it against the modes in the device tree. If a match is found the associated magic is written to the location referenced in the device tree node.

1.29.1. User API

Devices registered with the reboot mode API gain two parameters:

  • $dev_of_reboot_mode.prev (read-only): The reboot mode that was set previous to barebox startup
  • $dev_of_reboot_mode.next: The next reboot mode, for when the system is reset

The reboot mode driver core use the alias name if available to name the device. By convention, this should end with .reboot_mode, e.g.:

/ {
        aliases {
                gpr.reboot_name = &reboot_name_gpr;
        };
};

Reboot mode providers have priorities. The provider with the highest priority has its parameters aliased as $global.system.reboot_mode.prev and $global.system.reboot_mode.next.

1.29.2. Reset

Reboot modes can be stored on a syscon wrapping general purpose registers that survives warm resets. If the system instead did reset via an external power management IC, the registers may lose their value.

If such reboot mode storage is used, users must take care to use the correct reset provider. In barebox, multiple reset providers may co-exist. They reset command allows listing and choosing a specific reboot mode.

1.29.3. Disambiguation

Some uses of reboot modes partially overlap with other barebox functionality. They all ultimately serve different purposes, however.

1.29.3.1. Comparison to reset reason

The reset reason $global.system.reset is populated by different drivers to reflect the hardware cause of a reset, e.g. a watchdog. A reboot mode describes the OS intention behind a reset, e.g. to fall into a recovery mode. Reboot modes besides the default normal mode usually accompany a reset reason of RST (because the OS intentionally triggered a reset to activate the next reboot mode).

1.29.3.2. Comparison to bootsource

$bootsource reflects the current boot’s medium as indicated by the SoC. In cases where the reboot mode is used to communicate with the BootROM, $bootsource and $bootsource_instance may describe the same device as the reboot mode.

For cases, where the communication instead happens between barebox and an OS, they can be completely different, e.g. $bootsource may say barebox was booted from spi-nor, while the reboot mode describes that barebox should boot the Kernel off an USB flash drive.

1.29.3.3. Comparison to barebox state

barebox state also allows sharing information between barebox and the OS, but it does so while providing atomic updates, redundant storage and optionally wear leveling. In contrast to state, reboot mode is just that: a mode for a single reboot. barebox clears the reboot mode after reading it, so this can be reliably used across one reset only.