How ARM Systems are Booted: An Introduction to the ARM Boot Flow

In the first of our talks at ELC-E, Rouven Czerwinski gave an introduction into booting on ARM64 on Tuesday, as part of the "Embedded Essentials" talk series. The idea of this new series is to give newcomers a good insight into some more "general purpose", not highly specialized, topics. The concept seems to work, as the room was well filled.

In contrast to ARM32, where there are basically only the bootloader and the kernel and two exception levels, on ARM64 there are more components involved.

The first stage is the ROM code (BL1), and mass storage access is vendor specific. For development, they provide some sort of serial upload. This stage loads the 2nd stage (BL2), which is either U-Boot SPL, a barebox PBL or TF-A.

ARM Trusted Firmware, TF-A, implements several standard firmware runtime services these days, such as PSCI or SCMI. Unfortunately, TF-A is MIT licensed, so the vendors are not required to publish their source code or go through some well justified review system (such as the kernel's). Besides secure boot mechanics, the DDR frequency scaling is done here, as well as the Power State Coordination Interface (PSCI).

SCMI, the System Control and Memory Interface, takes care of switching on and off CPU cores and changing clocks; these services must live in TF-A, as even if Linux wants a clock to be off, the secure world might want to use it.

The actual bootloader then starts in exception level 2, is able to for example parse a devicetree hardware description and takes care of the usual interfaces that make it possible to get a kernel from somewhere.

In contrast to ARM32, there is no standard decompression in the kernel, so the bootloader needs to take care, then masks interrupts, initializes the standard ARM timer and loads the kernel to a defined offset. The MMU and data caches are disabled, the address of the device tree is put into register 0 and the bootloader jumps into the kernel.

All Attendees Reception

As in 2015 (when ELC-E was in Dublin for the last time), the attendee party took place in the Guinness Storehouse. The building is a great place to meet, but unfortunately, due to the ELC-E being part of the Open Source Summit, it's always a little bit difficult to find other embedded people among the thousands of cloud and Kubernetes and Hyperledger folks at these mega events.

However, the evening was a nice opportunity to talk and find out about what the other embedded Linux hackers did during the Corona years when meeting in person was not possible.