Boot chain overview

Applicable for

STM32MP13x lines, STM32MP15x lines

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1. Generic boot sequence

1.1. Linux start-up

Starting Linux^® on a processor is done in several steps that progressively initialize the platform peripherals and memories. These steps are explained in the following paragraphs and illustrated by the diagram on the right, which also gives typical memory sizes for each stage.

1.1.1. ROM code

The ROM code is a piece of software that takes its name from the read only memory (ROM) where it is stored. It fits in a few tens of Kbytes and maps its data in embedded RAM. It is the first code executed by the processor, and it embeds all the logic needed to select the boot device (serial link or flash) from which the first-stage bootloader (FSBL) is loaded to the embedded RAM.
Most products require to trust the application that is running on the device and the ROM code is the first link in the chain of trust that must be established across all started components: this trust is established by authenticating the FSBL before starting it. In turn, the FSBL and each following component authenticates the next one, up to a level defined by the product manufacturer.

1.1.2. First stage bootloader (FSBL)

Among other things, the first stage bootloader (FSBL) initializes (part of) the clock tree and the external RAM controller. Finally, the FSBL loads the second-stage bootloader (SSBL) into the external RAM and jumps to it.

The Trusted Firmware-A (TF-A BL2) is FSBL for Cortex-A developed by TrustedFirmware.org community.

1.1.3. Second-stage bootloader (SSBL)

The second-stage bootloader (SSBL) runs in a wide RAM so it can implement complex features (such as, USB, Ethernet, display), that are very useful to make Linux kernel loading more flexible (from a storage device on USB or on a network), and user-friendly (by showing a splash screen to the user). U-Boot is commonly used as a Linux bootloader in embedded systems.

1.1.4. Linux kernel space

The Linux kernel is started in the external memory and it initializes all the peripheral drivers that are needed on the platform.

1.1.5. Linux user space

Finally, the Linux kernel hands control to the user space starting the init process that runs all initialization actions described in the root file system (rootfs), including the application framework that exposes the user interface (UI) to the user.

1.2. Other services start-up

In addition to  Linux  startup, the boot chain also installs the secure monitor and may support coprocessor firmware loading.

For instance, for the STM32MP15, the boot chain starts:

• The  secure monitor , supported by the Arm^® Cortex^®-A secure context (TrustZone). Examples of use of a secure monitor are: user authentication, key storage, and tampering management.
• The  coprocessor  firmware, running on the Arm Cortex-M core. This can be used to offload real-time or low-power services.

The dotted lines in the diagram on the right mean that:

• The  coprocessor  can be started by the second stage bootloader (SSBL), known as “early boot”, or Linux kernel (by default).

Linux^® is a registered trademark of Linus Torvalds.

Read only memory

Random access memory (Early computer memories generally hadserial access. Memories where any given address can be accessed when desired were then called "random access" to distinguish them from the memories where contents can only be accessed in a fixed order. The term is used today for volatile random-acces ssemiconductor memories.)

First stage boot loader

Second stage boot loader

Cortex^®

User interface

Arm^® is a registered trademark of Arm Limited (or its subsidiaries) in the US and/or elsewhere.

TrustZone^®
Arm^® and TrustZone^® are registered trademarks of Arm Limited (or its subsidiaries) in the US and/or elsewhere.