How to configure TF-A SP-MIN

1. Article Purpose

This section details the TF-A SP-MIN (BL32) component. It will explain the usage in STM32 MPU runtime context and the build process from sources and how to deploy it on your target.
The build examples are based on the OpenSTLinux environment:

• Developer Package
• Distribution Package

2. Overview

TF-A SP-MIN is a secure partition that can be used for the ST boot chain.
This secure partition executed in monitor mode is a secure implementation for limited services:

• PSCI
• SCMI
• SiP services

Warning

TF-A SP-MIN images must be embedded in the FIP binary that is loaded by BL2 and can be automatically authentified

OP-TEE OS must be chosen for a most complete secure implementation.

3. Configuration

TF-A SP-MIN is based on device tree configuration.
Its device tree is also loaded by the TF-A BL2 (called TOS_FW_CONFIG) and the address is given to SP-MIN as second argument. It allows the SP-MIN code to remain generic and adapt the board compatibility based on the device tree.

4. Memory layout

SP-MIN binary (bl32.bin) is embedded in the FIP binary and identified as tos-fw.
The SP-MIN device tree is also embedded in the FIP and identified as tos-fw-config.

The SP-MIN is built with the PIE option which make the code executable from a configurable address. The load addresses for SP-MIN and the device tree is configurable thanks to TF-A firmware configuration framework. If required, the load address could be independently change by modifying the firmware configuration file.

5. Source code access and build process

Cross compilation of TF-A SP-MIN is only required if it is to be modified.
By default, in the Starter Package, the TF-A SP-MIN images are embedded in the trusted FIP: fip-<board>-trusted.bin.
If changes are made, you must rebuild TF-A (SP-MIN) and update the FIP. You must have to udpate the associated FIP partitions of your boot device with this new image.

The build process creates a TF-A SP-MIN image and its device tree.

5.1. Developer Package

5.1.1. Install sources

The Developer Package contains OpenSTLinux and TF-A sources: TF-A Installation

5.1.2. Official source tree

Download source code from the official Trusted Firmware-A github.

  git clone https://github.com/ARM-software/arm-trusted-firmware.git

Warning

The STM32MP1 platform is not yet fully upstreamed. Depending on the version used, some features may not be available.

For a full feature software, a STMicroelectronics github is available:

  git clone https://github.com/STMicroelectronics/arm-trusted-firmware.git

5.1.3. Build Process

5.1.3.1. Initialize the cross compile environment

Setup Cross compile environment

5.1.3.2. TF-A Build flags

Here is the list of the mandatory flags that needs to be specify to complete the BL2 build:

• ARM_ARCH_MAJOR=7: the major version of ARM Architecture to target (STM32MP1 is ARMv7 architecture based)
• ARCH=aarch32: specify aarch32 architecture to be built
• PLAT=stm32mp1: builds an stm32mp1 platform
• DTB_FILE_NAME=<fdt file name>.dtb: this must be defined to build the proper target and include the correct DTB file into the final file
• AARCH32_SP=sp_min: select SP-MIN as secure partition

Optional flags:

• DEBUG=1: add debug information in all binaries
• V=1: print verbose compilation traces

5.1.4. Build command

Warning

Please read carefully

You must add your own environment flags:

  unset LDFLAGS;
  unset CFLAGS;

Then you will have to compile the TF-A SP-MIN (BL32).
To avoid binary override, it is recommended to specify BUILD_PLAT per selected storage.

The default build command for STM32MP15 is:

  make -j4 ARM_ARCH_MAJOR=7 ARCH=aarch32 PLAT=stm32mp1 AARCH32_SP=sp_min \
         DTB_FILE_NAME=<board_name>.dtb BUILD_PLAT=build/sp_min bl32 dtbs

Here is build command for the stm32mp157c-ev1 board:

  make -j4 ARM_ARCH_MAJOR=7 ARCH=aarch32 PLAT=stm32mp1 AARCH32=sp_min \
         DTB_FILE_NAME=stm32mp157c-ev1.dtb BUILD_PLAT=build/sp_min/ bl32 dtbs
 

From the Developer Package tarball, a Makefile.sdk is present and must be used to build the target. It automatically sets the proper configuration for the TF-A build.

  make -f Makefile.sdk TF_A_CONFIG=trusted TF_A_DEVICETREE=<board>

The latest version of the helper file is also available in GitHub: README_HOWTO.txt .

5.1.5. Final image

Final images are available for updating the FIP binary (including the associated firmware configuration file):

<BUILD_PLAT>/sp_min/bl32.bin
<BUILD_PLAT>/sp_min/fdts/<board>.dtb
<BUILD_PLAT>/sp_min/fdts/<board>-fw-config.dtb
Ex:
build/sp_min/bl32.bin
build/sp_min/fdts/stm32mp157c-ev1.dtb
build/sp_min/fdts/stm32mp157c-ev1-fw-config.dtb

5.2. Distribution Package

For an OpenSTLinux distribution, the TF-A SP-MIN image is built in release mode by default. The yocto recipe can be found in:

meta-st/meta-st-stm32mp/recipes-bsp/trusted-firmware-a/tf-a-stm32mp_<version>.bb

If you want to modify the TF-A SP-MIN code source, use the following steps starting from an already downloaded and built OpenSTLinux distribution.

5.2.1. Access sources

You can use devtool to access the source.

  cd <baseline root directory>
  devtool modify tf-a-stm32mp sources/boot/tf-a

By going to the sources/boot/tf-a folder, you can manage and modify the TF-A sources. To rebuild it, go back to the build-<distribution> folder and launch the TF-A recipe:

  bitbake tf-a-stm32mp

The final image is deployed in the image default output folder.

5.3. Update software

SP_MIN binary and its associated firmware are part of the FIP binary.
The next step to deploy the SP-MIN firmware is to update the FIP binary following the FIP update process.

5.4. Extra tips

When generating the SP-MIN, it is also possible in a single line to generate the associated FIP with the following command.
Ex for STM32MP15 platform:

  make -j4 ARM_ARCH_MAJOR=7 ARCH=aarch32 PLAT=stm32mp1 AARCH32_SP=sp_min \
         DTB_FILE_NAME=<board_name>.dtb BL33=<u-boot_path>/u-boot-nodtb.bin \
         BL33_CFG=<u-boot_path>/u-boot.dtb <BUILD_PLAT=build/sp_min bl32 dtbs fip

Trusted Firmware for Arm^® Cortex^®-A

Secure Payload minimal

Boot Loader stage 3-2

Microprocessor Unit

Power State Coordination Interface

System control and management interface

Silicon Provider

Firmware Image Package is a packaging format used by TF-A

Position-Independent Executable - NEW

Device Tree Binary (or Blob)

Boot Loader stage 3-3