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Trusted Firmware-M Integration

The Trusted Firmware-M (TF-M) section contains information about the integration between TF-M and Zephyr RTOS. Use this information to help understand how to integrate TF-M with Zephyr for Cortex-M platforms and make use of its secure run-time services in Zephyr applications.

Board Definitions

TF-M will be built for the secure processing environment along with Zephyr if the CONFIG_BUILD_WITH_TFM flag is set to y.

Generally, this value should never be set at the application level, however, and all config flags required for TF-M should be set in a board variant with the _ns suffix.

This board variant must define an appropriate flash, SRAM and peripheral configuration that takes into account the initialisation process in the secure processing environment. CONFIG_TFM_BOARD must also be set via modules/trusted-firmware-m/Kconfig.tfm to the board name that TF-M expects for this target, so that it knows which target to build for the secure processing environment.

Example: mps2_an521_ns

The mps2_an521 target is a dual-core Arm Cortex-M33 evaluation board that, when using the default board variant, would generate a secure Zephyr binary.

The optional mps2_an521_ns target, however, sets these additional kconfig flags that indicate that Zephyr should be built as a non-secure image, linked with TF-M as an external project, and optionally the secure bootloader:

• CONFIG_TRUSTED_EXECUTION_NONSECURE y

• CONFIG_ARM_TRUSTZONE_M y

Comparing the mps2_an521.dts and mps2_an521_ns.dts files, we can see that the _ns version defines offsets in flash and SRAM memory, which leave the required space for TF-M and the secure bootloader:

reserved-memory {
            #address-cells = <1>;
            #size-cells = <1>;
            ranges;

            /* The memory regions defined below must match what the TF-M
             * project has defined for that board - a single image boot is
             * assumed. Please see the memory layout in:
             * https://git.trustedfirmware.org/TF-M/trusted-firmware-m.git/tree/platform/ext/target/mps2/an521/partition/flash_layout.h
             */

            code: memory@100000 {
                    reg = <0x00100000 DT_SIZE_K(512)>;
            };

            ram: memory@28100000 {
                    reg = <0x28100000 DT_SIZE_M(1)>;
            };
    };

This reserves 1 MB of code memory and 1 MB of RAM for secure boot and TF-M, such that our non-secure Zephyr application code will start at 0x10000, with RAM at 0x28100000. 512 KB code memory is available for the NS zephyr image, along with 1 MB of RAM.

This matches the flash memory layout we see in flash_layout.h in TF-M:

* 0x0000_0000 BL2 - MCUBoot (0.5 MB)
* 0x0008_0000 Secure image     primary slot (0.5 MB)
* 0x0010_0000 Non-secure image primary slot (0.5 MB)
* 0x0018_0000 Secure image     secondary slot (0.5 MB)
* 0x0020_0000 Non-secure image secondary slot (0.5 MB)
* 0x0028_0000 Scratch area (0.5 MB)
* 0x0030_0000 Protected Storage Area (20 KB)
* 0x0030_5000 Internal Trusted Storage Area (16 KB)
* 0x0030_9000 NV counters area (4 KB)
* 0x0030_A000 Unused (984 KB)

mps2/an521 will be passed in to Tf-M as the board target, specified via CONFIG_TFM_BOARD.