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/cpu0/ns
The mps2/an521/cpu0 board target is a dual-core Arm Cortex-M33 evaluation board that generates
a secure Zephyr binary.
The optional mps2/an521/cpu0/ns board 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:
Comparing the boards/arm/mps2/mps2_an521_cpu0.dts and
boards/arm/mps2/mps2_an521_cpu0_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.