STM32MP15 backup registers

1 Article purpose[edit]

The purpose of this article is to explain how the TAMP backup registers are used by STM32MPU Embedded Software.

2 Overview[edit]

The STM32MP15 embeds 32 backup registers of 32 bits. A programmable border allows to split those backup registers into a secure and a non-secure groupa.

3 Backup registers usage[edit]

This paragraph explains the default backup registers usage by the ROM code and STM32MPU Embedded Software distribution. Then, the next chapter shows the backup register mapping used to fulfill those needs.

Warning white.png Warning
It is important to notice that the backup registers are erased when a tamper detection occurs in TAMP internal peripheral
Info white.png Information
Backup register(s) may be used for another purpose by the application when the feature(s) listed below is/are not used by the said application

3.1 Boot counter feature[edit]

The BOOT_COUNTER is used by U-Boot to detect boot failures between its execution and before the complete Linux application initialization :

  • It is incremented by U-Boot
  • It is reset by the application

So, if U-Boot reads a non null value after a reset, this means that something went wrong at boot time and can be used to enter a fail safe mode.

3.2 Boot mode selection feature[edit]

The BOOT_MODE register is used to propagate boot mode information from one component to the next boot stage, on cold boot or after a reset:

  • The ROM code executes a serial boot if BOOT_MODE[7:0] is equal to 0xFF, as stated in the ROM code boot device selection strategy. In that case, the backup register is reset by the ROM code before proceeding with the serial boot mode. Other values are ignored by the ROM code.
  • TF-A gets the selected boot device from the ROM code context in SYSRAM and writes it into BOOT_MODE[15:8] for U-Boot[1].
  • U-Boot uses the BOOT_MODE register to get TF-A and Linux kernel (as explained in the next bullet) information[1] in order to select the wished boot mode (among "NORMAL", "STM32PROG", ...) and build the appropriate "chosen" node in the device tree given to the Linux kernel, as explained in "Runtime configuration" paragraph in Documentation/devicetree/usage-model.rst .
  • The Linux kernel can force a reboot-mode writing into the BOOT_MODE register. This writing is done via the "reboot" Linux command, that is configured via the compatible "syscon-reboot-mode" in the device tree [2].

3.3 DDR and CPU wake up management feature[edit]

The MAGIC_NUMBER and BRANCH_ADDRESS registers allow to control the DDR initialization and the Arm® Cortex®-A7 CPU cores behaviors on system transitions:

  • On cold boot, the ROM code sets the MAGIC_NUMBER register to 0x0. When the FSBL TF-A BL2 reads a value different from 0xCA7FACE0 (like 0x0) in MAGIC_NUMBER, it performs a complete DDR initialization before jumping to the SSBL (U-Boot).
  • Before entering in Standby with DDR in self-refresh low power mode, the PSCI framework writes the return address where the Arm® Cortex®-A7 core 0 should branch to on wake up into BRANCH_ADDRESS register and it sets the MAGIC_NUMBER register to 0xCA7FACE0.
  • On wakeup from Standby when the FSBL reads the value 0xCA7FACE0 from the MAGIC_NUMBER register, it has to perform a partial DDR initialization to exit Self-Refresh, before branching the Arm® Cortex®-A7 core 0 non-secure execution back to the address given by the BRANCH_ADDRESS register.
  • On startup, the Linux® kernel starts to run on the Arm® Cortex®-A7 core 0 and it uses the PSCI framework to write the address where the core 1 has to jump in BRANCH_ADDRESS register and to set MAGIC_NUMBER register to 0xCA7FACE1. Those values are then interpreted by the ROM code, as explained in secondary core boot paragraph.

3.4 Cortex-M4 wake up feature[edit]

The ROM code can autonomously start the Arm Cortex®-M4 with a firmware kept in RETRAM after a Standby low power mode period:

Warning white.png Warning
This feature is enabled in the ROM code but not implemented in STM32 MPU Embedded Software

3.5 Cortex-M4 management feature[edit]

Two registers are used to control the Arm Cortex®-M4 coprocessor from the Arm Cortex®-A7:

  • The COPRO_RSC_TBL_ADDRESS register owns the Coprocessor resource table base address: it is read by the Linux Kernel to retrieve the resource table when the Cortex-M4 firmware is loaded by U-Boot or OP-TEE.
  • The CORTEX_M_STATE register reflects the Cortex-M4 state : it is written by U-Boot or OP-TEE when loading the firmware, it can be updated by the Cortex-M4 firmware at runtime and it is read by the Linux Kernel to know the coprocessor state.

4 Memory mapping[edit]

The table below shows the backup register mapping used by STM32MPU Embedded Software.
The TAMP backup register base address is 0x5C00A100, corresponding to TAMP_BKP0R.

TAMP register Securitya ROM / software register name Comment
TAMP_BKP31R Non-secure M4_WAKEUP_AREA_HASH SHA-256 value, see Cortex-M4 wake up feature
TAMP_BKP30R Non-secure
TAMP_BKP29R Non-secure
TAMP_BKP28R Non-secure
TAMP_BKP27R Non-secure
TAMP_BKP26R Non-secure
TAMP_BKP25R Non-secure
TAMP_BKP24R Non-secure
TAMP_BKP23R Non-secure M4_WAKEUP_AREA_LENGTH See Cortex-M4 wake up feature
TAMP_BKP22R Non-secure M4_WAKEUP_AREA_START See Cortex-M4 wake up feature
TAMP_BKP21R Non-secure BOOT_COUNTER See Boot counter feature
TAMP_BKP20R Non-secure BOOT_MODE See Boot mode selection feature
TAMP_BKP19R Non-secure (Reserved for future use)
TAMP_BKP18R Non-secure CORTEX_M_STATE See Cortex-M4 management feature
TAMP_BKP17R Non-secure COPRO_RSC_TBL_ADDRESS See Cortex-M4 management feature
TAMP_BKP16R Non-secure (Reserved for future use)
TAMP_BKP15R Non-secure (Reserved for future use)
TAMP_BKP14R Non-secure (Reserved for future use)
TAMP_BKP13R Non-secure (Reserved for future use)
TAMP_BKP12R Non-secure (Reserved for future use)
TAMP_BKP11R Non-secure (Reserved for future use)
TAMP_BKP10R Non-secure (Reserved for future use)
TAMP_BKP9R Secure (Reserved for future use)
TAMP_BKP8R Secure (Reserved for future use)
TAMP_BKP7R Secure (Reserved for future use)
TAMP_BKP6R Secure (Reserved for future use)
TAMP_BKP5R Secure BRANCH_ADDRESS See DDR and CPU wake up management feature
TAMP_BKP4R Secure MAGIC_NUMBER See DDR and CPU wake up management feature
TAMP_BKP3R Secure M4_SECURITY_PERIMETER_EXTI3 See Cortex-M4 wake up feature
TAMP_BKP2R Secure M4_SECURITY_PERIMETER_EXTI2 See Cortex-M4 wake up feature
TAMP_BKP1R Secure M4_SECURITY_PERIMETER_EXTI1 See Cortex-M4 wake up feature
TAMP_BKP0R Secure WAKEUP_SEC See Cortex-M4 wake up feature

a: the security border is configured by the Secure OS (look for "zone" in plat/st/stm32mp1/sp_min/sp_min_setup.c for TF-A SP-MIN or in core/arch/arm/plat-stm32mp1/main.c for OP-TEE), so the source code has to be modified if a different border is needed.

5 References[edit]