What to modify in TF-A source code to define BKPSRAM as non-secure?

[JRobi.3]

We're are trying to use BKPSRAM in application code insecurely. According to the stm32mpu BKPSRAM_internal_memory page, we need to "set BKPSRAM as non-secure and degrade low power modes support, removing Standby mode"

Apart from setting the etzpc peripheral in the devicetree to non-secure [DECPROT(STM32MP1_ETZPC_BKPSRAM_ID, DECPROT_NS_RW, DECPROT_UNLOCK)]

what else is necessary in TF-A?

[Olivier GALLIEN]

Hi @JRobi.3​ ,

Take care of the warning banner :

"Default OpenSTLinux delivery prevents to define BKPSRAM as non-secure. This requires to modify TF-A source code with one of the following strategies:

• set BKPSRAM as non-secure and degrade low power modes support, removing Standby mode

This means removing STM32_PM_CSTOP_ALLOW_STANDBY_DDR_SR from the node below :

&pwr_regulators {
	system_suspend_supported_soc_modes = <
		STM32_PM_CSLEEP_RUN
		STM32_PM_CSTOP_ALLOW_LP_STOP
		STM32_PM_CSTOP_ALLOW_LPLV_STOP
		STM32_PM_CSTOP_ALLOW_LPLV_STOP2
		STM32_PM_CSTOP_ALLOW_STANDBY_DDR_SR
	>;
 
	system_off_soc_mode = <STM32_PM_SHUTDOWN>;
	vdd-supply = <&vdd>;
	vdd_3v3_usbfs-supply = <&vdd_usb>;
};

And as said this can imply further modification in TF-A or OP-TEE source code. ( eg : some check of secure state of the BKPSRAM.

Olivier