Weird flash usage report while using TF-M

[caspar.friedrich]

Hi!

If I build my project with TF-M I noticed that at least the flash usage report differs dramatically compared to the original firmware:

Original:

Memory region | Used Size | Region Size | %age Used
        FLASH:   206060 B      409088 B      50.37%
         SRAM:    83760 B      178968 B      46.80%

With TF-M enabled:

Memory region | Used Size | Region Size | %age Used
        FLASH:   193676 B        208 KB      90.93%
         SRAM:    79024 B      178968 B      44.16%

The actual firmware size makes sense so far and both variants boot fine. My expectation was that the sections should be more or less identical.

Regards

Caspar

[Carl Richard]

Hi, Caspar!

The TF-M partitions the flash regions differently compared to Zephyr resulting in your application using occupying a larger %age of the dedicated non-secure region. As mentioned in the Zephyr documentation on the nRF9160 DK you can adjust the size and start address of the flash image using DTS overlays. There are some samples for TF-M in Zephyr that may be worth checking out.

Best regards,
Carl Richard

[caspar.friedrich]

Okay, thanks. I'll look into that.

[Carl Richard]

Great! Please let me know if you face any other issues.

Best regards,
Carl Richard

[caspar.friedrich]

For my understanding the partitions should look like this (DT exerpt):

boot_partition: partition@0 {
    label = "mcuboot";
    reg = < 0x0 0x10000 >;
};
slot0_partition: partition@10000 {
    label = "image-0";
    reg = < 0x10000 0x50000 >;
};
slot0_ns_partition: partition@50000 {
    label = "image-0-nonsecure";
    reg = < 0x60000 0x20000 >;
};
slot1_partition: partition@80000 {
    label = "image-1";
    reg = < 0x80000 0x50000 >;
};
slot1_ns_partition: partition@c0000 {
    label = "image-1-nonsecure";
    reg = < 0xd0000 0x20000 >;
};

But regardless of what I put in here, I the ns-partition size is always 208 kB (aka 0x34000). This matches what I can see in pm_config.h but can't see where those values come from.

[Carl Richard]

Hi again!

I asked the developers about this. You can increase the non-secure partition by decreasing the TFM partition of the pm_manager KConfig. More specifically, you can open <zephyr_root>/nrf/subsys/partition_manager/Kconfig and tweak the partitizon-size following code:

partition=BL2
partition-size=0x8000
rsource "Kconfig.template.partition_size"

partition=TFM
partition-size=0x40000
rsource "Kconfig.template.partition_size"

partition=TFM_EXTRA
partition-size=0x10000
rsource "Kconfig.template.partition_size"

I tested by changing TFM to 0x30000, giving me 272 kB for the non-secure application. Be aware that TFM is large since "everything" is enabled.

Best regards,
Carl Richard