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

[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

[caspar.friedrich]

Can confirm, this works. Do I see that right that this mechanism ignores the partitions as defined in the device tree completely?

[Carl Richard]

Great! You do see it right. The partition manager basically ignores the device tree partitions.

Best regards,
Carl Richard

[caspar.friedrich]

Are there plans to change that in the future? Or do I need to adapt my projects accordingly?

[Carl Richard]

Not that I'm aware of and it's not likely as the PM is quite integral for handling applications with varying partitions and sizes. More detailed answers on roadmap questions can be provided by your Regional Sales Manager.

Best regards,
Carl Richard