Code relocation to the external flash with Mcuboot

[Shuhao Dong]

Hello,

I am using edge impulse library to achieve on-device classification. I now want to put the edge impulse related code to the external flash and use XIP function there rather than placing them inside my internal flash. I have learned something from the code relocation sample from NCS and zephyr. I now have my own linker script, modified CMakeList and prj.conf, which I show below. For cmakelist: 

zephyr_code_relocate(FILES src/feature/gait_analysis.c LOCATION EXTFLASH_TEXT NOCOPY)
zephyr_code_relocate(FILES src/feature/gait_analysis.c LOCATION EXTFLASH_RODATA NOCOPY)

For linker script: 

#include <zephyr/linker/sections.h>
#include <zephyr/devicetree.h>
#include <zephyr/linker/linker-defs.h>
#include <zephyr/linker/linker-tool.h>

MEMORY
{
    /* XIP region for Edge Impulse model code + rodata */
    EXTFLASH (rx) : ORIGIN = 0x10120000, LENGTH = 0x00400000
}
#include <zephyr/arch/arm/cortex_m/scripts/linker.ld>

and for prj.conf: 

CONFIG_CODE_DATA_RELOCATION=y
CONFIG_XIP=y
CONFIG_NORDIC_QSPI_NOR_XIP=y
CONFIG_HAVE_CUSTOM_LINKER_SCRIPT=y
CONFIG_CUSTOM_LINKER_SCRIPT="linker_arm_nocopy.ld"

. The build process can show part of the code is indeed placed into our external flash partition. But it is complaining about this: Error: Image size (0x10110be8) + trailer (0x630) exceeds requested size 0xe0000. I suspect this is because the relocated section is still included in the mcuboot binary image. We will need to keep mcuboot because we want to use dfu-util on our host machine for firmware upgrade. Is there a way to solve this issue? 

[PjGlasso]

Hi there,

SO ,

Key point: MCUboot assumes the application image is one contiguous region of flash.
It uses __rom_start__ and __rom_end__ from the linker to compute “image size”. That’s just:

image_size = __rom_end__ - __rom_start__

If any section is placed at 0x1012_0000 (external QSPI region), __rom_end__ becomes ~0x1012xxxx. So from the point of view of imgtool/MCUboot, your “image” appears to span from address 0x0000xxxx all the way up to 0x1012xxxx. That’s why the “image size” looks like 0x10110be8 – it’s not the amount of code, it’s the distance between the lowest and highest load address.

So:

• the relocated XIP section is still part of the MCUboot image, and it completely blows up the image size calculation.

• Stock MCUboot does not understand “one image spread over disjoint flash ranges (internal + QSPI XIP)” as a single signed binary.

   Short version:

You can’t just drop XIP sections into external flash and still expect a normal, single MCUboot image (with DFU, signatures, trailers, etc.) to “just work”.

You have to choose one of these design patterns:

Option 1 – Keep MCUboot simple, treat external flash as a separate asset

Concept:

• The “MCUboot image” lives entirely in internal flash.

• External QSPI flash holds model / feature code or data, but it is not part of the MCUboot image.

• You program/update external flash via some other mechanism (second DFU alt-setting, custom mcumgr command, dedicated updater image, etc.).

What to change technically:

1. Do not add EXTFLASH to MEMORY in your application linker script in a way that moves __rom_end__.

2. Keep the normal Zephyr/MCUboot linker flow for internal flash.

3. For QSPI XIP sections, either:

   • Build a second image or raw binary linked at 0x10120000, and flash it separately, or

   • Keep the model as data and explicitly read/execute from it (e.g., copy critical parts to RAM before running).

This is the least fragile route if you “must keep MCUboot + dfu-util”.

Option 2 – Multi-image / external-flash partition managed by NCS

If you really want a “single update story”, then:

• Use NCS Partition Manager (pm_static.yml) to define:

  • mcuboot, image_0 (primary), image_1 or scratch in internal flash.

  • A separate extflash_app (or ei_model) partition in external flash, not part of image_0.

HTH

GL :-) PJ :v:

[Shuhao Dong]

Hi, yes that bit I can understand now. I am following your option 2 now, which is to create another image but to save data. This works quite well for my self defined source code like gait_analysis.c. But when I try to move edge impulse library, this won't work. There is no error, but the edge impulse code is still there in the internal flash. 

However as I said earlier, I can see moving edge impulse library to the external flash is mentioned in nordic official page. So I was wondering how can I actually achieve that.