ports/stm32f4: Keep bootloader within first 32K

[petejohanson]

In order to free up sectors 2 and 3 for the application, limit the bootloader to the first 16K sector, config to the next 16K. We keep the application still starting at 0x10000 so that 0x8000 and 0xC000 sectors can be used by the application for settings/config storage.

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Description of Change

While trying to set up an stm32f401cb based board to run ZMK (using Zephyr under the hood), I ran into an issue trying to use Zephy's NVS driver for persistence of settings. Zephyr's NVS expects two sectors of 16K, which basically means I need to use sectors 2 and 3 for this:

I was able to make this work with the correct partitioning on the Zephyr/ZMK side, and properly fit the bootloader into the first two sectors with the changes in this PR. The device runs, and properly store settings as expected.

[petejohanson]

So, based on the CI failure... A few things are clear:

• tinyuf2 only fits with bare minimum features enabled. Feather build fails, for example, when the neopixel code gets built too. Setting the number of pixels to 0 for that board makes the build pass again here locally.
• I realize that changing this also means the config location would change for a lot of boards, probably breaking things for existing devices.

Thoughts on making this an optional different linker file/set of offsets that boards can opt in to?

[petejohanson]

Ok, I've made this change "opt in" by setting COMPACT_BOOTLOADER in board.mk, so existing boards don't have any change to the flash usage. Thoughts?

[kbladewht]

Great work. one question I tried, seems after this change , we still can't change starting at 0x10000 ? to 0x0C000 ? is there any restrict on APP Start Addr ?

[petejohanson]

Great work. one question I tried, seems after this change , we still can't change starting at 0x10000 ? to 0x0C000 ? is there any restrict on APP Start Addr ?

I haven't tried, but I don't see why you couldn't. In my case, I specifically need the two sectors starting at 0xC000 for application settings storage, so I want to keep the same start address.

[kbladewht]

Thanks your reply, yes, I am not sure where is the issue, after check log , it stuck on __set_MSP(sp); from board.c