Bootloader using modm

[ASMfreaK]

If I were to write a bootloader using modm or import some third-party one, like mentioned in private by @salkinium (https://github.com/mcu-tools/mcuboot), what steps should I follow?
Can I add a separate target to my firmware? Can modm produce an amalgamation firmware with bootloader attached? Do I need to write linkerscripts for it?

[salkinium]

Can I add a separate target to my firmware?

The bootloader and the application would be separate projects with their own project.xml files.

Can modm produce an amalgamation firmware with bootloader attached?

No, not natively. You would need to write a small bash script that calls scons/make bin for each and then concatenates the files correctly. Alternatively you can program the bootloader first, then use that to upload the application.

Do I need to write linkerscripts for it?

Not necessarily, the application would have the lbuild option modm:platform:cortex-m:linkerscript.flash_offset set to the bootloader size and then everything is shifted upwards automatically. The bootloader itself probably also doesn't need a special linkerscript.

[salkinium]

But I'm not sure if importing an existing bootloader project into modm is even useful?
Something like https://github.com/dmitrystu/sboot_stm32 already has a lot of devices supported, not sure if modm would help there.

[salkinium]

There's also a related issue here: #625

[ASMfreaK]

Not necessarily, the application would have the lbuild option modm:platform:cortex-m:linkerscript.flash_offset set to the bootloader size and then everything is shifted upwards automatically. The bootloader itself probably also doesn't need a special linkerscript.

So bootloader's main should look like this (psedocode and psaudoalgorithm):

int main(){
    initialize_needed_hardware();
    if(recieved_new_firmware()){
        unlock_flash();
        flash_firmware();
        lock_flash();
    }
    // now somehow reset the mcu and jump back to payload firmware?
}

I'm curious if can I reset the mcu with a different reset vector?

[salkinium]

Yes. To jump to the application you can reboot, then check a specific RAM location for the "boot application" magic number, then jump to the application. At least that's how some bootloaders do it.
You can even use the modm_initialize_platform() function, which is called very early, to save time, instead of waiting until main.

[salkinium]

I'm curious if can I reset the mcu with a different reset vector?

No, it's hardcoded to 0x0000'0004 by the Cortex-M core, then STM32 maps this to 0x0800'0000 for Flash or other addresses (system memory etc) via the BOOTn pins and user options bytes.