Update documentation on linking Zephyr within a flash partition

[mnkp]

PR #12108 "Shift DTS before Kconfig" modified how flash partitions are linked. Before #12108 was merged the process was as follows:

If the chosen node has no zephyr,code-partition property, the application image link uses the entire flash device. If a zephyr,code-partition property is defined, the application link will be restricted to that partition.

this is no longer true.

Following documentation needs to be updated:

• "Linking Zephyr Within a Partition"
• the help text of BOOTLOADER_MCUBOOT Kconfig option in ./Kconfig.zephyr
• majority of board dts files that define flash partitions find boards/ -name "*.dts" | xargs grep -l partitions document linking process within flash node.

[ioannisg]

Yeah, now it is more like all nodes have the code-partition, somehow defined in DTS of the boards, but it is only used if there is no BOOTLOADER, correct?

[mnkp]

It seems like the flash partition layout and setting of zephyr,code-partition property is taken into account only if BOOTLOADER_MCUBOOT Kconfig option is set.

[psidhu]

This affects adafruit_trinket_m0. It's required that the adafruit_trinket_m0 boots from 0x2000 due to their UF2 bootloader.

zephyr,code-partition isn't taken into account because BOOTALODER_MCUBOOT isn't set apparently. So currently, no one can boot zephyr on the adafruit_trinket_m0 unless they physically change where flash begins e.g. something like:

	soc {
		nvmctrl: nvmctrl@41004000  {
			flash0: flash@0 {
				reg = <0x2000 0x3E000>;
			};
		};
	};

I'm sure there are other boards affected by this.

[sslupsky]

See my comments PR #20740 and the documentation about flash partitions here https://github.com/zephyrproject-rtos/zephyr/blob/master/doc/guides/dts/flash_partitions.inc

The Adafruit board dts is correct. What is missing is the defconfig should include

CONFIG_USE_CODE_PARTITION=y