How to select page size when using MCUboot with external flash memory?

[diocorreia]

Hi!

I'm using an external flash (SPI NOR) to store the secondary partition slot of MCUboot. According to datasheet, the SPI NOR chip in use supports 4KB, 32KB and 64KB paging. Despite this, if I set any other value than 4096 for CONFIG_SPI_NOR_FLASH_LAYOUT_PAGE_SIZE, MCUboot fails to load. Everywhere I look, I see people using or suggesting to use pages with 4096, but cannot understand the underlying reason. One guess I can take is that paging needs to match between the internal (nrf9151) and external flash, but I can't find any info to sustain this hypothesis.

[00:00:00.782,562] <inf> lwm2m_firmware: Image is confirmed OK                                                                              
[00:00:00.782,562] <inf> lwm2m_firmware: Firmware failed to be updated 

[diocorreia]

I think I finally found out the reason behind it. But maybe some confirmation from your side would also be good :D

I think it has to do with the boot swap method I'm using (BOOT_PREFER_SWAP_MOVE) which requires "all sectors in both slots being of the same size". And since pages on internal flash have 4096KiB, that's why I only can use such value on the external flash with secondary slot.

[Hakon]

That seems like a very plausible reason indeed.