immutable bootloader vs immutable+upgradable bootloader

[ziv123]

hello Nordic

i am working with nrf52840, with zephyr and mcuboot

i have read that there can be first stage bootloader (immutable bootloader), and a second stage bootloader (upgradable bootloader) in the following links

https://developer.nordicsemi.com/nRF_Connect_SDK/doc/1.3.0/nrf/samples/bootloader/README.html

https://developer.nordicsemi.com/nRF_Connect_SDK/doc/1.3.0/nrf/ug_bootloader.html#ug-bootloader

my question is what should be my considerations when deciding to work with upgradable bootloader ?

(i assume having another bootloader takes more memory, in my system i have an additional external flash and i will DFU via OTA (BLE). 

what are the prose and cones for using only immutable bootloader vs using upgradable bootloader as well ?

(i thought that the use of secondary bootloader is for cases when there is not enough memory to run complete rtos so the bootloader is divided to stages of boot but to my understanding it is not the case with the nrf52840 running with zephyr .. am i correct ? )

hope to read from you soon

best regards

Ziv

[Amanda Hsieh]

Hi, 

If you want the bootloader to perform firmware updates for only upgrading an application, then use the single-stage, immutable bootloader solution.

If you want the bootloader to support firmware updates for both itself and the application, then use the two-stage, upgradable bootloader solution.

Regards,
Amanda

[ziv123]

hi Amanda

this much i understand

the question was when, or in what scenarios, or for what use cases will i want the ability to upgrade my bootloader 

considering i work with nrf52840 with zephyr which currently has no space issue when booting the whole zephyr application (including zephyr's ble stack implemented and compiled in it)

hope to read from you soon

best regards

Ziv

[Amanda Hsieh]

Hi,

ziv123 said:

the question was when, or in what scenarios, or for what use cases will i want the ability to upgrade my bootloader 

Maybe one day you would like to update the bootloader with new features, modifications, and so on. 

-Amanda 

[ziv123]

hi Amanda 

i am not that experienced, only few years as embedded engineer and i was actually hoping to get some examples as for what possible features i might want to add that are not possible to implement in the application itself, or better implemented in a second stage bootloader, that justify the extra space in memory that a second bootlodaer will take ?

( my system is currently built in zephyr using mcuboot as first stage bootloader )

 hope to read from you soon

best regards

Ziv

[Amanda Hsieh]

Hi, 

Not sure what feature you need, but I mean it's provided by Zephyr. If you don't get any issues, then don't worry about that. 

-Amanda