Bootloader customization limits

[fahirad]

Hello.

To what extent are we able to customize the bootloader? I am familiar with examples, and am using bootloader_override as a reference. If the size of the bootloader is not an issue, is it possible to connect to the internet, access a server and perform ota from the bootloader itself? This would mean using components such as esp_wifi, esp_https_ota...

Generally, what is the procedure of customizing it - from the readme file I gather you copy/paste the components you need in the bootloader_component folder, correct?

Thanks for the help.

[Sprite]

You're mostly limited by the fact that the bootloader does not have the MMU set up yet, so your bootloader can only be as big as fits into RAM. In practice, you're also limited by what the bootloader currently does: it does not have FreeRTOS or a memory allocator or even a standard C library set up, so anything that depends on that (=anything that has any blocking call, anything that calls malloc() and friends) won't compile. That includes most drivers, the WiFi stack and others.

By the way, OTA'ing the bootloader generally is not a smart idea. There's no 'fallback bootloader' (as there is for a proper OTA app) so if an OTA is interrupted (e.g. because of power loss) or when the OTA'ed bootloader is bad, the device won't work anymore.

[fahirad]

Got it, thank you a lot!