What *exactly* happens at boot?

[pfeerick]

It would be nice with precise documentation of what's happening during boot. So far, I couldn't find anything. Is there a BIOS or similar? What's the boot order? Can I modify it? What's the difference between with and without eMMC? And so on.

Processor on the sopine compute modules is the same as the pine64 - the A64. Meaning the boot order is

Code:

A64 boot-> check fel key pressed (yes)--> FEL mode (boot from USB OTG)
                          \  
                      (no) \
                            \-------> 1) try to boot from SMHC0 (SD card)
                                      2) try to boot from SMHC2 (eMMC)
                                      3) try to boot from SPI0 (SPI NOR Flash)
                                      4) FEL mode (boot from USB OTG)

These are ARM processors - there is no BIOS with arm. There is uboot, which performs a similar role. Hopefully you have a serial console cable, or 3v3 compatible usb to serial adapter, as you really need that when starting out / debugging these boards.

You can't change the boot order, it's hard coded in the A64. But it can be effectively changed if you boot uBoot from the SPI chip, at which point uBoot determines the boot order. However, I don't know if this has been done with the sopine/clusterboard yet, so it could be murky territory!

Only the first slot has access to the eMMC. So if you have the eMMC populated, you don't have to have a bootable microSD in the first sopine module, unless you want to boot from SD, and use the eMMC as bulk storage.

The idea was that the first slot would be the master, and provide PXE network boot to the remaining six modules, but there hasn't been enough developer-level interest in the clusterboard for it to really take off as it should have.