I bricked my mainboard… and recovered it using a Raspberry Pi!
So yeah, this is an MSI X370 SLI PLUS. A new firmware update came out, so I unpacked it onto a USB stick and started the upgrade process. With an overclocking profile that sometimes takes a few tries to boot (memory training) and with the stick inserted into a front panel USB port.
The process almost completed (or maybe completed?), the board tried to reboot… and it didn't boot. As in, power comes on, the CPU and RAM debug LEDs turn on and off, then nothing. Clearing CMOS didn't help.
So I looked at this MSI forum thread that contains the pinout and people arguing whether 3.3V is safe for a 1.8V EEPROM chip. I took a 5V-3.3V level shifter (the only one I had), plugging 3.3V into the "5V" side got me down to 2.9V.
And then I tried to connect to the tiny pins of the SPI header (thanks MSI for providing one!) using my large wires. This was rather painful. Couldn't connect ground at all (because that was 3 pins next to each other, it didn't fit). But without GND or VCC, flashrom actually detected an unknown chip! With some random experimentation I found that setting a low speed (1024) and plugging in only VCC totally works! Writing this from the recovered computer :)