I have a chip called CDC960 on my motherboard. It is responsible for generating the bus frequency for the cpu, the pci bus etc. I downloaded the datasheet and noticed that I can change the base frequency by +/-10% by sending an smbus command to the cdc960.
When I sent the smbus commands from a running linux the system froze for a few seconds and then rebooted. But after reboot the frequency was indeed changed (overclocked) and everything worked just fine.
I tried to add the smbus command to romstage.c in my coreboot, but that didn't work. The boot process just hanged immediately after the smbus command had been sent.
I guess the running CPU (or something else in the system) cannot handle the frequency change and needs to be reset immediately after the command is sent. Perhaps some watchdog under Linux resets the system after a few seconds of irresponsiveness. But what is the recommended way in coreboot?