[coreboot] How to change clockchip frequency?

[Marc Jones]

On Wed, Nov 2, 2011 at 6:37 AM, Oskar Enoksson <enok at lysator.liu.se> wrote:
> 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?

It would be hard to tell why Linux resets, but you can look in the
southbridge for a watchdog. You should also read the CPU spec about
changing clock speed etc.

Marc


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