[OpenPower-Firmware] Limit the CPU frequency

[Stewart Smith]

Artem Senichev <artemsen at gmail.com> writes:
> We have some hardware problem with 12-cores P8 CPU running at high
> load with maximum frequency.
> A workaround is to limit the frequency from the OS (for example, using
> `cpupower frequency-set`).
>
> Fixing this issue can take a lot of time, so I want to use the
> workaround as a temporary solution, but I want to set the upper limit
> in the firmware layer to prevent it from being changed by the OS
> kernel.
>
> Is it possible to limit the maximum CPU's frequency from hostboot or
> skiboot?

>From a skiboot perspective, we get possible frequencies from the OCC and
expose these to linux via the device tree. This is done in hw/occ.c

I *believe* the OCC will get this from processor VPD (but don't quote me
on it, that's mostly a pretty educated guess).


If you look at add_cpu_pstate_properties() in hw/occ.c, on p8 you'll see
that parse_pstates_v2() is what fills out the list.

As a hack for your workaround, you could filter the dt_id[] and
dt_freq[] arrays coming out of there to remove the states you don't want
enabled.

There *may* also be a way to do so via MRW.. but I'm not as familiar
with all the options there.

-- 
Stewart Smith
OPAL Architect, IBM.