POWER9 uses a single CPU register, per core, to change clock speed. Everything else is handled by the on-chip controller. This change necessitates a change to the cpufreq global kernel driver to bump supported levels, as the POWER9 device tree can have theoretically 256 different options. On my POWER9 Talos, the list consists of 100 items. At 16.67MHz intervals, that allows for a change of roughly 1.67GHz between lowest and highest.
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Currently it's very specific, because of the masking involved. I just checked on the POWER8, and that device tree also does include the requisite properties, but the format appears different from the POWER9, so I'm reluctant at this time to open this up to that as well. Once I figure out the POWER8 format, I can make it work for that with this same driver.