Linux Changes For CPPC On Zen 6 Clients - Hard-Coded Default Puts Low-Power Core At 0.47x

Following yesterday's Linux kernel patches for tuning ACPI EPP for Ryzen Zen 6 client processors, another patch series made it out on the mailing list with adjustments for ACPI Collaborative Processor Performance Control (CPPC) with upcoming Zen 6 client processors. CPPC is the ACPI standard for interacting between the OS and platform for managing CPU core frequencies and power scaling. AMD CPUs have supported CPPC for a while now but for Zen 6 client CPUs there are some changes abound. Zen 6 client (non-server) platforms are said to use CPPC "differently than previous platforms" and in turn needing to know a maximum frequency for a core type in order to determine capacity calculations. Future ACPI specification revisions are to add a new interface for reading the maximum frequency of a core type while for now there are hard-coded values being added to the kernel for Zen 6 client CPUs. "Zen6 client platforms use CPPC differently than previous platforms and knowing maximum frequency for a core type is needed for capacity calculations. Newer versions of the ACPI will introduce a proper interface for getting this information, but for now hardcode values so that these systems perform as intended and to unblock future scheduler work while we continue to wait." For Zen 6 and newer, the boost ratio is being calculated from frequency values rather than the arbitrary performance value scale. These hard-coded values put the Zen 6 performance cores at 5025, the Zen 6 efficiency cores at 3524, and the new low-power core type at 2399. This follows the Linux preparations for the new AMD low-power core type debuting in Zen 6. So for what it's worth, the low-power core type is 0.68x that of the efficiency (dense) core or 0.47x that of the full performance core. But it's just a hard-coded value for all Zen 6 clients in the Model 80 series and not known how accurately tuned this hard-coded default is. Plus it's just a hard-coded frequency scale value and no word yet on how "low power" these new low-power cores will be.... Should be interesting. We'll see if rough approximate lives up for roughly what to expect out of the new low-power core type once the next-gen Ryzen hardware launches. These new AMD Zen 6 CPPC patches are out for review on the Linux kernel mailing list. As they are too late for v7.3, the earliest we'll see them mainlined is for Linux 7.4. It's looking like Linux 7.4 will be important for next-gen Ryzen processor support. Linux 7.4 may also squeeze in for a late December release and in turn would then be this year's Linux LTS version.