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Index: website/content/en/status/report-2026-01-2026-03/cppc.adoc
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+=== Collaborative Processor Performance Control (CPPC)
+
+Contact: ShengYi Hung <aokblast@FreeBSD.org> +
+Contact: Olivier Certner <olce@FreeBSD.org>
+
+Collaborative Processor Performance Control (CPPC) is a standard introduced by
+ACPI to allow the OS to manage performance and, conversely, efficiency levels of
+CPUs thanks to an abstract performance scale in general uncorrelated to and more
+fine-grained than mere frequency levels.
+Intel and AMD have been providing CPU implementations in support of ACPI CPPC
+for several years now.
+
+FreeBSD had been supporting enabling CPPC but only for Intel processors and
+allowing to manage a useful but very limited subset of its functionality, thanks
+to the man:hwpstate_intel[4] driver added in 2020.
+Hardware autonomous selection of the performance target depending on the
+workload is forcibly enabled, and only the main corresponding hardware tunable,
+called Efficiency/Performance Preference (EPP), is exported to the administrator
+via a man:sysctl[8] knob.
+
+We have added support for AMD CPPC's implementation in the existing
+man:hwpstate_amd[4] driver which, contrary to man:hwpstate_intel[4], so far
+managed only "regular" P-states.
+The driver exports 4 man:sysctl[8] knobs: Minimum performance, maximum
+performance, desired performance and EPP.
+Minimum, maximum and desired performances are values between 0 and 255, but only
+a sub-range may have an effect depending on the hardware.
+Initial values of minimum and maximum performances are set to the effective
+sub-range bounds as instructed by the platform (if available).
+The EPP control serves to express a bias towards efficiency or performance, and
+is a value between 0 (maximum performance preference) and 255 (maximum
+efficiency preference).
+The desired performance may be set to any value between minimum and maximum
+performance, or to the special value 0 to enable hardware autonomous selection
+of target performance by the hardware depending on the current workload.
+The minimum performance, maximum performance and EPP controls apply regardless
+of whether autonomous selection is enabled or a specific desired performance
+specified.
+Note that the effect of each combination of these values depend on the CPU
+model, and we have already been able to observe wildly different behaviors on
+a few ones.
+Therefore, you should expect to have to experiment to find the values adapted to
+your use cases on a given machine.
+
+man:hwpstate_amd[4] is included by the `GENERIC` kernel (through man:cpufreq[4])
+and uses CPPC if the CPUs support it unless explicitly instructed otherwise
+(through the `machdep.hwpstate_amd_cppc_enable` tunable).
+Consequently, in order to avoid performance regressions, for the time being we
+have decided to set the above-mentioned controls for maximum performance, as
+this is the default behavior for traditional P-state support and also that of
+any other man:cpufreq[4] driver except for man:hwpstate_intel[4] (which
+currently forces hardware autonomous selection and sets EPP to 0x80 (50%) by
+default).
+This may be revised later depending on whether we can reliably determine if the
+running computer is a laptop.
+
+Next steps are:
+
+1. Modify man:hwpstate_intel[4] to be on par with man:hwpstate_amd[4]'s CPPC
+ support in terms of functionality and default behavior.
+ This includes:
+ - Better error-handling and debugging output
+ - Exporting knobs for all the above-mentioned controls
+ - Change the scale of EPP (from percents to an 8-bit value)
+ - Change the default values
+1. Write a manual page for man:hwpstate_amd[4] (in the meantime, the
+ explanations here and the embedded man:sysctl[8] knobs' documentation should
+ be enough).
+1. Teach man:powerd[8] the CPPC control knobs and some simple policies on how to
+ set them.
+1. Teach man:cpufreq[4] about the abstract performance values, to provide
+ a unified interface to retrieve or set them.
+1. Make man:cpufreq[4] support per-CPU settings.
+1. Select default control values based on the platform type (probably from
+ ACPI's ``FADT``'s `Preferred_PM_Profile` field).
+1. Possibly move man:powerd[8] policies to kernel space.
+
+Sponsor: The FreeBSD Foundation

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