linux-kernel - Re: [PATCH v7 4/7] ACPI: CPPC: add APIs and sysfs interface for min/max

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Message-ID: <CAJZ5v0if=tMiyLB-efkzB67SniJS-2pCVv1-eN+vzZxqrdAM8Q@mail.gmail.com>
Date: Tue, 3 Feb 2026 21:24:50 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Sumit Gupta <sumitg@...dia.com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Russell Haley <yumpusamongus@...il.com>, 
	"zhenglifeng (A)" <zhenglifeng1@...wei.com>, pierre.gondois@....com, viresh.kumar@...aro.org, 
	ionela.voinescu@....com, corbet@....net, rdunlap@...radead.org, 
	ray.huang@....com, gautham.shenoy@....com, mario.limonciello@....com, 
	perry.yuan@....com, zhanjie9@...ilicon.com, linux-pm@...r.kernel.org, 
	linux-acpi@...r.kernel.org, linux-doc@...r.kernel.org, 
	acpica-devel@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	linux-tegra@...r.kernel.org, treding@...dia.com, jonathanh@...dia.com, 
	vsethi@...dia.com, ksitaraman@...dia.com, sanjayc@...dia.com, 
	nhartman@...dia.com, bbasu@...dia.com
Subject: Re: [PATCH v7 4/7] ACPI: CPPC: add APIs and sysfs interface for min/max_perf

On Tue, Feb 3, 2026 at 3:32 PM Sumit Gupta <sumitg@...dia.com> wrote:
>
>
> On 03/02/26 18:24, Rafael J. Wysocki wrote:
> > External email: Use caution opening links or attachments
> >
> >
> > On Tue, Feb 3, 2026 at 1:45 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
> >> On Tue, Feb 3, 2026 at 10:41 AM Sumit Gupta <sumitg@...dia.com> wrote:
> >>>>>> Hi Sumit,
> >>>>>>
> >>>>>> I am thinking that maybe it is better to call these two sysfs interface
> >>>>>> 'min_freq' and 'max_freq' as users read and write khz instead of raw
> >>>>>> value.
> >>>>> Thanks for the suggestion.
> >>>>> Kept min_perf/max_perf to match the CPPC register names
> >>>>> (MIN_PERF/MAX_PERF), making it clear to users familiar with
> >>>>> CPPC what's being controlled.
> >>>>> The kHz unit is documented in the ABI.
> >>>>>
> >>>>> Thank you,
> >>>>> Sumit Gupta
> >>>> On my x86 machine with kernel 6.18.5, the kernel is exposing raw values:
> >>>>
> >>>>> grep . /sys/devices/system/cpu/cpu0/acpi_cppc/*
> >>>> /sys/devices/system/cpu/cpu0/acpi_cppc/feedback_ctrs:ref:342904018856568
> >>>> del:437439724183386
> >>>> /sys/devices/system/cpu/cpu0/acpi_cppc/guaranteed_perf:63
> >>>> /sys/devices/system/cpu/cpu0/acpi_cppc/highest_perf:88
> >>>> /sys/devices/system/cpu/cpu0/acpi_cppc/lowest_freq:0
> >>>> /sys/devices/system/cpu/cpu0/acpi_cppc/lowest_nonlinear_perf:36
> >>>> /sys/devices/system/cpu/cpu0/acpi_cppc/lowest_perf:1
> >>>> /sys/devices/system/cpu/cpu0/acpi_cppc/nominal_freq:3900
> >>>> /sys/devices/system/cpu/cpu0/acpi_cppc/nominal_perf:62
> >>>> /sys/devices/system/cpu/cpu0/acpi_cppc/reference_perf:62
> >>>> /sys/devices/system/cpu/cpu0/acpi_cppc/wraparound_time:18446744073709551615
> >>>>
> >>>> It would be surprising for a nearby sysfs interface with very similar
> >>>> names to use kHz instead.
> >>>>
> >>>> Thanks,
> >>>>
> >>>> Russell Haley
> >>> I can rename to either of the below:
> >>> - min/max_freq: might be confused with scaling_min/max_freq.
> >>> - min/max_perf_freq: keeps the CPPC register association clear.
> >>>
> >>> Rafael, Any preferences here?
> >> On x86 the units in CPPC are not kHz and there is no easy reliable way
> >> to convert them to kHz.
> >>
> >> Everything under /sys/devices/system/cpu/cpu0/acpi_cppc/ needs to be
> >> in CPPC units, not kHz (unless, of course, kHz are CPPC units).
>
>
> In v1 [1], these controls were added under acpi_cppc sysfs.
> After discussion, they were moved under cpufreq, and [2] was merged first.
> The decision to use frequency scale instead of raw perf was made
> for consistency with other cpufreq interfaces as per (v3 [3]).
>
> CPPC units in our case are also not in kHz. The kHz conversion uses the
> existing cppc_perf_to_khz()/cppc_khz_to_perf() helpers which are already
> used in cppc_cpufreq attributes. So the conversion behavior is consistent
> with existing cpufreq interfaces.
>
> [1]
> https://lore.kernel.org/lkml/076c199c-a081-4a7f-956c-f395f4d5e156@nvidia.com/
> [2]
> https://lore.kernel.org/all/20250507031941.2812701-1-zhenglifeng1@huawei.com/
> [3]
> https://lore.kernel.org/lkml/80e16de0-63e4-4ead-9577-4ebba9b1a02d@nvidia.com/
>
> > That said, the new attributes will show up elsewhere.
> >
> > So why do you need to add these things in the first place?
>
> Currently there's no sysfs interface to dynamically control the
> MIN_PERF/MAX_PERF bounds when using autonomous mode. This helps
> users tune power and performance at runtime.

So what about scaling_min_freq and scaling_max_freq?

intel_pstate uses them for an analogous purpose.