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

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Message-ID: <211d9dfa-26e6-4fc3-b70b-f5fbca49e5fd@nvidia.com>
Date: Tue, 3 Feb 2026 20:01:48 +0530
From: Sumit Gupta <sumitg@...dia.com>
To: "Rafael J. Wysocki" <rafael@...nel.org>
Cc: 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, sumitg@...dia.com
Subject: Re: [PATCH v7 4/7] ACPI: CPPC: add APIs and sysfs interface for
 min/max_perf

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.

Thank you,
Sumit Gupta