Message-ID: <b8603fcb-b0ab-47da-ae90-e82f7b9bcd67@nvidia.com>
Date: Wed, 4 Feb 2026 15:21:10 +0530
From: Sumit Gupta <sumitg@...dia.com>
To: Mario Limonciello <mario.limonciello@....com>,
 "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,
 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 04/02/26 01:58, Mario Limonciello wrote:
> External email: Use caution opening links or attachments
>
>
> On 2/3/26 2:24 PM, Rafael J. Wysocki wrote:
>> 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.
>
> FWIW same thing for amd_pstate.
>

intel_pstate and amd_pstate seem to use setpolicy() to update
scaling_min/max_freq and program MIN_PERF/MAX_PERF.
However, as discussed in v5 [1], cppc_cpufreq cannot switch to
a setpolicy based approach because:
- We need per-CPU control of auto_sel: With setpolicy, we can't
   dynamically disable auto_sel for individual CPUs and return to the
   target() (no target hook available).
   intel_pstate and amd_pstate seem to set HW autonomous mode for
   all CPUs, not per-CPU.
- We need to retain the target() callback - the CPPC spec allows
   desired_perf to be used even when autonomous selection is enabled.

[1] 
https://lore.kernel.org/lkml/66f58f43-631b-40a0-8d42-4e90cd24b757@arm.com/

Thank You,
Sumit Gupta

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