linux-kernel - Re: [PATCH v4 5/8] ACPI: CPPC: add APIs and sysfs interface for perf

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Message-ID: <c71152d2-acf3-4a06-aea5-164f661bfe2d@nvidia.com>
Date: Tue, 18 Nov 2025 15:50:00 +0530
From: Sumit Gupta <sumitg@...dia.com>
To: Ionela Voinescu <ionela.voinescu@....com>
Cc: rafael@...nel.org, viresh.kumar@...aro.org, lenb@...nel.org,
 robert.moore@...el.com, corbet@....net, pierre.gondois@....com,
 zhenglifeng1@...wei.com, 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 v4 5/8] ACPI: CPPC: add APIs and sysfs interface for
 perf_limited register


On 13/11/25 17:05, Ionela Voinescu wrote:
> External email: Use caution opening links or attachments
>
>
> Hi,
>
> On Wednesday 05 Nov 2025 at 17:08:41 (+0530), Sumit Gupta wrote:
>> Add sysfs interface to read/write the Performance Limited register.
>>
>> The Performance Limited register indicates to the OS that an
>> unpredictable event (like thermal throttling) has limited processor
>> performance. This register is sticky and remains set until reset or
>> OS clears it by writing 0.
>>
>> The interface is exposed as:
>>   /sys/devices/system/cpu/cpuX/cpufreq/perf_limited
> What is the intended use of this interface? The performance limited
> register has a specific format of status bits with feedback about
> performance being limited temporarily and the user can only clear it.
>
> "Contains a resource descriptor with a single Register() descriptor
> that describes the register to read to determine if performance was
> limited. A nonzero value indicates performance was limited. This
> register is sticky, and will remain set until reset or OSPM clears
> it by writing 0. See the section “Performance Limiting” for more
> details." Also, "The performance limited register should only be used
> to report short term, unpredictable events (e.g., PROCHOT being
> asserted)."
>
> Therefore, I'm not seeing the value of exposing this via sysfs.
>
> Thanks,
> Ionela.
>

This enables users to detect if platform throttling impacted a workload.
Users clear the register before execution, run the workload, then check
afterward if set, hardware throttling (thermal/power/current limits)
occurred during that time window. The write operation can be restricted
to only accept 0 (clearing) to prevent writing arbitrary values.

Thank you,
Sumit Gupta

....