linux-kernel - Re: [PATCH v4 6/6] cpufreq: CPPC: Support for autonomous selection in cppc

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Message-ID: <21654032-a394-4da9-8ee9-d7cb9df8c855@gmail.com>
Date: Thu, 16 Jan 2025 05:39:04 -0600
From: Russell Haley <yumpusamongus@...il.com>
To: Lifeng Zheng <zhenglifeng1@...wei.com>, rafael@...nel.org,
 lenb@...nel.org, robert.moore@...el.com, viresh.kumar@...aro.org,
 mario.limonciello@....com, gautham.shenoy@....com, ray.huang@....com,
 pierre.gondois@....com
Cc: acpica-devel@...ts.linux.dev, linux-acpi@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org, linuxarm@...wei.com,
 jonathan.cameron@...wei.com, zhanjie9@...ilicon.com, lihuisong@...wei.com,
 hepeng68@...wei.com, fanghao11@...wei.com
Subject: Re: [PATCH v4 6/6] cpufreq: CPPC: Support for autonomous selection in
 cppc_cpufreq

Hello,

I noticed something here just as a user casually browsing the mailing list.

On 1/13/25 6:21 AM, Lifeng Zheng wrote:
> Add sysfs interfaces for CPPC autonomous selection in the cppc_cpufreq
> driver.
> 
> Signed-off-by: Lifeng Zheng <zhenglifeng1@...wei.com>
> ---
>  .../ABI/testing/sysfs-devices-system-cpu      |  54 +++++++++
>  drivers/cpufreq/cppc_cpufreq.c                | 109 ++++++++++++++++++
>  2 files changed, 163 insertions(+)
> 
> diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
> index 206079d3bd5b..3d87c3bb3fe2 100644
> --- a/Documentation/ABI/testing/sysfs-devices-system-cpu
> +++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
> @@ -268,6 +268,60 @@ Description:	Discover CPUs in the same CPU frequency coordination domain
>  		This file is only present if the acpi-cpufreq or the cppc-cpufreq
>  		drivers are in use.
>  

[...snip...]

> +What:		/sys/devices/system/cpu/cpuX/cpufreq/energy_perf
> +Date:		October 2024
> +Contact:	linux-pm@...r.kernel.org
> +Description:	Energy performance preference
> +
> +		Read/write an 8-bit integer from/to this file. This file
> +		represents a range of values from 0 (performance preference) to
> +		0xFF (energy efficiency preference) that influences the rate of
> +		performance increase/decrease and the result of the hardware's
> +		energy efficiency and performance optimization policies.
> +
> +		Writing to this file only has meaning when Autonomous Selection is
> +		enabled.
> +
> +		This file only presents if the cppc-cpufreq driver is in use.

In intel_pstate driver, there is file with near-identical semantics:

/sys/devices/system/cpu/cpuX/cpufreq/energy_performance_preference

It also accepts a few string arguments and converts them to integers.

Perhaps the same name should be used, and the semantics made exactly
identical, and then it could be documented as present for either
cppc_cpufreq OR intel_pstate?

I think would be more elegant if userspace tooling could Just Work with
either driver.

One might object that the frequency selection behavior that results from
any particular value of the register itself might be different, but they
are *already* different between Intel's P and E-cores in the same CPU
package. (Ugh.)

--
Thanks,
Russell