Re: [PATCH v4 6/6] cpufreq: CPPC: Support for autonomous selection in cppc_cpufreq

Mario Limonciello
2025-01-17T14:30:48Z

On 1/16/2025 21:11, zhenglifeng (A) wrote:
> On 2025/1/16 19:39, Russell Haley wrote:
>
>> 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@huawei.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@vger.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.)
>
> Yes, I should use the same name. Thanks.
>
> As for accepting string arguments and converting them to integers, I don't
> think it is necessary. It'll be a litte confused if someone writes a raw
> value and reads a string I think. I prefer to let users freely set this
> value.
>
> In addition, there are many differences between the implementations of
> energy_performance_preference in intel_pstate and cppc_cpufreq (and
> amd-pstate...). It is really difficult to explain all this differences in
> this document. So I'll leave it to be documented as present for
> cppc_cpufreq only.

At least the interface to userspace I think we should do the best we can
to be the same between all the drivers if possible.

For example; I've got a patch that I may bring up in a future kernel
cycle that adds raw integer writes to amd-pstates
energy_performance_profile to behave the same way intel-pstate does.

>
>>
>> --
>> Thanks,
>> Russell
>>
>>
>>
>