linux-kernel - Re: [PATCH 2/2] cpufreq: amd-pstate: Use amd_get_highest

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Date: Thu, 27 Jun 2024 20:17:58 +0530
From: Gautham R.Shenoy <gautham.shenoy@....com>
To: Mario Limonciello <mario.limonciello@....com>, Borislav Petkov
	<bp@...en8.de>, Perry Yuan <perry.yuan@....com>
CC: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	"Dave Hansen" <dave.hansen@...ux.intel.com>, "maintainer:X86 ARCHITECTURE
 (32-BIT AND 64-BIT)" <x86@...nel.org>, "H . Peter Anvin" <hpa@...or.com>,
	Huang Rui <ray.huang@....com>, "Rafael J . Wysocki" <rafael@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>, Nikolay Borisov
	<nik.borisov@...e.com>, "Peter Zijlstra" <peterz@...radead.org>, "open
 list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <linux-kernel@...r.kernel.org>,
	"open list:AMD PSTATE DRIVER" <linux-pm@...r.kernel.org>
Subject: Re: [PATCH 2/2] cpufreq: amd-pstate: Use amd_get_highest_perf() to
 lookup perf values

Mario Limonciello <mario.limonciello@....com> writes:

> On 6/27/2024 00:12, Gautham R.Shenoy wrote:

[..snip..]
>> 
>>> -	return CPPC_HIGHEST_PERF_MAX;
>>> +	/*
>>> +	 * For AMD CPUs with Family ID 19H and Model ID range 0x70 to 0x7f,
>>> +	 * the highest performance level is set to 196.
>>> +	 * https://bugzilla.kernel.org/show_bug.cgi?id=218759
>>> +	 */
>>> +	if (cpu_feature_enabled(X86_FEATURE_ZEN4)) {
>>> +		switch (c->x86_model) {
>>> +		case 0x70 ... 0x7f:
>>> +			return CPPC_HIGHEST_PERF_PERFORMANCE;
>>> +		default:
>>> +			return CPPC_HIGHEST_PERF_DEFAULT;
>>                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>> Should this be CPPC_HIGHEST_PERF_MAX ?
>> 
>> Without this patchset, this function returns 255 on Genoa (0x10-0x1f)
>> and Bergamo (0xa0-0xaf) systems. This patchset changes the return value
>> to 166.
>> 
>> The acpi-cpufreq driver computes the max frequency based on the
>> boost-ratio, which is the ratio of the highest_perf (returned by this
>> function) to the nominal_perf.
>> 
>> So assuming a nominal_freq of 2000Mhz, nominal_perf of 159.
>> 
>> Previously the max_perf = (2000*255/159) ~ 3200Mhz
>> With this patch max_perf = (2000*166/159) ~ 2100Mhz.
>> 
>> Am I missing something ?
>
> Yeah; this is exactly what I'm worried about.
>
> How does Bergamo handle amd-pstate?  It should probably explode there
> too.

So amd-pstate driver calls amd_pstate_highest_perf_set() only when
hw_prefcore is set.

Thus for Genoa and Bergamo, since hw_prefcore is false, the highest_perf
is extracted from the MSR_AMD_CPPC_CAP1. See this fragment in
pstate_init_perf()


	/* For platforms that do not support the preferred core feature, the
	 * highest_pef may be configured with 166 or 255, to avoid max frequency
	 * calculated wrongly. we take the AMD_CPPC_HIGHEST_PERF(cap1) value as
	 * the default max perf.
	 */
	if (cpudata->hw_prefcore)
		highest_perf = amd_pstate_highest_perf_set(cpudata);
	else
		highest_perf = AMD_CPPC_HIGHEST_PERF(cap1);

Hence it doesn't blow up on amd-pstate. So it looks like it would be
better if the prefcore check is in the amd_get_highest_perf() function
so that it can be invoked from both acpi-cpufreq and amd-pstate drivers.

--
Thanks and Regards
gautham.