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

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Date: Thu, 27 Jun 2024 10:12:58 -0500
From: Mario Limonciello <mario.limonciello@....com>
To: "Gautham R.Shenoy" <gautham.shenoy@....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

On 6/27/2024 09:47, Gautham R.Shenoy wrote:
> 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.
> 

Ah; yes this makes more sense then.  I'll work on a modified series 
during next kernel cycle.