Re: [PATCH 2/2] cpufreq: amd-pstate: Use amd_get_highest_perf() to lookup perf values

Mario Limonciello
2024-06-27T15:13:04Z

On 6/27/2024 09:47, Gautham R.Shenoy wrote:
> Mario Limonciello <mario.limonciello@amd.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.