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

Gautham R.Shenoy
2024-06-27T14:48:19Z

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.

--
Thanks and Regards
gautham.