cpufreq: intel_pstate: Adjust _PSS[0] freqeuency if needed

The maximum turbo P-State used by the intel_pstate driver may be
limited by ACPI _PSS table entry 0. After commit 9522a2ff9cde
(cpufreq: intel_pstate: Enforce _PPC limits), the maximum performance
on servers will be capped by the _PSS table entry 0 by default.

Even though that is formally correct, it may lead to preformance
regressions in some cases. Namely, if the _PSS table entry 0 is
not the maximum turbo P-State, performance measured after commit
9522a2ff9cde will not match the performance measured before that
commit on the same system.

For this reason, modify the code to always use the maximum turbo
frequency as the one that corresponds to _PSS table entry 0 if turbo
is enabled in the BIOS. This way, the performance levels from
before commit 9522a2ff9cde will be restored on the affected systems.

Fixes: 9522a2ff9cde (cpufreq: intel_pstate: Enforce _PPC limits)
Suggested-by: Rafael J. Wysocki <[e-mail]>
Signed-off-by: Srinivas Pandruvada <[e-mail]>
[ rjw : Changelog ]
Signed-off-by: Rafael J. Wysocki <[e-mail]>

drivers/cpufreq/intel_pstate.c

···
 	return acpi_ppc;
 }
 
-/*
- * The max target pstate ratio is a 8 bit value in both PLATFORM_INFO MSR and
- * in TURBO_RATIO_LIMIT MSR, which pstate driver stores in max_pstate and
- * max_turbo_pstate fields. The PERF_CTL MSR contains 16 bit value for P state
- * ratio, out of it only high 8 bits are used. For example 0x1700 is setting
- * target ratio 0x17. The _PSS control value stores in a format which can be
- * directly written to PERF_CTL MSR. But in intel_pstate driver this shift
- * occurs during write to PERF_CTL (E.g. for cores core_set_pstate()).
- * This function converts the _PSS control value to intel pstate driver format
- * for comparison and assignment.
- */
-static int convert_to_native_pstate_format(struct cpudata *cpu, int index)
-{
-	return cpu->acpi_perf_data.states[index].control >> 8;
-}
-
 static void intel_pstate_init_acpi_perf_limits(struct cpufreq_policy *policy)
 {
 	struct cpudata *cpu;
-	int turbo_pss_ctl;
 	int ret;
 	int i;
 
···
 	 * max frequency, which will cause a reduced performance as
 	 * this driver uses real max turbo frequency as the max
 	 * frequency. So correct this frequency in _PSS table to
-	 * correct max turbo frequency based on the turbo ratio.
+	 * correct max turbo frequency based on the turbo state.
 	 * Also need to convert to MHz as _PSS freq is in MHz.
 	 */
-	turbo_pss_ctl = convert_to_native_pstate_format(cpu, 0);
-	if (turbo_pss_ctl > cpu->pstate.max_pstate)
+	if (!limits->turbo_disabled)
 		cpu->acpi_perf_data.states[0].core_frequency =
 					policy->cpuinfo.max_freq / 1000;
 	cpu->valid_pss_table = true;