cpufreq: ACPI: Fix max-frequency computation

Commit 3c55e94 ("cpufreq: ACPI: Extend frequency tables to cover
boost frequencies") introduced an assumption in acpi_cpufreq_cpu_init()
that the first entry in the P-state table was the nominal frequency.
This assumption is incorrect. The frequency corresponding to the P0
P-State need not be the same as the nominal frequency advertised via
CPPC.

Since the driver is using the CPPC.highest_perf and CPPC.nominal_perf
to compute the boost-ratio, it makes sense to use CPPC.nominal_freq to
compute the max-frequency. CPPC.nominal_freq is advertised on
platforms supporting CPPC revisions 3 or higher.

Hence, fallback to using the first entry in the P-State table only on
platforms that do not advertise CPPC.nominal_freq.

Fixes: 3c55e94 ("cpufreq: ACPI: Extend frequency tables to cover boost frequencies")
Tested-by: Dhananjay Ugwekar <Dhananjay.Ugwekar@amd.com>
Signed-off-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Link: https://patch.msgid.link/20250113044107.566-1-gautham.shenoy@amd.com
[ rjw: Retain reverse X-mas tree ordering of local variable declarations ]
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/cpufreq/acpi-cpufreq.c

@@ -623,7 +623,14 @@ static int acpi_cpufreq_blacklist(struct cpuinfo_x86 *c)
 #endif
 
 #ifdef CONFIG_ACPI_CPPC_LIB
-static u64 get_max_boost_ratio(unsigned int cpu)
+/*
+ * get_max_boost_ratio: Computes the max_boost_ratio as the ratio
+ * between the highest_perf and the nominal_perf.
+ *
+ * Returns the max_boost_ratio for @cpu. Returns the CPPC nominal
+ * frequency via @nominal_freq if it is non-NULL pointer.
+ */
+static u64 get_max_boost_ratio(unsigned int cpu, u64 *nominal_freq)
 {
 	struct cppc_perf_caps perf_caps;
 	u64 highest_perf, nominal_perf;
@@ -652,6 +659,9 @@ static u64 get_max_boost_ratio(unsigned int cpu)
 
 	nominal_perf = perf_caps.nominal_perf;
 
+	if (nominal_freq)
+		*nominal_freq = perf_caps.nominal_freq;
+
 	if (!highest_perf || !nominal_perf) {
 		pr_debug("CPU%d: highest or nominal performance missing\n", cpu);
 		return 0;
@@ -664,8 +674,12 @@ static u64 get_max_boost_ratio(unsigned int cpu)
 
 	return div_u64(highest_perf << SCHED_CAPACITY_SHIFT, nominal_perf);
 }
+
 #else
-static inline u64 get_max_boost_ratio(unsigned int cpu) { return 0; }
+static inline u64 get_max_boost_ratio(unsigned int cpu, u64 *nominal_freq)
+{
+	return 0;
+}
 #endif
 
 static int acpi_cpufreq_cpu_init(struct cpufreq_policy *policy)
@@ -675,9 +689,9 @@ static int acpi_cpufreq_cpu_init(struct cpufreq_policy *policy)
 	struct acpi_cpufreq_data *data;
 	unsigned int cpu = policy->cpu;
 	struct cpuinfo_x86 *c = &cpu_data(cpu);
+	u64 max_boost_ratio, nominal_freq = 0;
 	unsigned int valid_states = 0;
 	unsigned int result = 0;
-	u64 max_boost_ratio;
 	unsigned int i;
 #ifdef CONFIG_SMP
 	static int blacklisted;
@@ -827,16 +841,20 @@ static int acpi_cpufreq_cpu_init(struct cpufreq_policy *policy)
 	}
 	freq_table[valid_states].frequency = CPUFREQ_TABLE_END;
 
-	max_boost_ratio = get_max_boost_ratio(cpu);
+	max_boost_ratio = get_max_boost_ratio(cpu, &nominal_freq);
 	if (max_boost_ratio) {
-		unsigned int freq = freq_table[0].frequency;
+		unsigned int freq = nominal_freq;
 
 		/*
-		 * Because the loop above sorts the freq_table entries in the
-		 * descending order, freq is the maximum frequency in the table.
-		 * Assume that it corresponds to the CPPC nominal frequency and
-		 * use it to set cpuinfo.max_freq.
+		 * The loop above sorts the freq_table entries in the
+		 * descending order. If ACPI CPPC has not advertised
+		 * the nominal frequency (this is possible in CPPC
+		 * revisions prior to 3), then use the first entry in
+		 * the pstate table as a proxy for nominal frequency.
 		 */
+		if (!freq)
+			freq = freq_table[0].frequency;
+
 		policy->cpuinfo.max_freq = freq * max_boost_ratio >> SCHED_CAPACITY_SHIFT;
 	} else {
 		/*