cpufreq/amd-pstate: Set the initial min_freq to lowest_nonlinear_freq

According to the AMD architectural programmer's manual volume 2 [1], in
section "17.6.4.1 CPPC_CAPABILITY_1" lowest_nonlinear_perf is described
as "Reports the most energy efficient performance level (in terms of
performance per watt). Above this threshold, lower performance levels
generally result in increased energy efficiency. Reducing performance
below this threshold does not result in total energy savings for a given
computation, although it reduces instantaneous power consumption". So
lowest_nonlinear_perf is the most power efficient performance level, and
going below that would lead to a worse performance/watt.

Also, setting the minimum frequency to lowest_nonlinear_freq (instead of
lowest_freq) allows the CPU to idle at a higher frequency which leads
to more time being spent in a deeper idle state (as trivial idle tasks
are completed sooner). This has shown a power benefit in some systems,
in other systems, power consumption has increased but so has the
throughput/watt.

Modify the initial policy_data->min set by cpufreq-core to
lowest_nonlinear_freq, in the ->verify() callback. Also set the
cpudata->req[0] to FREQ_QOS_MIN_DEFAULT_VALUE (i.e. 0), so that it also
gets overriden by the check in verify function.

Link: https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/programmer-references/24593.pdf [1]

Signed-off-by: Dhananjay Ugwekar <[email protected]>
Reviewed-by: Mario Limonciello <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Mario Limonciello <[email protected]>

Author: dhananjay-AMD

drivers/cpufreq/amd-pstate.c

@@ -529,8 +529,27 @@ static void amd_pstate_update(struct amd_cpudata *cpudata, u32 min_perf,
 
 static int amd_pstate_verify(struct cpufreq_policy_data *policy_data)
 {
+	/*
+	 * Initialize lower frequency limit (i.e.policy->min) with
+	 * lowest_nonlinear_frequency which is the most energy efficient
+	 * frequency. Override the initial value set by cpufreq core and
+	 * amd-pstate qos_requests.
+	 */
+	if (policy_data->min == FREQ_QOS_MIN_DEFAULT_VALUE) {
+		struct cpufreq_policy *policy = cpufreq_cpu_get(policy_data->cpu);
+		struct amd_cpudata *cpudata;
+
+		if (!policy)
+			return -EINVAL;
+
+		cpudata = policy->driver_data;
+		policy_data->min = cpudata->lowest_nonlinear_freq;
+		cpufreq_cpu_put(policy);
+	}
+
 	cpufreq_verify_within_cpu_limits(policy_data);
 	pr_debug("policy_max =%d, policy_min=%d\n", policy_data->max, policy_data->min);
+
 	return 0;
 }
 
@@ -979,7 +998,7 @@ static int amd_pstate_cpu_init(struct cpufreq_policy *policy)
 		policy->fast_switch_possible = true;
 
 	ret = freq_qos_add_request(&policy->constraints, &cpudata->req[0],
-				   FREQ_QOS_MIN, policy->cpuinfo.min_freq);
+				   FREQ_QOS_MIN, FREQ_QOS_MIN_DEFAULT_VALUE);
 	if (ret < 0) {
 		dev_err(dev, "Failed to add min-freq constraint (%d)\n", ret);
 		goto free_cpudata1;