cpufreq: scmi: add support for fast frequency switching

The cpufreq core provides option for drivers to implement fast_switch
callback which is invoked for frequency switching from interrupt context.

This patch adds support for fast_switch callback in SCMI cpufreq driver
by making use of polling based SCMI transfer. It also sets the flag
fast_switch_possible.

Cc: linux-pm@vger.kernel.org
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

drivers/cpufreq/scmi-cpufreq.c

@@ -61,6 +61,22 @@ scmi_cpufreq_set_target(struct cpufreq_policy *policy, unsigned int index)
 	return ret;
 }
 
+static unsigned int scmi_cpufreq_fast_switch(struct cpufreq_policy *policy,
+					     unsigned int target_freq)
+{
+	struct scmi_data *priv = policy->driver_data;
+	struct scmi_perf_ops *perf_ops = handle->perf_ops;
+
+	if (!perf_ops->freq_set(handle, priv->domain_id,
+				target_freq * 1000, true)) {
+		arch_set_freq_scale(policy->related_cpus, target_freq,
+				    policy->cpuinfo.max_freq);
+		return target_freq;
+	}
+
+	return 0;
+}
+
 static int
 scmi_get_sharing_cpus(struct device *cpu_dev, struct cpumask *cpumask)
 {
@@ -160,6 +176,7 @@ static int scmi_cpufreq_init(struct cpufreq_policy *policy)
 
 	policy->cpuinfo.transition_latency = latency;
 
+	policy->fast_switch_possible = true;
 	return 0;
 
 out_free_cpufreq_table:
@@ -198,6 +215,7 @@ static struct cpufreq_driver scmi_cpufreq_driver = {
 	.verify	= cpufreq_generic_frequency_table_verify,
 	.attr	= cpufreq_generic_attr,
 	.target_index	= scmi_cpufreq_set_target,
+	.fast_switch	= scmi_cpufreq_fast_switch,
 	.get	= scmi_cpufreq_get_rate,
 	.init	= scmi_cpufreq_init,
 	.exit	= scmi_cpufreq_exit,