cpufreq: governor: Ensure sufficiently large sampling intervals

After commit aa7519af (cpufreq: Use transition_delay_us for legacy
governors as well) the sampling_rate field of struct dbs_data may be
less than the tick period which causes dbs_update() to produce
incorrect results, so make the code ensure that the value of that
field will always be sufficiently large.

Fixes: aa7519af (cpufreq: Use transition_delay_us for legacy governors as well)
Reported-by: Andy Tang <andy.tang@nxp.com>
Reported-by: Doug Smythies <dsmythies@telus.net>
Tested-by: Andy Tang <andy.tang@nxp.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>

drivers/cpufreq/cpufreq_governor.c

@@ -22,6 +22,8 @@
 
 #include "cpufreq_governor.h"
 
+#define CPUFREQ_DBS_MIN_SAMPLING_INTERVAL	(2 * TICK_NSEC / NSEC_PER_USEC)
+
 static DEFINE_PER_CPU(struct cpu_dbs_info, cpu_dbs);
 
 static DEFINE_MUTEX(gov_dbs_data_mutex);
@@ -47,11 +49,15 @@ ssize_t store_sampling_rate(struct gov_attr_set *attr_set, const char *buf,
 {
 	struct dbs_data *dbs_data = to_dbs_data(attr_set);
 	struct policy_dbs_info *policy_dbs;
+	unsigned int sampling_interval;
 	int ret;
-	ret = sscanf(buf, "%u", &dbs_data->sampling_rate);
-	if (ret != 1)
+
+	ret = sscanf(buf, "%u", &sampling_interval);
+	if (ret != 1 || sampling_interval < CPUFREQ_DBS_MIN_SAMPLING_INTERVAL)
 		return -EINVAL;
 
+	dbs_data->sampling_rate = sampling_interval;
+
 	/*
 	 * We are operating under dbs_data->mutex and so the list and its
 	 * entries can't be freed concurrently.
@@ -430,7 +436,14 @@ int cpufreq_dbs_governor_init(struct cpufreq_policy *policy)
 	if (ret)
 		goto free_policy_dbs_info;
 
-	dbs_data->sampling_rate = cpufreq_policy_transition_delay_us(policy);
+	/*
+	 * The sampling interval should not be less than the transition latency
+	 * of the CPU and it also cannot be too small for dbs_update() to work
+	 * correctly.
+	 */
+	dbs_data->sampling_rate = max_t(unsigned int,
+					CPUFREQ_DBS_MIN_SAMPLING_INTERVAL,
+					cpufreq_policy_transition_delay_us(policy));
 
 	if (!have_governor_per_policy())
 		gov->gdbs_data = dbs_data;