[PATCH v3 2/2] cpufreq: Warn if fast_switch is not set

[Wyes Karny <wyes.karny@amd.com>]

If fast_switch_possible flag is set by the scaling driver, the governor
is free to select fast_switch function even if adjust_perf is set. When
the frequency invariance is disabled due to some reason governor
fallbacks to fast_switch if fast_switch_possible is set. This could
crash the kernel if the driver didn't set the fast_switch function
pointer.

This issue becomes apparent when aperf/mperf overflow occurs with
amd_pstate (passive) + schedutil.  When this happens, kernel disables
frequency invariance calculation which causes schedutil to fallback to
sugov_update_single_freq which currently relies on the fast_switch
callback.

Normal flow:
  sugov_update_single_perf
    cpufreq_driver_adjust_perf
      cpufreq_driver->adjust_perf

Error case flow:
  sugov_update_single_perf
    sugov_update_single_freq  <-- This is chosen because the freq invariant is disabled due to aperf/mperf overflow
      cpufreq_driver_fast_switch
         cpufreq_driver->fast_switch <-- Here NULL pointer dereference is happening, because fast_switch is not set

Put up a warning message if the driver sets fast_switch_possible flag
but not fast_switch.

Signed-off-by: Wyes Karny <wyes.karny@amd.com>
---
 drivers/cpufreq/cpufreq.c | 18 ++++++++++++++++++
 include/linux/cpufreq.h   |  5 ++++-
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 6b52ebe5a890..180be9235b08 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -501,6 +501,13 @@ void cpufreq_enable_fast_switch(struct cpufreq_policy *policy)
 	if (!policy->fast_switch_possible)
 		return;
 
+	/**
+	 * It's not expected driver's fast_switch callback is not set
+	 * even fast_switch_possible is true.
+	 */
+	if (!cpufreq_driver_has_fast_switch())
+		pr_alert_once("fast_switch callback is not set\n");
+
 	mutex_lock(&cpufreq_fast_switch_lock);
 	if (cpufreq_fast_switch_count >= 0) {
 		cpufreq_fast_switch_count++;
@@ -2143,6 +2150,17 @@ unsigned int cpufreq_driver_fast_switch(struct cpufreq_policy *policy,
 }
 EXPORT_SYMBOL_GPL(cpufreq_driver_fast_switch);
 
+/**
+ * cpufreq_driver_has_fast_switch - Check "fast switch" callback.
+ *
+ * Return 'true' if the ->fast_switch callback is present for the
+ * current driver or 'false' otherwise.
+ */
+bool cpufreq_driver_has_fast_switch(void)
+{
+	return !!cpufreq_driver->fast_switch;
+}
+
 /**
  * cpufreq_driver_adjust_perf - Adjust CPU performance level in one go.
  * @cpu: Target CPU.
diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
index 26e2eb399484..4277d6f8e50c 100644
--- a/include/linux/cpufreq.h
+++ b/include/linux/cpufreq.h
@@ -340,7 +340,9 @@ struct cpufreq_driver {
 	/*
 	 * ->fast_switch() replacement for drivers that use an internal
 	 * representation of performance levels and can pass hints other than
-	 * the target performance level to the hardware.
+	 * the target performance level to the hardware. If driver is setting this,
+	 * then it needs to set fast_switch also. Because in certain scenario scale
+	 * invariance could be disabled and governor can switch back to fast_switch.
 	 */
 	void		(*adjust_perf)(unsigned int cpu,
 				       unsigned long min_perf,
@@ -604,6 +606,7 @@ struct cpufreq_governor {
 /* Pass a target to the cpufreq driver */
 unsigned int cpufreq_driver_fast_switch(struct cpufreq_policy *policy,
 					unsigned int target_freq);
+bool cpufreq_driver_has_fast_switch(void);
 void cpufreq_driver_adjust_perf(unsigned int cpu,
 				unsigned long min_perf,
 				unsigned long target_perf,
-- 
2.34.1