[PATCH v4 3/3] cpufreq: Return failure if fast_switch is not set and fast_switch_possible is 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.  Some
scaling drivers which use adjust_perf don't set fast_switch thinking
that the governor would never fall back to fast_switch. But the governor
can fall back to fast_switch even in runtime if frequency invariance is
disabled due to some reason. This could crash the kernel if the driver
didn't set the fast_switch function pointer.

Therefore, return failure in cpufreq_online function if fast_switch is
not set and fast_switch_possible is set.

Signed-off-by: Wyes Karny <wyes.karny@amd.com>
---
 drivers/cpufreq/cpufreq.c | 5 +++++
 include/linux/cpufreq.h   | 4 +++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 6b52ebe5a890..7835ba4fa34c 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -1376,6 +1376,11 @@ static int cpufreq_online(unsigned int cpu)
 			goto out_free_policy;
 		}
 
+		if (policy->fast_switch_possible && !cpufreq_driver->fast_switch) {
+			pr_err("fast_switch_possible is enabled but fast_switch callback is not set\n");
+			ret = -EINVAL;
+			goto out_destroy_policy;
+		}
 		/*
 		 * The initialization has succeeded and the policy is online.
 		 * If there is a problem with its frequency table, take it
diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
index 26e2eb399484..8cdf77bb3bc1 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,
-- 
2.34.1