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Subject[PATCH v4 3/3] cpufreq: Return failure if fast_switch is not set and fast_switch_possible is set
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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
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