[PATCH v4 1/2] arm64: topology: fix arch_freq_get_on_cpu() overflow above 4.19 GHz

[Oleg Keri <okerixx@gmail.com>]

arch_freq_get_on_cpu() computes the product of the frequency scale and
the reference frequency as a u64, but assigns it to an unsigned int
before shifting it back down:

	freq = scale * arch_scale_freq_ref(cpu);
	freq >>= SCHED_CAPACITY_SHIFT;

The product is truncated to 32 bits before the shift, so the result
wraps once arch_scale_freq_ref() exceeds 2^32 / SCHED_CAPACITY_SCALE,
i.e. 4194304 kHz.

On a Snapdragon X2 Elite (Glymur) laptop, whose boost OPP is 4723200
kHz, cpuinfo_avg_freq reports 524283 kHz instead of ~4723200 kHz while
the CPU demonstrably runs at the boost frequency: a fixed workload
completes in 1.72 s at the 4723200 kHz OPP versus 2.01 s at 4032000
kHz, matching the 1.171 frequency ratio.

Use cap_scale(), which the scheduler already has for exactly this
capacity scaling, so the multiply and the shift stay in 64 bits and
only the final value is narrowed by the return type. Move the macro
from the scheduler's private header to <linux/topology.h> so it can be
used outside kernel/sched.

Fixes: 16d1e27475f6 ("arm64: Provide an AMU-based version of arch_freq_get_on_cpu")
Signed-off-by: Oleg Keri <okerixx@gmail.com>
---
 arch/arm64/kernel/topology.c | 5 +----
 include/linux/topology.h     | 2 ++
 kernel/sched/sched.h         | 2 --
 3 files changed, 3 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
index d28438f8b83f..07b8c497c9e6 100644
--- a/arch/arm64/kernel/topology.c
+++ b/arch/arm64/kernel/topology.c
@@ -186,7 +186,6 @@ int arch_freq_get_on_cpu(int cpu)
 	struct amu_cntr_sample *amu_sample;
 	unsigned int start_cpu = cpu;
 	unsigned long last_update;
-	unsigned int freq = 0;
 	u64 scale;
 
 	if (!amu_fie_cpu_supported(cpu) || !arch_scale_freq_ref(cpu))
@@ -245,9 +244,7 @@ int arch_freq_get_on_cpu(int cpu)
 	 * (see amu_scale_freq_tick for details)
 	 */
 	scale = arch_scale_freq_capacity(cpu);
-	freq = scale * arch_scale_freq_ref(cpu);
-	freq >>= SCHED_CAPACITY_SHIFT;
-	return freq;
+	return cap_scale(arch_scale_freq_ref(cpu), scale);
 }
 
 static void amu_fie_setup(const struct cpumask *cpus)
diff --git a/include/linux/topology.h b/include/linux/topology.h
index 709a2dcf4c73..0a4ee12a98d5 100644
--- a/include/linux/topology.h
+++ b/include/linux/topology.h
@@ -351,4 +351,6 @@ static inline unsigned long topology_get_cpu_scale(int cpu)
 
 void topology_set_cpu_scale(unsigned int cpu, unsigned long capacity);
 
+#define cap_scale(v, s)		((v)*(s) >> SCHED_CAPACITY_SHIFT)
+
 #endif /* _LINUX_TOPOLOGY_H */
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 6c3ad70e58b8..45796fccdc84 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -236,8 +236,6 @@ static inline int task_has_dl_policy(struct task_struct *p)
 	return dl_policy(p->policy);
 }
 
-#define cap_scale(v, s)		((v)*(s) >> SCHED_CAPACITY_SHIFT)
-
 static inline void update_avg(u64 *avg, u64 sample)
 {
 	s64 diff = sample - *avg;
-- 
2.55.0