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arm64: topology: fix arch_freq_get_on_cpu() overflow above 4.19 GHz

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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.

Shift the u64 product and narrow only at the return.

Fixes: 16d1e27475 ("arm64: Provide an AMU-based version of arch_freq_get_on_cpu")
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Oleg Keri <okerixx@gmail.com>
Signed-off-by: Will Deacon <will@kernel.org>
This commit is contained in:
Oleg Keri 2026-09-23 14:56:04 +02:00 committed by Will Deacon
parent 1fef816691
commit 3872cc6b92
1 changed files with 2 additions and 4 deletions
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6
arch/arm64/kernel/topology.c
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@ -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 +244,8 @@ 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 (scale * arch_scale_freq_ref(cpu)) >> SCHED_CAPACITY_SHIFT;
}
static void amu_fie_setup(const struct cpumask *cpus)