[PATCH v1 1/2] arm64: topology: fix arch_freq_get_on_cpu() overflow above 4.19 GHz

[Oleg Keri]

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.

Keep the arithmetic in 64 bits until after the shift.

Fixes: 16d1e27475f6 ("arm64: Provide an AMU-based version of arch_freq_get_on_cpu")
Signed-off-by: Oleg Keri <okerixx@xxxxxxxxx>
---
 arch/arm64/kernel/topology.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
index d28438f8b83f..804758eeb63a 100644
--- a/arch/arm64/kernel/topology.c
+++ b/arch/arm64/kernel/topology.c
@@ -245,8 +245,9 @@ 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;
+	scale *= arch_scale_freq_ref(cpu);
+	scale >>= SCHED_CAPACITY_SHIFT;
+	freq = scale;
 	return freq;
 }
 
-- 
2.55.0