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

[Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>]

On Sun,  6 Sep 2026 18:37:14 +0200
Oleg Keri <okerixx@gmail.com> wrote:

> 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@gmail.com>
> ---
>  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;

Maybe just do it all one line?

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

I was going to suggest that you roll in the arch_scale_freq_capacity() as well
but then we'd need a cast to ensure maths was done in 64 bits and that was uglier.

I think this should end up the same as what you have.

Thanks,

Jonathan


> +	freq = scale;
>  	return freq;
>  }
>