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

From: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>
To: Oleg Keri <okerixx@gmail.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Beata Michalska <beata.michalska@arm.com>,
	Sumit Gupta <sumitg@nvidia.com>,
	Prasanna Kumar T S M <ptsm@linux.microsoft.com>,
	Russell King <linux@armlinux.org.uk>,
	Paul Walmsley <pjw@kernel.org>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Alexandre Ghiti <alex@ghiti.fr>,
	Sudeep Holla <sudeep.holla@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Danilo Krummrich <dakr@kernel.org>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org,
	driver-core@lists.linux.dev, linux-pm@vger.kernel.org
Subject: Re: [PATCH v1 1/2] arm64: topology: fix arch_freq_get_on_cpu() overflow above 4.19 GHz
Date: Wed, 9 Sep 2026 11:36:25 -0700	[thread overview]
Message-ID: <20260909113625.000063be@oss.qualcomm.com> (raw)
In-Reply-To: <08ee55292c1cedf26067d1a2b02fa5657d9650e8.1788712186.git.okerixx@gmail.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;
>  }
>  