linux-kernel - Re: [PATCH v2] PM: EM: Fix potential division-by-zero error in em_compute

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Message-ID: <143378b0-5740-4f2b-9a79-f04cf9ef1f77@arm.com>
Date: Mon, 14 Apr 2025 09:08:57 +0100
From: Lukasz Luba <lukasz.luba@....com>
To: Yaxiong Tian <iambestgod@...com>
Cc: linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Yaxiong Tian <tianyaxiong@...inos.cn>, rafael@...nel.org
Subject: Re: [PATCH v2] PM: EM: Fix potential division-by-zero error in
 em_compute_costs()

Hi Yaxiong,

On 4/11/25 02:28, Yaxiong Tian wrote:
> From: Yaxiong Tian <tianyaxiong@...inos.cn>
> 
> When the device is of a non-CPU type, table[i].performance won't be
> initialized in the previous em_init_performance(), resulting in division
> by zero when calculating costs in em_compute_costs().
> 
> Since the 'cost' algorithm is only used for EAS energy efficiency
> calculations and is currently not utilized by other device drivers, we
> should add the _is_cpu_device(dev) check to prevent this division-by-zero
> issue.
> 
> Fixes: <1b600da51073> ("PM: EM: Optimize em_cpu_energy() and remove division")
> Signed-off-by: Yaxiong Tian <tianyaxiong@...inos.cn>
> ---
>   kernel/power/energy_model.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
> index d9b7e2b38c7a..d1fa7e8787b5 100644
> --- a/kernel/power/energy_model.c
> +++ b/kernel/power/energy_model.c
> @@ -244,7 +244,7 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
>   					cost, ret);
>   				return -EINVAL;
>   			}
> -		} else {
> +		} else if (_is_cpu_device(dev)) {
>   			/* increase resolution of 'cost' precision */
>   			power_res = table[i].power * 10;
>   			cost = power_res / table[i].performance;


As the test robot pointed out, please set the 'cost' to 0
where it's declared.

The rest should be fine.

Regards,
Lukasz