linux-kernel - Re: [PATCH v3] PM: EM: Fix potential division-by-zero error in em_compute

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Message-ID: <0fc11aa1-e329-4f0d-a547-1ead73111088@arm.com>
Date: Tue, 15 Apr 2025 09:50:17 +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 v3] PM: EM: Fix potential division-by-zero error in
 em_compute_costs()



On 4/14/25 10:04, 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 | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
> index d9b7e2b38c7a..fc972cc1fc12 100644
> --- a/kernel/power/energy_model.c
> +++ b/kernel/power/energy_model.c
> @@ -235,7 +235,7 @@ static int em_compute_costs(struct device *dev, struct em_perf_state *table,
>   
>   	/* Compute the cost of each performance state. */
>   	for (i = nr_states - 1; i >= 0; i--) {
> -		unsigned long power_res, cost;
> +		unsigned long power_res, cost = 0;
>   
>   		if ((flags & EM_PERF_DOMAIN_ARTIFICIAL) && cb->get_cost) {
>   			ret = cb->get_cost(dev, table[i].frequency, &cost);
> @@ -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;

LGTM,

Reviewed-by: Lukasz Luba <lukasz.luba@....com>