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

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Message-ID: <CAJZ5v0iE_iw+pSBppEWnJw=2=DFNa-J2VPDorTNF=Mve+0PNCg@mail.gmail.com>
Date: Tue, 15 Apr 2025 19:17:45 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Yaxiong Tian <iambestgod@...com>
Cc: lukasz.luba@....com, rafael@...nel.org, linux-pm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Yaxiong Tian <tianyaxiong@...inos.cn>
Subject: Re: [PATCH v3] PM: EM: Fix potential division-by-zero error in em_compute_costs()

On Mon, Apr 14, 2025 at 11:09 AM Yaxiong Tian <iambestgod@...com> 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")

Please look at the Fixes: tags in the kernel git history.  They don't
look like the one above.

> 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)) {

Can't you just check this upfront at the beginning of the function and
make it bail out if dev is not a CPU device?

>                         /* increase resolution of 'cost' precision */
>                         power_res = table[i].power * 10;
>                         cost = power_res / table[i].performance;
> --