Re: [PATCH v2] PM: EM: Fix potential division-by-zero error in em_compute_costs()

[Yaxiong Tian]

在 2025/4/14 16:08, Lukasz Luba 写道:
> Hi Yaxiong,
>
> On 4/11/25 02:28, Yaxiong Tian wrote:
> > From: Yaxiong Tian <tianyaxiong@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> > ---
> >   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

Sorry, the V3 version with cost=0 still has issues.

I noticed that if the cost is set to 0, the condition "if (table[i].cost
 >= prev_cost)" in the following code will always evaluate to true. This
 will incorrectly set the flags to EM_PERF_STATE_INEFFICIENT.

Should we change ">=" to ">"?