Re: [PATCH v4 06/18] PM: EM: Check if the get_cost() callback is present in em_compute_costs()

[Lukasz Luba]

On 10/23/23 19:23, Daniel Lezcano wrote:
> On 25/09/2023 10:11, Lukasz Luba wrote:
> > The em_compute_cost() is going to be re-used in runtime modified EM
> > code path. Thus, make sure that this common code is safe and won't
> > try to use the NULL pointer. The former em_compute_cost() didn't have to
> > care about runtime modification code path. The upcoming changes introduce
> > such option, but with different callback. Those two paths which use
> > get_cost() (during first EM registration) or update_power() (during
> > runtime modification) need to be safely handled in em_compute_costs().
> >
> > Signed-off-by: Lukasz Luba <lukasz.luba@xxxxxxx>
> > ---
> >   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 7ea882401833..35e07933b34a 100644
> > --- a/kernel/power/energy_model.c
> > +++ b/kernel/power/energy_model.c
> > @@ -116,7 +116,7 @@ static int em_compute_costs(struct device *dev, 
> > struct em_perf_state *table,
> >       for (i = nr_states - 1; i >= 0; i--) {
> >           unsigned long power_res, cost;
> > -        if (flags & EM_PERF_DOMAIN_ARTIFICIAL) {
> > +        if (flags & EM_PERF_DOMAIN_ARTIFICIAL && cb->get_cost) {
> >               ret = cb->get_cost(dev, table[i].frequency, &cost);
> >               if (ret || !cost || cost > EM_MAX_POWER) {
> >                   dev_err(dev, "EM: invalid cost %lu %d\n",
>
> I do believe & operator has lower precedence than && operator, thus the 
> test is actually:
>
>      (flags & (EM_PERF_DOMAIN_ARTIFICIAL && cb->get_cost))
>
> but it should be
>
>      ((flags & EM_PERF_DOMAIN_ARTIFICIAL) && cb->get_cost)
>
> Right ?

The bitwise '&' is stronger than logical '&&', so the code will
work as in your 2nd example. Although, I will change it and add
parentheses for better reading.

Thanks!