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

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Date:   Tue, 26 Sep 2023 20:46:11 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        rafael@...nel.org, dietmar.eggemann@....com, rui.zhang@...el.com,
        amit.kucheria@...durent.com, amit.kachhap@...il.com,
        daniel.lezcano@...aro.org, viresh.kumar@...aro.org,
        len.brown@...el.com, pavel@....cz, mhiramat@...nel.org,
        qyousef@...alina.io, wvw@...gle.com
Subject: Re: [PATCH v4 06/18] PM: EM: Check if the get_cost() callback is
 present in em_compute_costs()

On Mon, Sep 25, 2023 at 10:11 AM Lukasz Luba <lukasz.luba@....com> 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().

I would just say something like this:

"Subsequent changes will introduce a case in which cb->get_cost may
not be set in em_compute_costs(), so add a check to ensure that it is
not NULL before attempting to dereference it."

The rest of the changelog is just redundant IMO.

>
> Signed-off-by: Lukasz Luba <lukasz.luba@....com>
> ---
>  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",
> --
> 2.25.1
>