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:   Mon, 23 Oct 2023 20:23:49 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Lukasz Luba <lukasz.luba@....com>, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, rafael@...nel.org
Cc:     dietmar.eggemann@....com, rui.zhang@...el.com,
        amit.kucheria@...durent.com, amit.kachhap@...il.com,
        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 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@....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",

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 ?

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