linux-kernel - Re: [PATCH] cpufreq, intel_pstate.c, Fix rounding errors

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Date:	Thu, 19 Nov 2015 10:16:39 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Srinivas Pandruvada <srinivas.pandruvada@...el.com>,
	Len Brown <len.brown@...el.com>,
	Alexandra Yates <alexandra.yates@...el.com>,
	Kristen Carlson Accardi <kristen@...ux.intel.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>, linux-pm@...r.kernel.org
Subject: Re: [PATCH] cpufreq, intel_pstate.c, Fix rounding errors

On 18-11-15, 10:55, Prarit Bhargava wrote:
> diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
> index 2e31d09..686f024 100644
> --- a/drivers/cpufreq/intel_pstate.c
> +++ b/drivers/cpufreq/intel_pstate.c
> @@ -1233,6 +1233,8 @@ static int intel_pstate_set_policy(struct cpufreq_policy *policy)
>  	struct cpudata *cpu;
>  	int i;
>  #endif
> +	int max_policy_calc;
> +
>  	pr_debug("intel_pstate: %s max %u policy->max %u\n", __func__,
>  		 policy->cpuinfo.max_freq, policy->max);
>  	if (!policy->cpuinfo.max_freq)
> @@ -1249,7 +1251,10 @@ static int intel_pstate_set_policy(struct cpufreq_policy *policy)
>  	limits = &powersave_limits;
>  	limits->min_policy_pct = (policy->min * 100) / policy->cpuinfo.max_freq;
>  	limits->min_policy_pct = clamp_t(int, limits->min_policy_pct, 0 , 100);
> -	limits->max_policy_pct = (policy->max * 100) / policy->cpuinfo.max_freq;
> +
> +	max_policy_calc = (policy->max * 1000) / policy->cpuinfo.max_freq;
> +	limits->max_policy_pct = DIV_ROUND_UP(max_policy_calc, 10);
> +

Nice catch, but why can't we do this instead:

	limits->max_policy_pct = DIV_ROUND_UP(policy->max * 100, policy->cpuinfo.max_freq);

>  	limits->max_policy_pct = clamp_t(int, limits->max_policy_pct, 0 , 100);
>  
>  	/* Normalize user input to [min_policy_pct, max_policy_pct] */
> @@ -1269,6 +1274,7 @@ static int intel_pstate_set_policy(struct cpufreq_policy *policy)
>  				  int_tofp(100));
>  	limits->max_perf = div_fp(int_tofp(limits->max_perf_pct),
>  				  int_tofp(100));
> +	limits->max_perf = round_up(limits->max_perf, 8);

Perhaps you should fix this in a separate patch.

-- 
viresh
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