cpufreq: intel_pstate: Avoid percentages in limits-related computations

Currently, intel_pstate_update_perf_limits() first converts the
policy minimum and maximum limits into percentages of the maximum
turbo frequency (rounding up to an integer) and then converts these
percentages to fractions (by using fixed-point arithmetic to divide
them by 100).

That introduces a rounding error unnecessarily, because the fractions
can be obtained by carrying out fixed-point divisions directly on the
input numbers.

Rework the computations in intel_pstate_hwp_set() to use fractions
instead of percentages (and drop redundant local variables from
there) and modify intel_pstate_update_perf_limits() to compute the
fractions directly and percentages out of them.

While at it, introduce percent_ext_fp() for converting percentages
to fractions (with extended number of fraction bits) and use it in
the computations.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index ee12641..08e134f 100644 (file)
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -84,6 +84,11 @@ static inline u64 div_ext_fp(u64 x, u64 y)
        return div64_u64(x << EXT_FRAC_BITS, y);
 }
 
+static inline int32_t percent_ext_fp(int percent)
+{
+       return div_ext_fp(percent, 100);
+}
+
 /**
  * struct sample -     Store performance sample
  * @core_avg_perf:     Ratio of APERF/MPERF which is the actual average
@@ -850,7 +855,6 @@ static void intel_pstate_hwp_set(struct cpufreq_policy *policy)
        u64 value, cap;
 
        for_each_cpu(cpu, policy->cpus) {
-               int max_perf_pct, min_perf_pct;
                struct cpudata *cpu_data = all_cpu_data[cpu];
                s16 epp;
 
@@ -864,16 +868,14 @@ static void intel_pstate_hwp_set(struct cpufreq_policy *policy)
                else
                        hw_max = HWP_HIGHEST_PERF(cap);
 
-               max_perf_pct = perf_limits->max_perf_pct;
-               min_perf_pct = perf_limits->min_perf_pct;
-               min = hw_max * min_perf_pct / 100;
+               min = fp_ext_toint(hw_max * perf_limits->min_perf);
 
                rdmsrl_on_cpu(cpu, MSR_HWP_REQUEST, &value);
 
                value &= ~HWP_MIN_PERF(~0L);
                value |= HWP_MIN_PERF(min);
 
-               max = hw_max * max_perf_pct / 100;
+               max = fp_ext_toint(hw_max * perf_limits->max_perf);
                value &= ~HWP_MAX_PERF(~0L);
                value |= HWP_MAX_PERF(max);
 
@@ -1223,7 +1225,7 @@ static ssize_t store_max_perf_pct(struct kobject *a, struct attribute *b,
                                   limits->max_perf_pct);
        limits->max_perf_pct = max(limits->min_perf_pct,
                                   limits->max_perf_pct);
-       limits->max_perf = div_ext_fp(limits->max_perf_pct, 100);
+       limits->max_perf = percent_ext_fp(limits->max_perf_pct);
 
        intel_pstate_update_policies();
 
@@ -1260,7 +1262,7 @@ static ssize_t store_min_perf_pct(struct kobject *a, struct attribute *b,
                                   limits->min_perf_pct);
        limits->min_perf_pct = min(limits->max_perf_pct,
                                   limits->min_perf_pct);
-       limits->min_perf = div_ext_fp(limits->min_perf_pct, 100);
+       limits->min_perf = percent_ext_fp(limits->min_perf_pct);
 
        intel_pstate_update_policies();
 
@@ -2078,36 +2080,34 @@ static void intel_pstate_clear_update_util_hook(unsigned int cpu)
 static void intel_pstate_update_perf_limits(struct cpufreq_policy *policy,
                                            struct perf_limits *limits)
 {
+       int32_t max_policy_perf, min_policy_perf;
 
-       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);
+       max_policy_perf = div_ext_fp(policy->max, policy->cpuinfo.max_freq);
+       max_policy_perf = clamp_t(int32_t, max_policy_perf, 0, int_ext_tofp(1));
        if (policy->max == policy->min) {
-               limits->min_policy_pct = limits->max_policy_pct;
+               min_policy_perf = max_policy_perf;
        } else {
-               limits->min_policy_pct = DIV_ROUND_UP(policy->min * 100,
-                                                     policy->cpuinfo.max_freq);
-               limits->min_policy_pct = clamp_t(int, limits->min_policy_pct,
-                                                0, 100);
+               min_policy_perf = div_ext_fp(policy->min,
+                                            policy->cpuinfo.max_freq);
+               min_policy_perf = clamp_t(int32_t, min_policy_perf,
+                                         0, max_policy_perf);
        }
 
-       /* Normalize user input to [min_policy_pct, max_policy_pct] */
-       limits->min_perf_pct = max(limits->min_policy_pct,
-                                  limits->min_sysfs_pct);
-       limits->min_perf_pct = min(limits->max_policy_pct,
-                                  limits->min_perf_pct);
-       limits->max_perf_pct = min(limits->max_policy_pct,
-                                  limits->max_sysfs_pct);
-       limits->max_perf_pct = max(limits->min_policy_pct,
-                                  limits->max_perf_pct);
+       /* Normalize user input to [min_perf, max_perf] */
+       limits->min_perf = max(min_policy_perf,
+                              percent_ext_fp(limits->min_sysfs_pct));
+       limits->min_perf = min(limits->min_perf, max_policy_perf);
+       limits->max_perf = min(max_policy_perf,
+                              percent_ext_fp(limits->max_sysfs_pct));
+       limits->max_perf = max(min_policy_perf, limits->max_perf);
 
-       /* Make sure min_perf_pct <= max_perf_pct */
-       limits->min_perf_pct = min(limits->max_perf_pct, limits->min_perf_pct);
+       /* Make sure min_perf <= max_perf */
+       limits->min_perf = min(limits->min_perf, limits->max_perf);
 
-       limits->min_perf = div_ext_fp(limits->min_perf_pct, 100);
-       limits->max_perf = div_ext_fp(limits->max_perf_pct, 100);
        limits->max_perf = round_up(limits->max_perf, EXT_FRAC_BITS);
        limits->min_perf = round_up(limits->min_perf, EXT_FRAC_BITS);
+       limits->max_perf_pct = fp_ext_toint(limits->max_perf * 100);
+       limits->min_perf_pct = fp_ext_toint(limits->min_perf * 100);
 
        pr_debug("cpu:%d max_perf_pct:%d min_perf_pct:%d\n", policy->cpu,
                 limits->max_perf_pct, limits->min_perf_pct);