intel_pstate: Improve accuracy by not truncating until final result

This patch addresses Bug 60727
(https://bugzilla.kernel.org/show_bug.cgi?id=60727)
which was due to the truncation of intermediate values in the
calculations, which causes the code to consistently underestimate the
current cpu frequency, specifically 100% cpu utilization was truncated
down to the setpoint of 97%. This patch fixes the problem by keeping
the results of all intermediate calculations as fixed point numbers
rather scaling them back and forth between integers and fixed point.

References: https://bugzilla.kernel.org/show_bug.cgi?id=60727
Signed-off-by: Brennan Shacklett <bpshacklett@gmail.com>
Acked-by: Dirk Brandewie <dirk.j.brandewie@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

1 files changed, 15 insertions, 18 deletions

diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index badf6206b2b2..8b8677f92700 100644
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -48,7 +48,7 @@ static inline int32_t div_fp(int32_t x, int32_t y)
 }
 
 struct sample {
-        int core_pct_busy;
+        int32_t core_pct_busy;
         u64 aperf;
         u64 mperf;
         int freq;
@@ -68,7 +68,7 @@ struct _pid {
         int32_t i_gain;
         int32_t d_gain;
         int deadband;
-        int last_err;
+        int32_t last_err;
 };
 
 struct cpudata {
@@ -153,16 +153,15 @@ static inline void pid_d_gain_set(struct _pid *pid, int percent)
         pid->d_gain = div_fp(int_tofp(percent), int_tofp(100));
 }
 
-static signed int pid_calc(struct _pid *pid, int busy)
+static signed int pid_calc(struct _pid *pid, int32_t busy)
 {
-        signed int err, result;
+        signed int result;
         int32_t pterm, dterm, fp_error;
         int32_t integral_limit;
 
-        err = pid->setpoint - busy;
-        fp_error = int_tofp(err);
+        fp_error = int_tofp(pid->setpoint) - busy;
 
-        if (abs(err) <= pid->deadband)
+        if (abs(fp_error) <= int_tofp(pid->deadband))
                 return 0;
 
         pterm = mul_fp(pid->p_gain, fp_error);
@@ -176,8 +175,8 @@ static signed int pid_calc(struct _pid *pid, int busy)
         if (pid->integral < -integral_limit)
                 pid->integral = -integral_limit;
 
-        dterm = mul_fp(pid->d_gain, (err - pid->last_err));
-        pid->last_err = err;
+        dterm = mul_fp(pid->d_gain, fp_error - pid->last_err);
+        pid->last_err = fp_error;
 
         result = pterm + mul_fp(pid->integral, pid->i_gain) + dterm;
 
@@ -436,8 +435,9 @@ static inline void intel_pstate_calc_busy(struct cpudata *cpu,
                                         struct sample *sample)
 {
         u64 core_pct;
-        core_pct = div64_u64(sample->aperf * 100, sample->mperf);
-        sample->freq = cpu->pstate.max_pstate * core_pct * 1000;
+        core_pct = div64_u64(int_tofp(sample->aperf * 100),
+                             sample->mperf);
+        sample->freq = fp_toint(cpu->pstate.max_pstate * core_pct * 1000);
 
         sample->core_pct_busy = core_pct;
 }
@@ -469,22 +469,19 @@ static inline void intel_pstate_set_sample_time(struct cpudata *cpu)
         mod_timer_pinned(&cpu->timer, jiffies + delay);
 }
 
-static inline int intel_pstate_get_scaled_busy(struct cpudata *cpu)
+static inline int32_t intel_pstate_get_scaled_busy(struct cpudata *cpu)
 {
-        int32_t busy_scaled;
         int32_t core_busy, max_pstate, current_pstate;
 
-        core_busy = int_tofp(cpu->samples[cpu->sample_ptr].core_pct_busy);
+        core_busy = cpu->samples[cpu->sample_ptr].core_pct_busy;
         max_pstate = int_tofp(cpu->pstate.max_pstate);
         current_pstate = int_tofp(cpu->pstate.current_pstate);
-        busy_scaled = mul_fp(core_busy, div_fp(max_pstate, current_pstate));
-
-        return fp_toint(busy_scaled);
+        return mul_fp(core_busy, div_fp(max_pstate, current_pstate));
 }
 
 static inline void intel_pstate_adjust_busy_pstate(struct cpudata *cpu)
 {
-        int busy_scaled;
+        int32_t busy_scaled;
         struct _pid *pid;
         signed int ctl = 0;
         int steps;