cpufreq: CPPC: Fix performance/frequency conversion

CPUfreq governors request CPU frequencies using information
on current CPU usage. The CPPC driver converts them to
performance requests. Frequency targets are computed as:
	target_freq = (util / cpu_capacity) * max_freq
target_freq is then clamped between [policy->min, policy->max].

The CPPC driver converts performance values to frequencies
(and vice-versa) using cppc_cpufreq_perf_to_khz() and
cppc_cpufreq_khz_to_perf(). These functions both use two different
factors depending on the range of the input value. For
cppc_cpufreq_khz_to_perf():
- (NOMINAL_PERF / NOMINAL_FREQ) or
- (LOWEST_PERF / LOWEST_FREQ)
and for cppc_cpufreq_perf_to_khz():
- (NOMINAL_FREQ / NOMINAL_PERF) or
- ((NOMINAL_PERF - LOWEST_FREQ) / (NOMINAL_PERF - LOWEST_PERF))

This means:
1- the functions are not inverse for some values:
   (perf_to_khz(khz_to_perf(x)) != x)
2- cppc_cpufreq_perf_to_khz(LOWEST_PERF) can sometimes give
   a different value from LOWEST_FREQ due to integer approximation
3- it is implied that performance and frequency are proportional
   (NOMINAL_FREQ / NOMINAL_PERF) == (LOWEST_PERF / LOWEST_FREQ)

This patch changes the conversion functions to an affine function.
This fixes the 3 points above.

Suggested-by: Lukasz Luba <lukasz.luba@arm.com>
Suggested-by: Morten Rasmussen <morten.rasmussen@arm.com>
Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

drivers/cpufreq/cppc_cpufreq.c

@@ -303,52 +303,48 @@ static u64 cppc_get_dmi_max_khz(void)
 
 /*
  * If CPPC lowest_freq and nominal_freq registers are exposed then we can
- * use them to convert perf to freq and vice versa
+ * use them to convert perf to freq and vice versa. The conversion is
- *
+ * extrapolated as an affine function passing by the 2 points:
- * If the perf/freq point lies between Nominal and Lowest, we can treat
+ *  - (Low perf, Low freq)
- * (Low perf, Low freq) and (Nom Perf, Nom freq) as 2D co-ordinates of a line
+ *  - (Nominal perf, Nominal perf)
- * and extrapolate the rest
- * For perf/freq > Nominal, we use the ratio perf:freq at Nominal for conversion
  */
 static unsigned int cppc_cpufreq_perf_to_khz(struct cppc_cpudata *cpu_data,
 					     unsigned int perf)
 {
 	struct cppc_perf_caps *caps = &cpu_data->perf_caps;
+	s64 retval, offset = 0;
 	static u64 max_khz;
 	u64 mul, div;
 
 	if (caps->lowest_freq && caps->nominal_freq) {
-		if (perf >= caps->nominal_perf) {
+		mul = caps->nominal_freq - caps->lowest_freq;
-			mul = caps->nominal_freq;
+		div = caps->nominal_perf - caps->lowest_perf;
-			div = caps->nominal_perf;
+		offset = caps->nominal_freq - div64_u64(caps->nominal_perf * mul, div);
-		} else {
-			mul = caps->nominal_freq - caps->lowest_freq;
-			div = caps->nominal_perf - caps->lowest_perf;
-		}
 	} else {
 		if (!max_khz)
 			max_khz = cppc_get_dmi_max_khz();
 		mul = max_khz;
 		div = caps->highest_perf;
 	}
-	return (u64)perf * mul / div;
+
+	retval = offset + div64_u64(perf * mul, div);
+	if (retval >= 0)
+		return retval;
+	return 0;
 }
 
 static unsigned int cppc_cpufreq_khz_to_perf(struct cppc_cpudata *cpu_data,
 					     unsigned int freq)
 {
 	struct cppc_perf_caps *caps = &cpu_data->perf_caps;
+	s64 retval, offset = 0;
 	static u64 max_khz;
 	u64  mul, div;
 
 	if (caps->lowest_freq && caps->nominal_freq) {
-		if (freq >= caps->nominal_freq) {
+		mul = caps->nominal_perf - caps->lowest_perf;
-			mul = caps->nominal_perf;
+		div = caps->nominal_freq - caps->lowest_freq;
-			div = caps->nominal_freq;
+		offset = caps->nominal_perf - div64_u64(caps->nominal_freq * mul, div);
-		} else {
-			mul = caps->lowest_perf;
-			div = caps->lowest_freq;
-		}
 	} else {
 		if (!max_khz)
 			max_khz = cppc_get_dmi_max_khz();
@@ -356,7 +352,10 @@ static unsigned int cppc_cpufreq_khz_to_perf(struct cppc_cpudata *cpu_data,
 		div = max_khz;
 	}
 
-	return (u64)freq * mul / div;
+	retval = offset + div64_u64(freq * mul, div);
+	if (retval >= 0)
+		return retval;
+	return 0;
 }
 
 static int cppc_cpufreq_set_target(struct cpufreq_policy *policy,