Patch "cpufreq: CPPC: Fix performance/frequency conversion" has been added to the 5.16-stable tree

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This is a note to let you know that I've just added the patch titled

    cpufreq: CPPC: Fix performance/frequency conversion

to the 5.16-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     cpufreq-cppc-fix-performance-frequency-conversion.patch
and it can be found in the queue-5.16 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 7526e71b68e2f761d50c9945567af236d0d4baa0
Author: Pierre Gondois <Pierre.Gondois@xxxxxxx>
Date:   Tue Feb 8 09:01:09 2022 +0100

    cpufreq: CPPC: Fix performance/frequency conversion
    
    [ Upstream commit ec1c7ad47664f964c1101fe555b6fde0cb124b38 ]
    
    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@xxxxxxx>
    Suggested-by: Morten Rasmussen <morten.rasmussen@xxxxxxx>
    Signed-off-by: Pierre Gondois <Pierre.Gondois@xxxxxxx>
    Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
index db17196266e4..82d370ae6a4a 100644
--- a/drivers/cpufreq/cppc_cpufreq.c
+++ b/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
- *
- * If the perf/freq point lies between Nominal and Lowest, we can treat
- * (Low perf, Low freq) and (Nom Perf, Nom freq) as 2D co-ordinates of a line
- * and extrapolate the rest
- * For perf/freq > Nominal, we use the ratio perf:freq at Nominal for conversion
+ * use them to convert perf to freq and vice versa. The conversion is
+ * extrapolated as an affine function passing by the 2 points:
+ *  - (Low perf, Low freq)
+ *  - (Nominal perf, Nominal perf)
  */
 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;
-			div = caps->nominal_perf;
-		} else {
-			mul = caps->nominal_freq - caps->lowest_freq;
-			div = caps->nominal_perf - caps->lowest_perf;
-		}
+		mul = caps->nominal_freq - caps->lowest_freq;
+		div = caps->nominal_perf - caps->lowest_perf;
+		offset = caps->nominal_freq - div64_u64(caps->nominal_perf * mul, div);
 	} 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;
-			div = caps->nominal_freq;
-		} else {
-			mul = caps->lowest_perf;
-			div = caps->lowest_freq;
-		}
+		mul = caps->nominal_perf - caps->lowest_perf;
+		div = caps->nominal_freq - caps->lowest_freq;
+		offset = caps->nominal_perf - div64_u64(caps->nominal_freq * mul, div);
 	} 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,



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