PM: EM: convert power field to micro-Watts precision and align drivers

The milli-Watts precision causes rounding errors while calculating
efficiency cost for each OPP. This is especially visible in the 'simple'
Energy Model (EM), where the power for each OPP is provided from OPP
framework. This can cause some OPPs to be marked inefficient, while
using micro-Watts precision that might not happen.

Update all EM users which access 'power' field and assume the value is
in milli-Watts.

Solve also an issue with potential overflow in calculation of energy
estimation on 32bit machine. It's needed now since the power value
(thus the 'cost' as well) are higher.

Example calculation which shows the rounding error and impact:

power = 'dyn-power-coeff' * volt_mV * volt_mV * freq_MHz

power_a_uW = (100 * 600mW * 600mW * 500MHz) / 10^6 = 18000
power_a_mW = (100 * 600mW * 600mW * 500MHz) / 10^9 = 18

power_b_uW = (100 * 605mW * 605mW * 600MHz) / 10^6 = 21961
power_b_mW = (100 * 605mW * 605mW * 600MHz) / 10^9 = 21

max_freq = 2000MHz

cost_a_mW = 18 * 2000MHz/500MHz = 72
cost_a_uW = 18000 * 2000MHz/500MHz = 72000

cost_b_mW = 21 * 2000MHz/600MHz = 70 // <- artificially better
cost_b_uW = 21961 * 2000MHz/600MHz = 73203

The 'cost_b_mW' (which is based on old milli-Watts) is misleadingly
better that the 'cost_b_uW' (this patch uses micro-Watts) and such
would have impact on the 'inefficient OPPs' information in the Cpufreq
framework. This patch set removes the rounding issue.

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

1 files changed, 38 insertions, 16 deletions

diff --git a/include/linux/energy_model.h b/include/linux/energy_model.h
index 8419bffb4398..b9caa01dfac4 100644
--- a/include/linux/energy_model.h
+++ b/include/linux/energy_model.h
@@ -62,7 +62,7 @@ struct em_perf_domain {
 /*
  *  em_perf_domain flags:
  *
- *  EM_PERF_DOMAIN_MILLIWATTS: The power values are in milli-Watts or some
+ *  EM_PERF_DOMAIN_MICROWATTS: The power values are in micro-Watts or some
  *  other scale.
  *
  *  EM_PERF_DOMAIN_SKIP_INEFFICIENCIES: Skip inefficient states when estimating
@@ -71,7 +71,7 @@ struct em_perf_domain {
  *  EM_PERF_DOMAIN_ARTIFICIAL: The power values are artificial and might be
  *  created by platform missing real power information
  */
-#define EM_PERF_DOMAIN_MILLIWATTS BIT(0)
+#define EM_PERF_DOMAIN_MICROWATTS BIT(0)
 #define EM_PERF_DOMAIN_SKIP_INEFFICIENCIES BIT(1)
 #define EM_PERF_DOMAIN_ARTIFICIAL BIT(2)
 
@@ -79,22 +79,44 @@ struct em_perf_domain {
 #define em_is_artificial(em) ((em)->flags & EM_PERF_DOMAIN_ARTIFICIAL)
 
 #ifdef CONFIG_ENERGY_MODEL
-#define EM_MAX_POWER 0xFFFF
+/*
+ * The max power value in micro-Watts. The limit of 64 Watts is set as
+ * a safety net to not overflow multiplications on 32bit platforms. The
+ * 32bit value limit for total Perf Domain power implies a limit of
+ * maximum CPUs in such domain to 64.
+ */
+#define EM_MAX_POWER (64000000) /* 64 Watts */
+
+/*
+ * To avoid possible energy estimation overflow on 32bit machines add
+ * limits to number of CPUs in the Perf. Domain.
+ * We are safe on 64bit machine, thus some big number.
+ */
+#ifdef CONFIG_64BIT
+#define EM_MAX_NUM_CPUS 4096
+#else
+#define EM_MAX_NUM_CPUS 16
+#endif
 
 /*
- * Increase resolution of energy estimation calculations for 64-bit
- * architectures. The extra resolution improves decision made by EAS for the
- * task placement when two Performance Domains might provide similar energy
- * estimation values (w/o better resolution the values could be equal).
+ * To avoid an overflow on 32bit machines while calculating the energy
+ * use a different order in the operation. First divide by the 'cpu_scale'
+ * which would reduce big value stored in the 'cost' field, then multiply by
+ * the 'sum_util'. This would allow to handle existing platforms, which have
+ * e.g. power ~1.3 Watt at max freq, so the 'cost' value > 1mln micro-Watts.
+ * In such scenario, where there are 4 CPUs in the Perf. Domain the 'sum_util'
+ * could be 4096, then multiplication: 'cost' * 'sum_util'  would overflow.
+ * This reordering of operations has some limitations, we lose small
+ * precision in the estimation (comparing to 64bit platform w/o reordering).
  *
- * We increase resolution only if we have enough bits to allow this increased
- * resolution (i.e. 64-bit). The costs for increasing resolution when 32-bit
- * are pretty high and the returns do not justify the increased costs.
+ * We are safe on 64bit machine.
  */
 #ifdef CONFIG_64BIT
-#define em_scale_power(p) ((p) * 1000)
+#define em_estimate_energy(cost, sum_util, scale_cpu) \
+	(((cost) * (sum_util)) / (scale_cpu))
 #else
-#define em_scale_power(p) (p)
+#define em_estimate_energy(cost, sum_util, scale_cpu) \
+	(((cost) / (scale_cpu)) * (sum_util))
 #endif
 
 struct em_data_callback {
@@ -112,7 +134,7 @@ struct em_data_callback {
 	 * and frequency.
 	 *
 	 * In case of CPUs, the power is the one of a single CPU in the domain,
-	 * expressed in milli-Watts or an abstract scale. It is expected to
+	 * expressed in micro-Watts or an abstract scale. It is expected to
 	 * fit in the [0, EM_MAX_POWER] range.
 	 *
 	 * Return 0 on success.
@@ -148,7 +170,7 @@ struct em_perf_domain *em_cpu_get(int cpu);
 struct em_perf_domain *em_pd_get(struct device *dev);
 int em_dev_register_perf_domain(struct device *dev, unsigned int nr_states,
 				struct em_data_callback *cb, cpumask_t *span,
-				bool milliwatts);
+				bool microwatts);
 void em_dev_unregister_perf_domain(struct device *dev);
 
 /**
@@ -273,7 +295,7 @@ static inline unsigned long em_cpu_energy(struct em_perf_domain *pd,
 	 *   pd_nrg = ------------------------                       (4)
 	 *                  scale_cpu
 	 */
-	return ps->cost * sum_util / scale_cpu;
+	return em_estimate_energy(ps->cost, sum_util, scale_cpu);
 }
 
 /**
@@ -297,7 +319,7 @@ struct em_data_callback {};
 static inline
 int em_dev_register_perf_domain(struct device *dev, unsigned int nr_states,
 				struct em_data_callback *cb, cpumask_t *span,
-				bool milliwatts)
+				bool microwatts)
 {
 	return -EINVAL;
 }