energy_model: Use a fixed reference frequency

The last item of a performance domain is not always the performance point
that has been used to compute CPU's capacity. This can lead to different
target frequency compared with other part of the system like schedutil and
would result in wrong energy estimation.

A new arch_scale_freq_ref() is available to return a fixed and coherent
frequency reference that can be used when computing the CPU's frequency
for an level of utilization. Use this function to get this reference
frequency.

Energy model is never used without defining arch_scale_freq_ref() but
can be compiled. Define a default arch_scale_freq_ref() returning 0
in such case.

Signed-off-by: Vincent Guittot <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Tested-by: Lukasz Luba <[e-mail]>
Reviewed-by: Lukasz Luba <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

include/linux/energy_model.h

···
 				unsigned long max_util, unsigned long sum_util,
 				unsigned long allowed_cpu_cap)
 {
-	unsigned long freq, scale_cpu;
+	unsigned long freq, ref_freq, scale_cpu;
 	struct em_perf_state *ps;
 	int cpu;
 
···
 	 */
 	cpu = cpumask_first(to_cpumask(pd->cpus));
 	scale_cpu = arch_scale_cpu_capacity(cpu);
-	ps = &pd->table[pd->nr_perf_states - 1];
+	ref_freq = arch_scale_freq_ref(cpu);
 
 	max_util = min(max_util, allowed_cpu_cap);
-	freq = map_util_freq(max_util, ps->frequency, scale_cpu);
+	freq = map_util_freq(max_util, ref_freq, scale_cpu);
 
 	/*
 	 * Find the lowest performance state of the Energy Model above the