sched: Document arch_scale_*_capacity()

Rather that hide their purpose in some dark, damp corner of Documentation/,
add some documentation to the default implementations.
Signed-off-by: NValentin Schneider <valentin.schneider@arm.com>
Signed-off-by: NIngo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200731192016.7484-2-valentin.schneider@arm.com

include/linux/sched/topology.h

@@ -217,6 +217,16 @@ static inline bool cpus_share_cache(int this_cpu, int that_cpu)
 #endif	/* !CONFIG_SMP */
 
 #ifndef arch_scale_cpu_capacity
+/**
+ * arch_scale_cpu_capacity - get the capacity scale factor of a given CPU.
+ * @cpu: the CPU in question.
+ *
+ * Return: the CPU scale factor normalized against SCHED_CAPACITY_SCALE, i.e.
+ *
+ *             max_perf(cpu)
+ *      ----------------------------- * SCHED_CAPACITY_SCALE
+ *      max(max_perf(c) : c \in CPUs)
+ */
 static __always_inline
 unsigned long arch_scale_cpu_capacity(int cpu)
 {

kernel/sched/sched.h

@@ -2049,6 +2049,16 @@ void arch_scale_freq_tick(void)
 #endif
 
 #ifndef arch_scale_freq_capacity
+/**
+ * arch_scale_freq_capacity - get the frequency scale factor of a given CPU.
+ * @cpu: the CPU in question.
+ *
+ * Return: the frequency scale factor normalized against SCHED_CAPACITY_SCALE, i.e.
+ *
+ *     f_curr
+ *     ------ * SCHED_CAPACITY_SCALE
+ *     f_max
+ */
 static __always_inline
 unsigned long arch_scale_freq_capacity(int cpu)
 {