Documentation: EM: Add artificial EM registration description

Add description about new artificial EM registration and use cases.
Update also the documentation with the new .get_cost() callback
description and usage.

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Reviewed-by: Ionela Voinescu <ionela.voinescu@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/power/energy-model.rst

@@ -123,6 +123,26 @@ allows a platform to register EM power values which are reflecting total power
 (static + dynamic). These power values might be coming directly from
 experiments and measurements.
 
+Registration of 'artificial' EM
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+There is an option to provide a custom callback for drivers missing detailed
+knowledge about power value for each performance state. The callback
+.get_cost() is optional and provides the 'cost' values used by the EAS.
+This is useful for platforms that only provide information on relative
+efficiency between CPU types, where one could use the information to
+create an abstract power model. But even an abstract power model can
+sometimes be hard to fit in, given the input power value size restrictions.
+The .get_cost() allows to provide the 'cost' values which reflect the
+efficiency of the CPUs. This would allow to provide EAS information which
+has different relation than what would be forced by the EM internal
+formulas calculating 'cost' values. To register an EM for such platform, the
+driver must set the flag 'milliwatts' to 0, provide .get_power() callback
+and provide .get_cost() callback. The EM framework would handle such platform
+properly during registration. A flag EM_PERF_DOMAIN_ARTIFICIAL is set for such
+platform. Special care should be taken by other frameworks which are using EM
+to test and treat this flag properly.
+
 Registration of 'simple' EM
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~