PM: EM: Clarify abstract scale usage for power values in Energy Model

The Energy Model (EM) can store power values in milli-Watts or in abstract
scale. This might cause issues in the subsystems which use the EM for
estimating the device power, such as:

 - mixing of different scales in a subsystem which uses multiple
   (cooling) devices (e.g. thermal Intelligent Power Allocation (IPA))

 - assuming that energy [milli-Joules] can be derived from the EM power
   values which might not be possible since the power scale doesn't have
   to be in milli-Watts

To avoid misconfiguration add the requisite documentation to the EM and
related subsystems: EAS and IPA.
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/driver-api/thermal/power_allocator.rst

@@ -71,7 +71,9 @@ to the speed-grade of the silicon.  `sustainable_power` is therefore
 simply an estimate, and may be tuned to affect the aggressiveness of
 the thermal ramp. For reference, the sustainable power of a 4" phone
 is typically 2000mW, while on a 10" tablet is around 4500mW (may vary
-depending on screen size).
+depending on screen size). It is possible to have the power value
+expressed in an abstract scale. The sustained power should be aligned
+to the scale used by the related cooling devices.
 
 If you are using device tree, do add it as a property of the
 thermal-zone.  For example::
@@ -269,3 +271,11 @@ won't be very good.  Note that this is not particular to this
 governor, step-wise will also misbehave if you call its throttle()
 faster than the normal thermal framework tick (due to interrupts for
 example) as it will overreact.
+
+Energy Model requirements
+=========================
+
+Another important thing is the consistent scale of the power values
+provided by the cooling devices. All of the cooling devices in a single
+thermal zone should have power values reported either in milli-Watts
+or scaled to the same 'abstract scale'.

Documentation/power/energy-model.rst

@@ -20,6 +20,19 @@ possible source of information on its own, the EM framework intervenes as an
 abstraction layer which standardizes the format of power cost tables in the
 kernel, hence enabling to avoid redundant work.
 
+The power values might be expressed in milli-Watts or in an 'abstract scale'.
+Multiple subsystems might use the EM and it is up to the system integrator to
+check that the requirements for the power value scale types are met. An example
+can be found in the Energy-Aware Scheduler documentation
+Documentation/scheduler/sched-energy.rst. For some subsystems like thermal or
+powercap power values expressed in an 'abstract scale' might cause issues.
+These subsystems are more interested in estimation of power used in the past,
+thus the real milli-Watts might be needed. An example of these requirements can
+be found in the Intelligent Power Allocation in
+Documentation/driver-api/thermal/power_allocator.rst.
+Important thing to keep in mind is that when the power values are expressed in
+an 'abstract scale' deriving real energy in milli-Joules would not be possible.
+
 The figure below depicts an example of drivers (Arm-specific here, but the
 approach is applicable to any architecture) providing power costs to the EM
 framework, and interested clients reading the data from it::

Documentation/scheduler/sched-energy.rst

@@ -350,6 +350,11 @@ independent EM framework in Documentation/power/energy-model.rst.
 Please also note that the scheduling domains need to be re-built after the
 EM has been registered in order to start EAS.
 
+EAS uses the EM to make a forecasting decision on energy usage and thus it is
+more focused on the difference when checking possible options for task
+placement. For EAS it doesn't matter whether the EM power values are expressed
+in milli-Watts or in an 'abstract scale'.
+
 
 6.3 - Energy Model complexity
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^