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>

1 files changed, 11 insertions, 1 deletions

diff --git a/Documentation/driver-api/thermal/power_allocator.rst b/Documentation/driver-api/thermal/power_allocator.rst
index 67b6a3297238..aa5f66552d6f 100644
--- a/Documentation/driver-api/thermal/power_allocator.rst
+++ b/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'.