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Commit 72e29a67 authored by Quentin Perret's avatar Quentin Perret Committed by Todd Kjos
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ANDROID: PM / EM: Document the support for legacy (deprecated) EM 



Android 4.19 has a compatibility driver enabling to load legacy EMs (now
DEPRECATED) into the new framework.

Although it is NOT RECOMMENDED to use this feature, it *is* there, so we
should at least document it. It's also an opportunity to make clear it's
deprecated, and to point at the alternative solutions.

Bug: 120440300
Change-Id: I1ec43208c2727b878d31c508153e2a0a0e7fa55b
Signed-off-by: default avatarQuentin Perret <quentin.perret@arm.com>
parent 50597ab1

Documentation/power/energy-model.txt

+25 −0
Original line number Diff line number Diff line
@@ -142,3 +142,28 @@ EM framework. 
37

38	        return 0;

39	}





4. Support for legacy Energy Models (DEPRECATED)

------------------------------------------------



The Android kernel version 4.14 and before used a different type of EM for EAS,

referred to as the 'legacy' EM. The legacy EM relies on the out-of-tree

'sched-energy-costs' devicetree bindings to provide the kernel with power costs.

The usage of such bindings in Android has now been DEPRECATED in favour of the

mainline equivalents.



The currently supported alternatives to populate the EM include:

 - using a firmware-based solution such as Arm SCMI (supported in

   drivers/cpufreq/scmi-cpufreq.c);

 - using the 'dynamic-power-coefficient' devicetree binding together with

   PM_OPP. See the of_dev_pm_opp_get_cpu_power() helper in PM_OPP, and the

   reference implementation in drivers/cpufreq/cpufreq-dt.c.



In order to ease the transition to the new EM format, Android 4.19 also provides

a compatibility driver able to load a legacy EM from DT into the EM framework.

*** Please note that THIS FEATURE WILL NOT BE AVAILABLE in future Android

kernels, and as such it must be considered only as a temporary workaround. ***



If you know what you're doing and still want to use this driver, you need to set

CONFIG_LEGACY_ENERGY_MODEL_DT=y in your kernel configuration to enable it.