For thermal management strategy you might be interested on limit the
input power for a power supply. We already have current limit but
basically what we probably want is to limit power. So, introduce the
input_power_limit property.

Although the common use case is limit the input power, in some
specific cases it is the voltage that is problematic (i.e some regulators
have different efficiencies at higher voltage resulting in more heat).
So introduce also the input_voltage_limit property.

This happens in one Chromebook and is used on the Pixel C's thermal
management strategy to effectively limit the input power to 5V 3A when
the screen is on. When the screen is on, the display, the CPU, and the GPU
all contribute more heat to the system than while the screen is off, and
we made a tradeoff to throttle the charger in order to give more of the
thermal budget to those other components.

So there's nothing fundamentally broken about the hardware that would
cause the Pixel C to malfunction if we were charging at 9V or 12V instead
of 5V when the screen is on, i.e. if userspace doesn't change this.

What would happen is that you wouldn't meet Google's skin temperature
targets on the system if the charger was allowed to run at 9V or 12V with
the screen on.

For folks hacking on Pixel Cs (which is now outside of Google's official
support window for Android) and customizing their own kernel and userspace
this would be acceptable, but we wanted to expose this feature in the
power supply properties because the feature does exist in the Emedded
Controller firmware of the Pixel C and all of Google's Chromebooks with
USB-C made since 2015 in case someone running an up to date kernel wanted
to limit the charging power for thermal or other reasons.

This patch exposes a new property, similar to input current limit, to
re-configure the maximum voltage from the external supply at runtime
based on system-level knowledge or user input.
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Acked-by: Adam Thomson <Adam.Thomson.Opensource@diasemi.com>
Reviewed-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>