[PATCH v3 1/2] power: supply: add input voltage limit property

[Enric Balletbo i Serra]

This is part of 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, ie 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@xxxxxxxxxxxxx>
Reviewed-by: Guenter Roeck <groeck@xxxxxxxxxxxx>
---

Changes in v3:
- Improve commit log and documentation with Benson comments.

Changes in v2:
- Document the new property in ABI/testing/sysfs-class-power.
- Add the Reviewed-by Guenter Roeck tag.

 Documentation/ABI/testing/sysfs-class-power | 15 +++++++++++++++
 Documentation/power/power_supply_class.txt  |  2 ++
 drivers/power/supply/power_supply_sysfs.c   |  1 +
 include/linux/power_supply.h                |  1 +
 4 files changed, 19 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-class-power b/Documentation/ABI/testing/sysfs-class-power
index 5e23e22dce1b..6dee5c105a28 100644
--- a/Documentation/ABI/testing/sysfs-class-power
+++ b/Documentation/ABI/testing/sysfs-class-power
@@ -335,6 +335,21 @@ Description:
 		Access: Read, Write
 		Valid values: Represented in microamps
 
+What:		/sys/class/power_supply/<supply_name>/input_voltage_limit
+Date:		Nov 2018
+Contact:	linux-pm@xxxxxxxxxxxxxxx
+Description:
+		This entry configures the incoming VBUS voltage limit currently
+		set in the supply. Normally this is configured based on
+		system-level knowledge or user input (e.g. This is part of the
+		Pixel C's thermal management strategy to effectively limit the
+		input power to 5V when the screen is on to meet Google's skin
+		temperature targets). Note that this feature should not be
+		used for safety critical things.
+
+		Access: Read, Write
+		Valid values: Represented in microvolts
+
 What:		/sys/class/power_supply/<supply_name>/online,
 Date:		May 2007
 Contact:	linux-pm@xxxxxxxxxxxxxxx
diff --git a/Documentation/power/power_supply_class.txt b/Documentation/power/power_supply_class.txt
index 300d37896e51..7b4be615b4f8 100644
--- a/Documentation/power/power_supply_class.txt
+++ b/Documentation/power/power_supply_class.txt
@@ -137,6 +137,8 @@ power supply object.
 
 INPUT_CURRENT_LIMIT - input current limit programmed by charger. Indicates
 the current drawn from a charging source.
+INPUT_VOLTAGE_LIMIT - input voltage limit programmed by charger. Indicates
+the voltage limit from a charging source.
 
 CHARGE_CONTROL_LIMIT - current charge control limit setting
 CHARGE_CONTROL_LIMIT_MAX - maximum charge control limit setting
diff --git a/drivers/power/supply/power_supply_sysfs.c b/drivers/power/supply/power_supply_sysfs.c
index dce24f596160..5848742ebb59 100644
--- a/drivers/power/supply/power_supply_sysfs.c
+++ b/drivers/power/supply/power_supply_sysfs.c
@@ -275,6 +275,7 @@ static struct device_attribute power_supply_attrs[] = {
 	POWER_SUPPLY_ATTR(charge_control_limit),
 	POWER_SUPPLY_ATTR(charge_control_limit_max),
 	POWER_SUPPLY_ATTR(input_current_limit),
+	POWER_SUPPLY_ATTR(input_voltage_limit),
 	POWER_SUPPLY_ATTR(energy_full_design),
 	POWER_SUPPLY_ATTR(energy_empty_design),
 	POWER_SUPPLY_ATTR(energy_full),
diff --git a/include/linux/power_supply.h b/include/linux/power_supply.h
index f80769175c56..608ba88e32ee 100644
--- a/include/linux/power_supply.h
+++ b/include/linux/power_supply.h
@@ -122,6 +122,7 @@ enum power_supply_property {
 	POWER_SUPPLY_PROP_CHARGE_CONTROL_LIMIT,
 	POWER_SUPPLY_PROP_CHARGE_CONTROL_LIMIT_MAX,
 	POWER_SUPPLY_PROP_INPUT_CURRENT_LIMIT,
+	POWER_SUPPLY_PROP_INPUT_VOLTAGE_LIMIT,
 	POWER_SUPPLY_PROP_ENERGY_FULL_DESIGN,
 	POWER_SUPPLY_PROP_ENERGY_EMPTY_DESIGN,
 	POWER_SUPPLY_PROP_ENERGY_FULL,
-- 
2.19.2