power: supply: Add battery driver for Surface Aggregator Module

On newer Microsoft Surface models (specifically 7th-generation, i.e.
Surface Pro 7, Surface Book 3, Surface Laptop 3, and Surface Laptop Go),
battery and AC status/information is no longer handled via standard ACPI
devices, but instead directly via the Surface System Aggregator Module
(SSAM), i.e. the embedded controller on those devices.

While on previous generation models, battery status is also handled via
SSAM, an ACPI shim was present to translate the standard ACPI battery
interface to SSAM requests. The SSAM interface itself, which is modeled
closely after the ACPI interface, has not changed.

This commit introduces a new SSAM client device driver to support
battery status/information via the aforementioned interface on said
Surface models. It is in parts based on the standard ACPI battery
driver.
Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

Documentation/ABI/testing/sysfs-class-power-surface

+What:		/sys/class/power_supply/<supply_name>/alarm
+Date:		April 2021
+KernelVersion:	5.13
+Contact:	Maximilian Luz <luzmaximilian@gmail.com>
+Description:
+		Battery trip point. When the remaining battery capacity crosses this
+		value in either direction, the system will be notified and if
+		necessary woken.
+
+		Set to zero to clear/disable.
+
+		Access: Read, Write
+
+		Valid values: In micro-Wh or micro-Ah, depending on the power unit
+		of the battery

MAINTAINERS

@@ -11861,6 +11861,13 @@ F:	drivers/scsi/smartpqi/smartpqi*.[ch]
 F:	include/linux/cciss*.h
 F:	include/uapi/linux/cciss*.h
 
+MICROSOFT SURFACE BATTERY AND AC DRIVERS
+M:	Maximilian Luz <luzmaximilian@gmail.com>
+L:	linux-pm@vger.kernel.org
+L:	platform-driver-x86@vger.kernel.org
+S:	Maintained
+F:	drivers/power/supply/surface_battery.c
+
 MICROSOFT SURFACE GPE LID SUPPORT DRIVER
 M:	Maximilian Luz <luzmaximilian@gmail.com>
 L:	platform-driver-x86@vger.kernel.org

drivers/power/supply/Kconfig

@@ -801,4 +801,20 @@ config BATTERY_ACER_A500
 	help
 	  Say Y to include support for Acer Iconia Tab A500 battery fuel gauge.
 
+config BATTERY_SURFACE
+	tristate "Battery driver for 7th-generation Microsoft Surface devices"
+	depends on SURFACE_AGGREGATOR_REGISTRY
+	help
+	  Driver for battery devices connected via/managed by the Surface System
+	  Aggregator Module (SSAM).
+
+	  This driver provides battery-information and -status support for
+	  Surface devices where said data is not exposed via the standard ACPI
+	  devices. On those models (7th-generation), battery-information is
+	  instead handled directly via SSAM client devices and this driver.
+
+	  Say M or Y here to include battery status support for 7th-generation
+	  Microsoft Surface devices, i.e. Surface Pro 7, Surface Laptop 3,
+	  Surface Book 3, and Surface Laptop Go.
+
 endif # POWER_SUPPLY

drivers/power/supply/Makefile

@@ -101,3 +101,4 @@ obj-$(CONFIG_CHARGER_BD99954)	+= bd99954-charger.o
 obj-$(CONFIG_CHARGER_WILCO)	+= wilco-charger.o
 obj-$(CONFIG_RN5T618_POWER)	+= rn5t618_power.o
 obj-$(CONFIG_BATTERY_ACER_A500)	+= acer_a500_battery.o
+obj-$(CONFIG_BATTERY_SURFACE)	+= surface_battery.o