drivers/rtc/rtc-palmas.c: support for backup battery charging

Palmas series device like TPS65913, TPS80036 supports the backup battery
for powering the RTC when no other energy source is available.

The backup battery is optional, connected to the VBACKUP pin, and can be
nonrechargeable or rechargeable. The rechargeable battery can be charged
from the system supply using the backup battery charger.

Add support for enabling charging of this backup battery. Also add the DT
binding document and the new properties to have this support.

Signed-off-by: Laxman Dewangan <[e-mail]>
Reviewed-by: Felipe Balbi <[e-mail]>
Acked-by: Kumar Gala <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/devicetree/bindings/rtc/rtc-palmas.txt

···
+Palmas RTC controller bindings
+
+Required properties:
+- compatible:
+  - "ti,palmas-rtc" for palma series of the RTC controller
+- interrupt-parent: Parent interrupt device, must be handle of palmas node.
+- interrupts: Interrupt number of RTC submodule on device.
+
+Optional properties:
+
+- ti,backup-battery-chargeable: The Palmas series device like TPS65913 or
+	TPS80036 supports the backup battery for powering the RTC when main
+	battery is removed or in very low power state. The backup battery
+	can be chargeable or non-chargeable. This flag will tells whether
+	battery is chargeable or not. If charging battery then driver can
+	enable the charging.
+- ti,backup-battery-charge-high-current: Enable high current charging in
+	backup battery. Device supports the < 100mA and > 100mA charging.
+	The high current will be > 100mA. Absence of this property will
+	charge battery to lower current i.e. < 100mA.
+
+Example:
+	palmas: tps65913@58 {
+		...
+		palmas_rtc: rtc {
+			compatible = "ti,palmas-rtc";
+			interrupt-parent = <&palmas>;
+			interrupts = <8 0>;
+			ti,backup-battery-chargeable;
+			ti,backup-battery-charge-high-current;
+		};
+		...
+	};

drivers/rtc/rtc-palmas.c

···
 	struct palmas *palmas = dev_get_drvdata(pdev->dev.parent);
 	struct palmas_rtc *palmas_rtc = NULL;
 	int ret;
+	bool enable_bb_charging = false;
+	bool high_bb_charging;
+
+	if (pdev->dev.of_node) {
+		enable_bb_charging = of_property_read_bool(pdev->dev.of_node,
+					"ti,backup-battery-chargeable");
+		high_bb_charging = of_property_read_bool(pdev->dev.of_node,
+					"ti,backup-battery-charge-high-current");
+	}
 
 	palmas_rtc = devm_kzalloc(&pdev->dev, sizeof(struct palmas_rtc),
 			GFP_KERNEL);
···
 
 	palmas_rtc->dev = &pdev->dev;
 	platform_set_drvdata(pdev, palmas_rtc);
+
+	if (enable_bb_charging) {
+		unsigned reg = PALMAS_BACKUP_BATTERY_CTRL_BBS_BBC_LOW_ICHRG;
+
+		if (high_bb_charging)
+			reg = 0;
+
+		ret = palmas_update_bits(palmas, PALMAS_PMU_CONTROL_BASE,
+			PALMAS_BACKUP_BATTERY_CTRL,
+			PALMAS_BACKUP_BATTERY_CTRL_BBS_BBC_LOW_ICHRG, reg);
+		if (ret < 0) {
+			dev_err(&pdev->dev,
+				"BACKUP_BATTERY_CTRL update failed, %d\n", ret);
+			return ret;
+		}
+
+		ret = palmas_update_bits(palmas, PALMAS_PMU_CONTROL_BASE,
+			PALMAS_BACKUP_BATTERY_CTRL,
+			PALMAS_BACKUP_BATTERY_CTRL_BB_CHG_EN,
+			PALMAS_BACKUP_BATTERY_CTRL_BB_CHG_EN);
+		if (ret < 0) {
+			dev_err(&pdev->dev,
+				"BACKUP_BATTERY_CTRL update failed, %d\n", ret);
+			return ret;
+		}
+	}
 
 	/* Start RTC */
 	ret = palmas_update_bits(palmas, PALMAS_RTC_BASE, PALMAS_RTC_CTRL_REG,