rtc: isl12022: implement support for the #clock-cells DT property

If device tree implies that the chip's IRQ/F_OUT pin is used as a
clock, expose that in the driver. For now, pretend it is a
fixed-rate (32kHz) clock; if other use cases appear the driver can be
updated to provide its own clk_ops etc.

When the clock output is not used on a given board, one can prolong
the battery life by ensuring that the FOx bits are 0. For the hardware
I'm currently working on, the RTC draws 1.2uA with the FOx bits at
their default 0001 value, dropping to 0.88uA when those bits are
cleared.
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/r/20230615105826.411953-9-linux@rasmusvillemoes.dkSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-isl12022.c

@@ -10,6 +10,7 @@
 
 #include <linux/bcd.h>
 #include <linux/bitfield.h>
+#include <linux/clk-provider.h>
 #include <linux/err.h>
 #include <linux/hwmon.h>
 #include <linux/i2c.h>
@@ -44,6 +45,9 @@
 #define ISL12022_SR_LBAT75	(1 << 1)
 
 #define ISL12022_INT_WRTC	(1 << 6)
+#define ISL12022_INT_FO_MASK	GENMASK(3, 0)
+#define ISL12022_INT_FO_OFF	0x0
+#define ISL12022_INT_FO_32K	0x1
 
 #define ISL12022_REG_VB85_MASK	GENMASK(5, 3)
 #define ISL12022_REG_VB75_MASK	GENMASK(2, 0)
@@ -242,6 +246,41 @@ static const struct regmap_config regmap_config = {
 	.use_single_write = true,
 };
 
+static int isl12022_register_clock(struct device *dev)
+{
+	struct regmap *regmap = dev_get_drvdata(dev);
+	struct clk_hw *hw;
+	int ret;
+
+	if (!device_property_present(dev, "#clock-cells")) {
+		/*
+		 * Disabling the F_OUT pin reduces the power
+		 * consumption in battery mode by ~25%.
+		 */
+		regmap_update_bits(regmap, ISL12022_REG_INT, ISL12022_INT_FO_MASK,
+				   ISL12022_INT_FO_OFF);
+
+		return 0;
+	}
+
+	if (!IS_ENABLED(CONFIG_COMMON_CLK))
+		return 0;
+
+	/*
+	 * For now, only support a fixed clock of 32768Hz (the reset default).
+	 */
+	ret = regmap_update_bits(regmap, ISL12022_REG_INT,
+				 ISL12022_INT_FO_MASK, ISL12022_INT_FO_32K);
+	if (ret)
+		return ret;
+
+	hw = devm_clk_hw_register_fixed_rate(dev, "isl12022", NULL, 0, 32768);
+	if (IS_ERR(hw))
+		return PTR_ERR(hw);
+
+	return devm_of_clk_add_hw_provider(dev, of_clk_hw_simple_get, hw);
+}
+
 static const u32 trip_levels[2][7] = {
 	{ 2125000, 2295000, 2550000, 2805000, 3060000, 4250000, 4675000 },
 	{ 1875000, 2025000, 2250000, 2475000, 2700000, 3750000, 4125000 },
@@ -288,6 +327,7 @@ static int isl12022_probe(struct i2c_client *client)
 {
 	struct rtc_device *rtc;
 	struct regmap *regmap;
+	int ret;
 
 	if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C))
 		return -ENODEV;
@@ -300,6 +340,10 @@ static int isl12022_probe(struct i2c_client *client)
 
 	dev_set_drvdata(&client->dev, regmap);
 
+	ret = isl12022_register_clock(&client->dev);
+	if (ret)
+		return ret;
+
 	isl12022_set_trip_levels(&client->dev);
 	isl12022_hwmon_register(&client->dev);