rtc: bq32000: add support to enable disable the trickle charge FET bypass

The bq32000 includes a trickle charge circuit to maintain the charge of the
backup supply when a super capacitor is used.

You can enable the charging circuit by setting 'trickle-resistor-ohms',
additionally you can set TCFE to 1 to bypass the internal diode and boost
the charge voltage of the backup supply. You might want to enable/disable
the TCFE switch from userspace (e.g when device is only connected to a
battery)

This patch introduces a new sysfs entry to enable and disable this FET
form userspace.
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

Documentation/ABI/testing/sysfs-bus-i2c-devices-bq32k

+What:		/sys/bus/i2c/devices/.../trickle_charge_bypass
+Date:		Jan 2017
+KernelVersion:	4.11
+Contact:	Enric Balletbo i Serra <eballetbo@gmail.com>
+Description:    Attribute for enable/disable the trickle charge bypass
+		The trickle_charge_bypass attribute allows the userspace to
+                enable/disable the Trickle charge FET bypass.

drivers/rtc/rtc-bq32k.c

@@ -34,6 +34,7 @@
 #define BQ32K_CALIBRATION	0x07	/* CAL_CFG1, calibration and control */
 #define BQ32K_TCH2		0x08	/* Trickle charge enable */
 #define BQ32K_CFG2		0x09	/* Trickle charger control */
+#define BQ32K_TCFE		BIT(6)	/* Trickle charge FET bypass */
 
 struct bq32k_regs {
 	uint8_t		seconds;
@@ -188,6 +189,65 @@ static int trickle_charger_of_init(struct device *dev, struct device_node *node)
 	return 0;
 }
 
+static ssize_t bq32k_sysfs_show_tricklecharge_bypass(struct device *dev,
+					       struct device_attribute *attr,
+					       char *buf)
+{
+	int reg, error;
+
+	error = bq32k_read(dev, &reg, BQ32K_CFG2, 1);
+	if (error)
+		return error;
+
+	return sprintf(buf, "%d\n", (reg & BQ32K_TCFE) ? 1 : 0);
+}
+
+static ssize_t bq32k_sysfs_store_tricklecharge_bypass(struct device *dev,
+						struct device_attribute *attr,
+						const char *buf, size_t count)
+{
+	int reg, enable, error;
+
+	if (kstrtoint(buf, 0, &enable))
+		return -EINVAL;
+
+	error = bq32k_read(dev, &reg, BQ32K_CFG2, 1);
+	if (error)
+		return error;
+
+	if (enable) {
+		reg |= BQ32K_TCFE;
+		error = bq32k_write(dev, &reg, BQ32K_CFG2, 1);
+		if (error)
+			return error;
+
+		dev_info(dev, "Enabled trickle charge FET bypass.\n");
+	} else {
+		reg &= ~BQ32K_TCFE;
+		error = bq32k_write(dev, &reg, BQ32K_CFG2, 1);
+		if (error)
+			return error;
+
+		dev_info(dev, "Disabled trickle charge FET bypass.\n");
+	}
+
+	return count;
+}
+
+static DEVICE_ATTR(trickle_charge_bypass, 0644,
+		   bq32k_sysfs_show_tricklecharge_bypass,
+		   bq32k_sysfs_store_tricklecharge_bypass);
+
+static int bq32k_sysfs_register(struct device *dev)
+{
+	return device_create_file(dev, &dev_attr_trickle_charge_bypass);
+}
+
+static void bq32k_sysfs_unregister(struct device *dev)
+{
+	device_remove_file(dev, &dev_attr_trickle_charge_bypass);
+}
+
 static int bq32k_probe(struct i2c_client *client,
 				const struct i2c_device_id *id)
 {
@@ -224,11 +284,26 @@ static int bq32k_probe(struct i2c_client *client,
 	if (IS_ERR(rtc))
 		return PTR_ERR(rtc);
 
+	error = bq32k_sysfs_register(&client->dev);
+	if (error) {
+		dev_err(&client->dev,
+			"Unable to create sysfs entries for rtc bq32000\n");
+		return error;
+	}
+
+
 	i2c_set_clientdata(client, rtc);
 
 	return 0;
 }
 
+static int bq32k_remove(struct i2c_client *client)
+{
+	bq32k_sysfs_unregister(&client->dev);
+
+	return 0;
+}
+
 static const struct i2c_device_id bq32k_id[] = {
 	{ "bq32000", 0 },
 	{ }
@@ -240,6 +315,7 @@ static struct i2c_driver bq32k_driver = {
 		.name	= "bq32k",
 	},
 	.probe		= bq32k_probe,
+	.remove		= bq32k_remove,
 	.id_table	= bq32k_id,
 };