rtc: isl1208: Add isl1208_set_xtoscb()

As per the HW manual, set the XTOSCB bit as follows:

If using an external clock signal, set the XTOSCB bit as 1 to
disable the crystal oscillator.

If using an external crystal, the XTOSCB bit needs to be set at 0
to enable the crystal oscillator.

Add isl1208_set_xtoscb() to set XTOSCB bit based on the clock-names
property. Fallback is enabling the internal crystal oscillator.

While at it, introduce a variable "sr" for reading the status register
in probe() as it is reused for writing and also remove the unnecessary
blank line.
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/20230623140948.384762-10-biju.das.jz@bp.renesas.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-isl1208.c

@@ -6,6 +6,7 @@
  */
 
 #include <linux/bcd.h>
+#include <linux/clk.h>
 #include <linux/i2c.h>
 #include <linux/module.h>
 #include <linux/of_device.h>
@@ -175,6 +176,20 @@ isl1208_i2c_validate_client(struct i2c_client *client)
 	return 0;
 }
 
+static int isl1208_set_xtoscb(struct i2c_client *client, int sr, int xtosb_val)
+{
+	/* Do nothing if bit is already set to desired value */
+	if ((sr & ISL1208_REG_SR_XTOSCB) == xtosb_val)
+		return 0;
+
+	if (xtosb_val)
+		sr |= ISL1208_REG_SR_XTOSCB;
+	else
+		sr &= ~ISL1208_REG_SR_XTOSCB;
+
+	return i2c_smbus_write_byte_data(client, ISL1208_REG_SR, sr);
+}
+
 static int
 isl1208_i2c_get_sr(struct i2c_client *client)
 {
@@ -511,7 +526,6 @@ isl1208_i2c_set_time(struct i2c_client *client, struct rtc_time const *tm)
 	return 0;
 }
 
-
 static int
 isl1208_rtc_set_time(struct device *dev, struct rtc_time *tm)
 {
@@ -805,12 +819,26 @@ static int isl1208_setup_irq(struct i2c_client *client, int irq)
 	return rc;
 }
 
+static int
+isl1208_clk_present(struct i2c_client *client, const char *name)
+{
+	struct clk *clk;
+
+	clk = devm_clk_get_optional(&client->dev, name);
+	if (IS_ERR(clk))
+		return PTR_ERR(clk);
+
+	return !!clk;
+}
+
 static int
 isl1208_probe(struct i2c_client *client)
 {
-	int rc = 0;
 	struct isl1208_state *isl1208;
 	int evdet_irq = -1;
+	int xtosb_val = 0;
+	int rc = 0;
+	int sr;
 
 	if (!i2c_check_functionality(client->adapter, I2C_FUNC_I2C))
 		return -ENODEV;
@@ -837,6 +865,19 @@ isl1208_probe(struct i2c_client *client)
 		isl1208->config = (struct isl1208_config *)id->driver_data;
 	}
 
+	rc = isl1208_clk_present(client, "xin");
+	if (rc < 0)
+		return rc;
+
+	if (!rc) {
+		rc = isl1208_clk_present(client, "clkin");
+		if (rc < 0)
+			return rc;
+
+		if (rc)
+			xtosb_val = 1;
+	}
+
 	isl1208->rtc = devm_rtc_allocate_device(&client->dev);
 	if (IS_ERR(isl1208->rtc))
 		return PTR_ERR(isl1208->rtc);
@@ -848,13 +889,17 @@ isl1208_probe(struct i2c_client *client)
 	isl1208->nvmem_config.size = isl1208->config->nvmem_length;
 	isl1208->nvmem_config.priv = isl1208;
 
-	rc = isl1208_i2c_get_sr(client);
-	if (rc < 0) {
+	sr = isl1208_i2c_get_sr(client);
+	if (sr < 0) {
 		dev_err(&client->dev, "reading status failed\n");
-		return rc;
+		return sr;
 	}
 
-	if (rc & ISL1208_REG_SR_RTCF)
+	rc = isl1208_set_xtoscb(client, sr, xtosb_val);
+	if (rc)
+		return rc;
+
+	if (sr & ISL1208_REG_SR_RTCF)
 		dev_warn(&client->dev, "rtc power failure detected, "
 			 "please set clock.\n");