serial: max310x: Try to get crystal clock rate from property

In some configurations, mainly ACPI-based, the clock frequency of the device
is supplied by very well established 'clock-frequency' property. Hence, try
to get it from the property at last if no other providers are available.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210517172930.83353-1-andriy.shevchenko@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/max310x.c

@@ -552,7 +552,7 @@ static int max310x_update_best_err(unsigned long f, long *besterr)
 	return 1;
 }
 
-static int max310x_set_ref_clk(struct device *dev, struct max310x_port *s,
+static u32 max310x_set_ref_clk(struct device *dev, struct max310x_port *s,
 			       unsigned long freq, bool xtal)
 {
 	unsigned int div, clksrc, pllcfg = 0;
@@ -618,7 +618,7 @@ static int max310x_set_ref_clk(struct device *dev, struct max310x_port *s,
 		}
 	}
 
-	return (int)bestfreq;
+	return bestfreq;
 }
 
 static void max310x_batch_write(struct uart_port *port, u8 *txbuf, unsigned int len)
@@ -1253,9 +1253,10 @@ static int max310x_gpio_set_config(struct gpio_chip *chip, unsigned int offset,
 static int max310x_probe(struct device *dev, const struct max310x_devtype *devtype,
 			 struct regmap *regmap, int irq)
 {
-	int i, ret, fmin, fmax, freq, uartclk;
+	int i, ret, fmin, fmax, freq;
 	struct max310x_port *s;
-	bool xtal = false;
+	u32 uartclk = 0;
+	bool xtal;
 
 	if (IS_ERR(regmap))
 		return PTR_ERR(regmap);
@@ -1267,24 +1268,20 @@ static int max310x_probe(struct device *dev, const struct max310x_devtype *devty
 		return -ENOMEM;
 	}
 
+	/* Always ask for fixed clock rate from a property. */
+	device_property_read_u32(dev, "clock-frequency", &uartclk);
+
 	s->clk = devm_clk_get_optional(dev, "osc");
 	if (IS_ERR(s->clk))
 		return PTR_ERR(s->clk);
 	if (s->clk) {
-		fmin = 500000;
-		fmax = 35000000;
+		xtal = false;
 	} else {
 		s->clk = devm_clk_get_optional(dev, "xtal");
 		if (IS_ERR(s->clk))
 			return PTR_ERR(s->clk);
-		if (s->clk) {
-			fmin = 1000000;
-			fmax = 4000000;
-			xtal = true;
-		} else {
-			dev_err(dev, "Cannot get clock\n");
-			return -EINVAL;
-		}
+
+		xtal = true;
 	}
 
 	ret = clk_prepare_enable(s->clk);
@@ -1292,6 +1289,21 @@ static int max310x_probe(struct device *dev, const struct max310x_devtype *devty
 		return ret;
 
 	freq = clk_get_rate(s->clk);
+	if (freq == 0)
+		freq = uartclk;
+	if (freq == 0) {
+		dev_err(dev, "Cannot get clock rate\n");
+		return -EINVAL;
+	}
+
+	if (xtal) {
+		fmin = 1000000;
+		fmax = 4000000;
+	} else {
+		fmin = 500000;
+		fmax = 35000000;
+	}
+
 	/* Check frequency limits */
 	if (freq < fmin || freq > fmax) {
 		ret = -ERANGE;