serial: max310x: Check the clock readiness

This chip has a diagnostics status bit informing about the state and
stability of the clock subsystem. According to the datasheet (STSint
register, bit 5, ClockReady), this bit works with the crystal
oscillator, but even without the PLL. Therefore:

- ensure that the clock check is done even when PLL is not active
- warn when the chip thinks that the clock is not ready yet

There are HW features which would let us wait asynchronously (there's a
maskable IRQ for that bit), but I think that even this simple check is a
net improvement. It would have saved me two days of debugging :).

Signed-off-by: Jan Kundrát <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/max310x.c

···
 	return 1;
 }
 
-static int max310x_set_ref_clk(struct max310x_port *s, unsigned long freq,
-			       bool xtal)
+static int max310x_set_ref_clk(struct device *dev, struct max310x_port *s,
+			       unsigned long freq, bool xtal)
 {
 	unsigned int div, clksrc, pllcfg = 0;
 	long besterr = -1;
···
 	regmap_write(s->regmap, MAX310X_CLKSRC_REG, clksrc);
 
 	/* Wait for crystal */
-	if (pllcfg && xtal)
+	if (xtal) {
+		unsigned int val;
 		msleep(10);
+		regmap_read(s->regmap, MAX310X_STS_IRQSTS_REG, &val);
+		if (!(val & MAX310X_STS_CLKREADY_BIT)) {
+			dev_warn(dev, "clock is not stable yet\n");
+		}
+	}
 
 	return (int)bestfreq;
 }
···
 				   MAX310X_MODE1_AUTOSLEEP_BIT);
 	}
 
-	uartclk = max310x_set_ref_clk(s, freq, xtal);
+	uartclk = max310x_set_ref_clk(dev, s, freq, xtal);
 	dev_dbg(dev, "Reference clock set to %i Hz\n", uartclk);
 
 	mutex_init(&s->mutex);