serial: max310x: improve crystal stable clock detection

Some people are seeing a warning similar to this when using a crystal:

    max310x 11-006c: clock is not stable yet

The datasheet doesn't mention the maximum time to wait for the clock to be
stable when using a crystal, and it seems that the 10ms delay in the driver
is not always sufficient.

Jan Kundrát reported that it took three tries (each separated by 10ms) to
get a stable clock.

Modify behavior to check stable clock ready bit multiple times (20), and
waiting 10ms between each try.

Note: the first draft of the driver originally used a 50ms delay, without
checking the clock stable bit.
Then a loop with 1000 retries was implemented, each time reading the clock
stable bit.

Fixes: 4cf9a88 ("serial: max310x: Check the clock readiness")
Cc: [email protected]
Suggested-by: Jan Kundrát <[email protected]>
Link: https://www.spinics.net/lists/linux-serial/msg35773.html
Link: https://lore.kernel.org/all/[email protected]/raw
Link: boundarydevices/linux@e5dfe3e
Signed-off-by: Hugo Villeneuve <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: hvilleneuvedoo

drivers/tty/serial/max310x.c

@@ -237,6 +237,10 @@
 #define MAX310x_REV_MASK		(0xf8)
 #define MAX310X_WRITE_BIT		0x80
 
+/* Crystal-related definitions */
+#define MAX310X_XTAL_WAIT_RETRIES	20 /* Number of retries */
+#define MAX310X_XTAL_WAIT_DELAY_MS	10 /* Delay between retries */
+
 /* MAX3107 specific */
 #define MAX3107_REV_ID			(0xa0)
 
@@ -641,12 +645,19 @@ static u32 max310x_set_ref_clk(struct device *dev, struct max310x_port *s,
 
 	/* Wait for crystal */
 	if (xtal) {
-		unsigned int val = 0;
-		msleep(10);
-		regmap_read(s->regmap, MAX310X_STS_IRQSTS_REG, &val);
-		if (!(val & MAX310X_STS_CLKREADY_BIT)) {
+		bool stable = false;
+		unsigned int try = 0, val = 0;
+
+		do {
+			msleep(MAX310X_XTAL_WAIT_DELAY_MS);
+			regmap_read(s->regmap, MAX310X_STS_IRQSTS_REG, &val);
+
+			if (val & MAX310X_STS_CLKREADY_BIT)
+				stable = true;
+		} while (!stable && (++try < MAX310X_XTAL_WAIT_RETRIES));
+
+		if (!stable)
 			dev_warn(dev, "clock is not stable yet\n");
-		}
 	}
 
 	return bestfreq;