sc16is7xx: Read the LSR register for basic device presence check

Currently, the driver probes just fine and binds all its resources even
if the physical device is not present.

As the device lacks an identification register, let's at least read the
LSR register to check whether a device at the configured address responds
to the request at all.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Link: https://lore.kernel.org/r/20200521091152.404404-7-daniel@zonque.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sc16is7xx.c

@@ -1172,6 +1172,7 @@ static int sc16is7xx_probe(struct device *dev,
 {
 	struct sched_param sched_param = { .sched_priority = MAX_RT_PRIO / 2 };
 	unsigned long freq = 0, *pfreq = dev_get_platdata(dev);
+	unsigned int val;
 	u32 uartclk = 0;
 	int i, ret;
 	struct sc16is7xx_port *s;
@@ -1179,6 +1180,16 @@ static int sc16is7xx_probe(struct device *dev,
 	if (IS_ERR(regmap))
 		return PTR_ERR(regmap);
 
+	/*
+	 * This device does not have an identification register that would
+	 * tell us if we are really connected to the correct device.
+	 * The best we can do is to check if communication is at all possible.
+	 */
+	ret = regmap_read(regmap,
+			  SC16IS7XX_LSR_REG << SC16IS7XX_REG_SHIFT, &val);
+	if (ret < 0)
+		return ret;
+
 	/* Alloc port structure */
 	s = devm_kzalloc(dev, struct_size(s, p, devtype->nr_uart), GFP_KERNEL);
 	if (!s) {