[PATCH v3 2/2] serial: sc16is7xx: hardware reset chip if reset-gpios is defined in DT

[Hui Wang <hui.wang@canonical.com>]

Some boards connect a GPIO to the reset pin, and the reset pin needs
to be setup correctly before accessing the chip.

Add a function to handle the chip reset. If the reset-gpios is defined
in the DT, do hardware reset through this GPIO, othwerwise do software
reset as before.

Signed-off-by: Hui Wang <hui.wang@canonical.com>
---
In the v3:
 - drop Reviewed-by
 - adjust the sequence of if, else if and else
 - replace PTR_ERR(reset_gpiod) with dev_err_probe
 - change GPIOD_OUT_LOW to GPIOD_OUT_HIGH, this will assert the reset pin after requesting the GPIO
 - change the 2nd parameter struct regmap *regmap[] to struct regmap *regmap
 - address all spelling and description issues in the v2
 
 drivers/tty/serial/sc16is7xx.c | 33 +++++++++++++++++++++++++++++----
 1 file changed, 29 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/serial/sc16is7xx.c b/drivers/tty/serial/sc16is7xx.c
index bf0065d1c8e9..8c7e0fe76049 100644
--- a/drivers/tty/serial/sc16is7xx.c
+++ b/drivers/tty/serial/sc16is7xx.c
@@ -14,6 +14,7 @@
 #include <linux/delay.h>
 #include <linux/device.h>
 #include <linux/export.h>
+#include <linux/gpio/consumer.h>
 #include <linux/gpio/driver.h>
 #include <linux/idr.h>
 #include <linux/kthread.h>
@@ -1467,6 +1468,30 @@ static const struct serial_rs485 sc16is7xx_rs485_supported = {
 	.delay_rts_after_send = 1,	/* Not supported but keep returning -EINVAL */
 };
 
+/* Reset device, purging any pending irq / data */
+static int sc16is7xx_reset(struct device *dev, struct regmap *regmap)
+{
+	struct gpio_desc *reset_gpio;
+
+	/* The reset GPIO is ACTIVE_LOW, flag GPIOD_OUT_HIGH asserts the reset GPIO */
+	reset_gpio = devm_gpiod_get_optional(dev, "reset", GPIOD_OUT_HIGH);
+	if (IS_ERR(reset_gpio))
+		return dev_err_probe(dev, PTR_ERR(reset_gpio), "Failed to get reset GPIO\n");
+
+	if (reset_gpio) {
+		/* The minimum reset pulse width is 3 us. */
+		udelay(5);
+		gpiod_set_value_cansleep(reset_gpio, 0);
+		/* Deassert GPIO */
+	} else {
+		/* Software reset */
+		regmap_write(regmap, SC16IS7XX_IOCONTROL_REG,
+			     SC16IS7XX_IOCONTROL_SRESET_BIT);
+	}
+
+	return 0;
+}
+
 int sc16is7xx_probe(struct device *dev, const struct sc16is7xx_devtype *devtype,
 		    struct regmap *regmaps[], int irq)
 {
@@ -1527,6 +1552,10 @@ int sc16is7xx_probe(struct device *dev, const struct sc16is7xx_devtype *devtype,
 	s->devtype = devtype;
 	dev_set_drvdata(dev, s);
 
+	ret = sc16is7xx_reset(dev, regmaps[0]);
+	if (ret)
+		goto out_clk;
+
 	kthread_init_worker(&s->kworker);
 	s->kworker_task = kthread_run(kthread_worker_fn, &s->kworker,
 				      "sc16is7xx");
@@ -1536,10 +1565,6 @@ int sc16is7xx_probe(struct device *dev, const struct sc16is7xx_devtype *devtype,
 	}
 	sched_set_fifo(s->kworker_task);
 
-	/* reset device, purging any pending irq / data */
-	regmap_write(regmaps[0], SC16IS7XX_IOCONTROL_REG,
-		     SC16IS7XX_IOCONTROL_SRESET_BIT);
-
 	/* Mark each port line and status as uninitialised. */
 	for (i = 0; i < devtype->nr_uart; ++i) {
 		s->p[i].port.line = SC16IS7XX_MAX_DEVS;
-- 
2.34.1