iio: addac: ad74413r: add support for reset-gpio

We have a board where the reset pin of the ad74412 is connected to a
gpio, but also pulled low (i.e., asserted) by default. Hence to get
the chip out of reset, the driver needs to know about that gpio and
deassert the reset signal before attempting to communicate with the
chip.

When a reset-gpio is given in device tree, use that instead of the
software reset. According to the data sheet, the two methods are
functionally equivalent.
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/r/20221115095517.1008632-4-linux@rasmusvillemoes.dkSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/addac/ad74413r.c

@@ -71,6 +71,7 @@ struct ad74413r_state {
 	struct regmap			*regmap;
 	struct device			*dev;
 	struct iio_trigger		*trig;
+	struct gpio_desc		*reset_gpio;
 
 	size_t			adc_active_channels;
 	struct spi_message	adc_samples_msg;
@@ -393,6 +394,13 @@ static int ad74413r_reset(struct ad74413r_state *st)
 {
 	int ret;
 
+	if (st->reset_gpio) {
+		gpiod_set_value_cansleep(st->reset_gpio, 1);
+		fsleep(50);
+		gpiod_set_value_cansleep(st->reset_gpio, 0);
+		return 0;
+	}
+
 	ret = regmap_write(st->regmap, AD74413R_REG_CMD_KEY,
 			   AD74413R_CMD_KEY_RESET1);
 	if (ret)
@@ -1322,6 +1330,10 @@ static int ad74413r_probe(struct spi_device *spi)
 	if (IS_ERR(st->regmap))
 		return PTR_ERR(st->regmap);
 
+	st->reset_gpio = devm_gpiod_get_optional(st->dev, "reset", GPIOD_OUT_LOW);
+	if (IS_ERR(st->reset_gpio))
+		return PTR_ERR(st->reset_gpio);
+
 	st->refin_reg = devm_regulator_get(st->dev, "refin");
 	if (IS_ERR(st->refin_reg))
 		return dev_err_probe(st->dev, PTR_ERR(st->refin_reg),