iio: magnetometer: ak8975: Add gpio reset support

According to AK09911 datasheet, if reset gpio is provided then
deassert reset on ak8975_power_on() and assert reset on ak8975_power_off().

Without reset's deassertion during ak8975_power_on(), driver's probe fails
on ak8975_who_i_am() while checking for device identity for AK09911 chip.

AK09911 has an active low reset gpio to handle register's reset.
AK09911 datasheet says that, if not used, reset pin should be connected
to VID. This patch emulates this situation.
Signed-off-by: Jonathan Albrieux <jonathan.albrieux@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/magnetometer/ak8975.c

@@ -358,6 +358,7 @@ struct ak8975_data {
 	u8			asa[3];
 	long			raw_to_gauss[3];
 	struct gpio_desc	*eoc_gpiod;
+	struct gpio_desc	*reset_gpiod;
 	int			eoc_irq;
 	wait_queue_head_t	data_ready_queue;
 	unsigned long		flags;
@@ -384,6 +385,9 @@ static int ak8975_power_on(const struct ak8975_data *data)
 			 "Failed to enable specified Vid supply\n");
 		return ret;
 	}
+
+	gpiod_set_value_cansleep(data->reset_gpiod, 0);
+
 	/*
 	 * According to the datasheet the power supply rise time is 200us
 	 * and the minimum wait time before mode setting is 100us, in
@@ -396,6 +400,8 @@ static int ak8975_power_on(const struct ak8975_data *data)
 /* Disable attached power regulator if any. */
 static void ak8975_power_off(const struct ak8975_data *data)
 {
+	gpiod_set_value_cansleep(data->reset_gpiod, 1);
+
 	regulator_disable(data->vid);
 	regulator_disable(data->vdd);
 }
@@ -839,6 +845,7 @@ static int ak8975_probe(struct i2c_client *client,
 	struct ak8975_data *data;
 	struct iio_dev *indio_dev;
 	struct gpio_desc *eoc_gpiod;
+	struct gpio_desc *reset_gpiod;
 	const void *match;
 	unsigned int i;
 	int err;
@@ -856,6 +863,16 @@ static int ak8975_probe(struct i2c_client *client,
 	if (eoc_gpiod)
 		gpiod_set_consumer_name(eoc_gpiod, "ak_8975");
 
+	/*
+	 * According to AK09911 datasheet, if reset GPIO is provided then
+	 * deassert reset on ak8975_power_on() and assert reset on
+	 * ak8975_power_off().
+	 */
+	reset_gpiod = devm_gpiod_get_optional(&client->dev,
+					      "reset", GPIOD_OUT_HIGH);
+	if (IS_ERR(reset_gpiod))
+		return PTR_ERR(reset_gpiod);
+
 	/* Register with IIO */
 	indio_dev = devm_iio_device_alloc(&client->dev, sizeof(*data));
 	if (indio_dev == NULL)
@@ -866,6 +883,7 @@ static int ak8975_probe(struct i2c_client *client,
 
 	data->client = client;
 	data->eoc_gpiod = eoc_gpiod;
+	data->reset_gpiod = reset_gpiod;
 	data->eoc_irq = 0;
 
 	err = iio_read_mount_matrix(&client->dev, "mount-matrix", &data->orientation);