iio: bmg160: reset chip when probing

The gyroscope chip might need to be reset to be used.

Without the chip being reset, the driver stopped at the first
regmap_read (to get the CHIP_ID) and failed to probe.

The datasheet of the gyroscope says that a minimum wait of 30ms after
the reset has to be done.

This patch has been checked on a BMX055 and the datasheet of the BMG160
and the BMI055 give the same reset register and bits.
Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/gyro/bmg160_core.c

@@ -27,6 +27,7 @@
 #include <linux/iio/trigger_consumer.h>
 #include <linux/iio/triggered_buffer.h>
 #include <linux/regmap.h>
+#include <linux/delay.h>
 #include "bmg160.h"
 
 #define BMG160_IRQ_NAME		"bmg160_event"
@@ -52,6 +53,9 @@
 #define BMG160_DEF_BW			100
 #define BMG160_REG_PMU_BW_RES		BIT(7)
 
+#define BMG160_GYRO_REG_RESET		0x14
+#define BMG160_GYRO_RESET_VAL		0xb6
+
 #define BMG160_REG_INT_MAP_0		0x17
 #define BMG160_INT_MAP_0_BIT_ANY	BIT(1)
 
@@ -236,6 +240,14 @@ static int bmg160_chip_init(struct bmg160_data *data)
 	int ret;
 	unsigned int val;
 
+	/*
+	 * Reset chip to get it in a known good state. A delay of 30ms after
+	 * reset is required according to the datasheet.
+	 */
+	regmap_write(data->regmap, BMG160_GYRO_REG_RESET,
+		     BMG160_GYRO_RESET_VAL);
+	usleep_range(30000, 30700);
+
 	ret = regmap_read(data->regmap, BMG160_REG_CHIP_ID, &val);
 	if (ret < 0) {
 		dev_err(dev, "Error reading reg_chip_id\n");