iio: imu: inv_mpu6050: skip first sample when gyro is on

Implement generic skip first samples mechanism and use it to
filter out first sample when gyro is on.

The problem for these chips is that the first sample of the gyro
is out of specs, because gyro is not completely stabilized. To
ensure all data are within sensor specs, we just skip the first
sample when turning gyro on.
Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/imu/inv_mpu6050/inv_mpu_iio.h

@@ -148,6 +148,7 @@ struct inv_mpu6050_state {
 	struct regmap *map;
 	int irq;
 	u8 irq_mask;
+	unsigned skip_samples;
 };
 
 /*register and associated bit definition*/

drivers/iio/imu/inv_mpu6050/inv_mpu_ring.c

@@ -185,7 +185,12 @@ irqreturn_t inv_mpu6050_read_fifo(int irq, void *p)
 		if (result == 0)
 			timestamp = 0;
 
-		iio_push_to_buffers_with_timestamp(indio_dev, data, timestamp);
+		/* skip first samples if needed */
+		if (st->skip_samples)
+			st->skip_samples--;
+		else
+			iio_push_to_buffers_with_timestamp(indio_dev, data,
+							   timestamp);
 
 		fifo_count -= bytes_per_datum;
 	}

drivers/iio/imu/inv_mpu6050/inv_mpu_trigger.c

@@ -49,11 +49,14 @@ static int inv_mpu6050_set_enable(struct iio_dev *indio_dev, bool enable)
 		if (result)
 			return result;
 		inv_scan_query(indio_dev);
+		st->skip_samples = 0;
 		if (st->chip_config.gyro_fifo_enable) {
 			result = inv_mpu6050_switch_engine(st, true,
 					INV_MPU6050_BIT_PWR_GYRO_STBY);
 			if (result)
 				goto error_power_off;
+			/* gyro first sample is out of specs, skip it */
+			st->skip_samples = 1;
 		}
 		if (st->chip_config.accl_fifo_enable) {
 			result = inv_mpu6050_switch_engine(st, true,