iio: imu: inv_mpu6050: switch to use sample rate divider

Instead of storing fifo rate in Hz, store the chip internal sample
rate divider. This will be more useful for timestamping. There
are both equivalent.
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_core.c

@@ -82,7 +82,7 @@ static const struct inv_mpu6050_reg_map reg_set_6050 = {
 static const struct inv_mpu6050_chip_config chip_config_6050 = {
 	.fsr = INV_MPU6050_FSR_2000DPS,
 	.lpf = INV_MPU6050_FILTER_20HZ,
-	.fifo_rate = INV_MPU6050_INIT_FIFO_RATE,
+	.divider = INV_MPU6050_FIFO_RATE_TO_DIVIDER(INV_MPU6050_INIT_FIFO_RATE),
 	.gyro_fifo_enable = false,
 	.accl_fifo_enable = false,
 	.accl_fs = INV_MPU6050_FS_02G,
@@ -278,7 +278,7 @@ static int inv_mpu6050_init_config(struct iio_dev *indio_dev)
 	if (result)
 		goto error_power_off;
 
-	d = INV_MPU6050_ONE_K_HZ / INV_MPU6050_INIT_FIFO_RATE - 1;
+	d = INV_MPU6050_FIFO_RATE_TO_DIVIDER(INV_MPU6050_INIT_FIFO_RATE);
 	result = regmap_write(st->map, st->reg->sample_rate_div, d);
 	if (result)
 		goto error_power_off;
@@ -628,7 +628,7 @@ static ssize_t
 inv_mpu6050_fifo_rate_store(struct device *dev, struct device_attribute *attr,
 			    const char *buf, size_t count)
 {
-	s32 fifo_rate;
+	int fifo_rate;
 	u8 d;
 	int result;
 	struct iio_dev *indio_dev = dev_to_iio_dev(dev);
@@ -644,8 +644,13 @@ inv_mpu6050_fifo_rate_store(struct device *dev, struct device_attribute *attr,
 	if (result)
 		return result;
 
+	/* compute the chip sample rate divider */
+	d = INV_MPU6050_FIFO_RATE_TO_DIVIDER(fifo_rate);
+	/* compute back the fifo rate to handle truncation cases */
+	fifo_rate = INV_MPU6050_DIVIDER_TO_FIFO_RATE(d);
+
 	mutex_lock(&st->lock);
-	if (fifo_rate == st->chip_config.fifo_rate) {
+	if (d == st->chip_config.divider) {
 		result = 0;
 		goto fifo_rate_fail_unlock;
 	}
@@ -653,11 +658,10 @@ inv_mpu6050_fifo_rate_store(struct device *dev, struct device_attribute *attr,
 	if (result)
 		goto fifo_rate_fail_unlock;
 
-	d = INV_MPU6050_ONE_K_HZ / fifo_rate - 1;
 	result = regmap_write(st->map, st->reg->sample_rate_div, d);
 	if (result)
 		goto fifo_rate_fail_power_off;
-	st->chip_config.fifo_rate = fifo_rate;
+	st->chip_config.divider = d;
 
 	result = inv_mpu6050_set_lpf(st, fifo_rate);
 	if (result)
@@ -685,7 +689,7 @@ inv_fifo_rate_show(struct device *dev, struct device_attribute *attr,
 	unsigned fifo_rate;
 
 	mutex_lock(&st->lock);
-	fifo_rate = st->chip_config.fifo_rate;
+	fifo_rate = INV_MPU6050_DIVIDER_TO_FIFO_RATE(st->chip_config.divider);
 	mutex_unlock(&st->lock);
 
 	return scnprintf(buf, PAGE_SIZE, "%u\n", fifo_rate);

drivers/iio/imu/inv_mpu6050/inv_mpu_iio.h

@@ -86,7 +86,7 @@ enum inv_devices {
  *  @accl_fs:		accel full scale range.
  *  @accl_fifo_enable:	enable accel data output
  *  @gyro_fifo_enable:	enable gyro data output
- *  @fifo_rate:		FIFO update rate.
+ *  @divider:		chip sample rate divider (sample rate divider - 1)
  */
 struct inv_mpu6050_chip_config {
 	unsigned int fsr:2;
@@ -94,7 +94,7 @@ struct inv_mpu6050_chip_config {
 	unsigned int accl_fs:2;
 	unsigned int accl_fifo_enable:1;
 	unsigned int gyro_fifo_enable:1;
-	u16 fifo_rate;
+	u8 divider;
 	u8 user_ctrl;
 };
 
@@ -228,7 +228,17 @@ struct inv_mpu6050_state {
 #define INV_MPU6050_INIT_FIFO_RATE           50
 #define INV_MPU6050_MAX_FIFO_RATE            1000
 #define INV_MPU6050_MIN_FIFO_RATE            4
-#define INV_MPU6050_ONE_K_HZ                 1000
+
+/* chip internal frequency: 1KHz */
+#define INV_MPU6050_INTERNAL_FREQ_HZ		1000
+/* return the frequency divider (chip sample rate divider + 1) */
+#define INV_MPU6050_FREQ_DIVIDER(st)					\
+	((st)->chip_config.divider + 1)
+/* chip sample rate divider to fifo rate */
+#define INV_MPU6050_FIFO_RATE_TO_DIVIDER(fifo_rate)			\
+	((INV_MPU6050_INTERNAL_FREQ_HZ / (fifo_rate)) - 1)
+#define INV_MPU6050_DIVIDER_TO_FIFO_RATE(divider)			\
+	(INV_MPU6050_INTERNAL_FREQ_HZ / ((divider) + 1))
 
 #define INV_MPU6050_REG_WHOAMI			117