iio: st_lsm6dsx: Fixed calibrated timestamp calculation

The calibrated timestamp is calculated from the nominal value using the
formula:
ts_gain[ns] ≈ ts_sensitivity - (ts_trim_coeff * val) / 1000.

The values of ts_sensitivity and ts_trim_coeff are not the same for all
devices, so it is necessary to differentiate them based on the part name.
For the correct values please consult the relevant AN.

Fixes: cb3b6b8e1bc0 ("iio: imu: st_lsm6dsx: add odr calibration feature")
Signed-off-by: Mario Tesi <[e-mail]>
Acked-by: Lorenzo Bianconi <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/imu/st_lsm6dsx/st_lsm6dsx.h

···
  * @fifo_en: Hw timer FIFO enable register info (addr + mask).
  * @decimator: Hw timer FIFO decimator register info (addr + mask).
  * @freq_fine: Difference in % of ODR with respect to the typical.
+ * @ts_sensitivity: Nominal timestamp sensitivity.
+ * @ts_trim_coeff: Coefficient for calculating the calibrated timestamp gain.
+ *                 This coefficient comes into play when linearizing the formula
+ *                 used to calculate the calibrated timestamp (please see the
+ *                 relevant formula in the AN for the specific IMU).
+ *                 For example, in the case of LSM6DSO we have:
+ *
+ *                  1 / (1 + x) ~= 1 - x (Taylor’s Series)
+ *                  ttrim[s] = 1 / (40000 * (1 + 0.0015 * val)) (from AN5192)
+ *                  ttrim[ns] ~= 25000 - 37.5 * val
+ *                  ttrim[ns] ~= 25000 - (37500 * val) / 1000
+ *
+ *                  so, replacing ts_sensitivity = 25000 and
+ *                  ts_trim_coeff = 37500
+ *
+ *                  ttrim[ns] ~= ts_sensitivity - (ts_trim_coeff * val) / 1000
  */
 struct st_lsm6dsx_hw_ts_settings {
 	struct st_lsm6dsx_reg timer_en;
···
 	struct st_lsm6dsx_reg fifo_en;
 	struct st_lsm6dsx_reg decimator;
 	u8 freq_fine;
+	u16 ts_sensitivity;
+	u16 ts_trim_coeff;
 };
 
 /**

drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c

···
 
 #define ST_LSM6DSX_REG_WHOAMI_ADDR		0x0f
 
-#define ST_LSM6DSX_TS_SENSITIVITY		25000UL /* 25us */
-
 static const struct iio_chan_spec st_lsm6dsx_acc_channels[] = {
 	ST_LSM6DSX_CHANNEL_ACC(IIO_ACCEL, 0x28, IIO_MOD_X, 0),
 	ST_LSM6DSX_CHANNEL_ACC(IIO_ACCEL, 0x2a, IIO_MOD_Y, 1),
···
 				.mask = GENMASK(7, 6),
 			},
 			.freq_fine = 0x63,
+			.ts_sensitivity = 25000,
+			.ts_trim_coeff = 37500,
 		},
 		.shub_settings = {
 			.page_mux = {
···
 				.mask = GENMASK(7, 6),
 			},
 			.freq_fine = 0x63,
+			.ts_sensitivity = 25000,
+			.ts_trim_coeff = 37500,
 		},
 		.event_settings = {
 			.enable_reg = {
···
 				.mask = GENMASK(7, 6),
 			},
 			.freq_fine = 0x4f,
+			.ts_sensitivity = 21701,
+			.ts_trim_coeff = 28212,
 		},
 		.shub_settings = {
 			.page_mux = {
···
 	}
 
 	/* calibrate timestamp sensitivity */
-	hw->ts_gain = ST_LSM6DSX_TS_SENSITIVITY;
+	hw->ts_gain = ts_settings->ts_sensitivity;
 	if (ts_settings->freq_fine) {
 		err = regmap_read(hw->regmap, ts_settings->freq_fine, &val);
 		if (err < 0)
 			return err;
 
-		/*
-		 * linearize the AN5192 formula:
-		 * 1 / (1 + x) ~= 1 - x (Taylor’s Series)
-		 * ttrim[s] = 1 / (40000 * (1 + 0.0015 * val))
-		 * ttrim[ns] ~= 25000 - 37.5 * val
-		 * ttrim[ns] ~= 25000 - (37500 * val) / 1000
-		 */
-		hw->ts_gain -= ((s8)val * 37500) / 1000;
+		hw->ts_gain -= ((s8)val * ts_settings->ts_trim_coeff) / 1000;
 	}
 
 	return 0;