iio: imu: st_lsm6dsx: discard samples during filters settling time

During digital filters settling time the driver is expected to drop
samples since they can be corrupted. Introduce the capability to drop
a given number of samples according to the configured ODR.
Add sample_to_discard for LSM6DSM-like sensors since new generation
devices (e.g. LSM6DSO) support DRDY mask where corrupted samples are
masked in hw with values greather than 0x7ffd so the driver can easily
discard them.
I have not added sample_to_discard support for LSM6DS3 or LSM6DS3H since
I do not have any sample for testing at the moment.

Reported-by: Philippe De Muyter <[e-mail]>
Tested-by: Philippe De Muyter <[e-mail]>
Signed-off-by: Lorenzo Bianconi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/imu/st_lsm6dsx/st_lsm6dsx.h

···
 	int odr_len;
 };
 
+struct st_lsm6dsx_samples_to_discard {
+	struct {
+		u32 milli_hz;
+		u16 samples;
+	} val[ST_LSM6DSX_ODR_LIST_SIZE];
+};
+
 struct st_lsm6dsx_fs {
 	u32 gain;
 	u8 val;
···
  * @irq_config: interrupts related registers.
  * @drdy_mask: register info for data-ready mask (addr + mask).
  * @odr_table: Hw sensors odr table (Hz + val).
+ * @samples_to_discard: Number of samples to discard for filters settling time.
  * @fs_table: Hw sensors gain table (gain + val).
  * @decimator: List of decimator register info (addr + mask).
  * @batch: List of FIFO batching register info (addr + mask).
···
 	} irq_config;
 	struct st_lsm6dsx_reg drdy_mask;
 	struct st_lsm6dsx_odr_table_entry odr_table[2];
+	struct st_lsm6dsx_samples_to_discard samples_to_discard[2];
 	struct st_lsm6dsx_fs_table_entry fs_table[2];
 	struct st_lsm6dsx_reg decimator[ST_LSM6DSX_MAX_ID];
 	struct st_lsm6dsx_reg batch[ST_LSM6DSX_MAX_ID];
···
  * @hw: Pointer to instance of struct st_lsm6dsx_hw.
  * @gain: Configured sensor sensitivity.
  * @odr: Output data rate of the sensor [Hz].
+ * @samples_to_discard: Number of samples to discard for filters settling time.
  * @watermark: Sensor watermark level.
  * @decimator: Sensor decimation factor.
  * @sip: Number of samples in a given pattern.
···
 	u32 gain;
 	u32 odr;
 
+	u16 samples_to_discard;
 	u16 watermark;
 	u8 decimator;
 	u8 sip;

drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_buffer.c

···
 			}
 
 			if (gyro_sip > 0 && !(sip % gyro_sensor->decimator)) {
-				iio_push_to_buffers_with_timestamp(
-					hw->iio_devs[ST_LSM6DSX_ID_GYRO],
-					&hw->scan[ST_LSM6DSX_ID_GYRO],
-					gyro_sensor->ts_ref + ts);
+				/*
+				 * We need to discards gyro samples during
+				 * filters settling time
+				 */
+				if (gyro_sensor->samples_to_discard > 0)
+					gyro_sensor->samples_to_discard--;
+				else
+					iio_push_to_buffers_with_timestamp(
+						hw->iio_devs[ST_LSM6DSX_ID_GYRO],
+						&hw->scan[ST_LSM6DSX_ID_GYRO],
+						gyro_sensor->ts_ref + ts);
 				gyro_sip--;
 			}
 			if (acc_sip > 0 && !(sip % acc_sensor->decimator)) {
-				iio_push_to_buffers_with_timestamp(
-					hw->iio_devs[ST_LSM6DSX_ID_ACC],
-					&hw->scan[ST_LSM6DSX_ID_ACC],
-					acc_sensor->ts_ref + ts);
+				/*
+				 * We need to discards accel samples during
+				 * filters settling time
+				 */
+				if (acc_sensor->samples_to_discard > 0)
+					acc_sensor->samples_to_discard--;
+				else
+					iio_push_to_buffers_with_timestamp(
+						hw->iio_devs[ST_LSM6DSX_ID_ACC],
+						&hw->scan[ST_LSM6DSX_ID_ACC],
+						acc_sensor->ts_ref + ts);
 				acc_sip--;
 			}
 			if (ext_sip > 0 && !(sip % ext_sensor->decimator)) {
···
 	return err;
 }
 
+static void
+st_lsm6dsx_update_samples_to_discard(struct st_lsm6dsx_sensor *sensor)
+{
+	const struct st_lsm6dsx_samples_to_discard *data;
+	struct st_lsm6dsx_hw *hw = sensor->hw;
+	int i;
+
+	if (sensor->id != ST_LSM6DSX_ID_GYRO &&
+	    sensor->id != ST_LSM6DSX_ID_ACC)
+		return;
+
+	/* check if drdy mask is supported in hw */
+	if (hw->settings->drdy_mask.addr)
+		return;
+
+	data = &hw->settings->samples_to_discard[sensor->id];
+	for (i = 0; i < ST_LSM6DSX_ODR_LIST_SIZE; i++) {
+		if (data->val[i].milli_hz == sensor->odr) {
+			sensor->samples_to_discard = data->val[i].samples;
+			return;
+		}
+	}
+}
+
 int st_lsm6dsx_update_fifo(struct st_lsm6dsx_sensor *sensor, bool enable)
 {
 	struct st_lsm6dsx_hw *hw = sensor->hw;
···
 		if (err < 0)
 			goto out;
 	}
+
+	if (enable)
+		st_lsm6dsx_update_samples_to_discard(sensor);
 
 	err = st_lsm6dsx_device_set_enable(sensor, enable);
 	if (err < 0)

drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c

···
 				.fs_len = 4,
 			},
 		},
+		.samples_to_discard = {
+			[ST_LSM6DSX_ID_ACC] = {
+				.val[0] = {  12500, 1 },
+				.val[1] = {  26000, 1 },
+				.val[2] = {  52000, 1 },
+				.val[3] = { 104000, 2 },
+				.val[4] = { 208000, 2 },
+				.val[5] = { 416000, 2 },
+			},
+			[ST_LSM6DSX_ID_GYRO] = {
+				.val[0] = {  12500,  2 },
+				.val[1] = {  26000,  5 },
+				.val[2] = {  52000,  7 },
+				.val[3] = { 104000, 12 },
+				.val[4] = { 208000, 20 },
+				.val[5] = { 416000, 36 },
+			},
+		},
 		.irq_config = {
 			.irq1 = {
 				.addr = 0x0d,