linux-kernel - Re: [PATCH] iio: st_lsm6dsx: Fixed calibrated timestamp calculation

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Message-ID: <aO_e2DEGsO-kJQFx@lore-desk>
Date: Wed, 15 Oct 2025 19:50:16 +0200
From: Lorenzo Bianconi <lorenzo@...nel.org>
To: Mario Tesi <martepisa@...il.com>
Cc: linux-iio@...r.kernel.org, jic23@...nel.org, dlechner@...libre.com,
	nuno.sa@...log.com, andy@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] 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.
> 
> Signed-off-by: Mario Tesi <mario.tesi@...com>

Acked-by: Lorenzo Bianconi <lorenzo@...nel.org>

> ---
>  drivers/iio/imu/st_lsm6dsx/st_lsm6dsx.h      | 18 ++++++++++++++++++
>  drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c | 19 ++++++++-----------
>  2 files changed, 26 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx.h b/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx.h
> index c225b246c8a5..f8486a1b02d0 100644
> --- a/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx.h
> +++ b/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx.h
> @@ -192,6 +192,22 @@ struct st_lsm6dsx_fifo_ops {
>   * @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;
> @@ -199,6 +215,8 @@ struct st_lsm6dsx_hw_ts_settings {
>  	struct st_lsm6dsx_reg fifo_en;
>  	struct st_lsm6dsx_reg decimator;
>  	u8 freq_fine;
> +	u16 ts_sensitivity;
> +	u16 ts_trim_coeff;
>  };
>  
>  /**
> diff --git a/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c b/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c
> index d8cb4b0218d5..a2daf0c14d96 100644
> --- a/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c
> +++ b/drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c
> @@ -94,8 +94,6 @@
>  
>  #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),
> @@ -983,6 +981,8 @@ static const struct st_lsm6dsx_settings st_lsm6dsx_sensor_settings[] = {
>  				.mask = GENMASK(7, 6),
>  			},
>  			.freq_fine = 0x63,
> +			.ts_sensitivity = 25000,
> +			.ts_trim_coeff = 37500,
>  		},
>  		.shub_settings = {
>  			.page_mux = {
> @@ -1196,6 +1196,8 @@ static const struct st_lsm6dsx_settings st_lsm6dsx_sensor_settings[] = {
>  				.mask = GENMASK(7, 6),
>  			},
>  			.freq_fine = 0x63,
> +			.ts_sensitivity = 25000,
> +			.ts_trim_coeff = 37500,
>  		},
>  		.event_settings = {
>  			.enable_reg = {
> @@ -1371,6 +1373,8 @@ static const struct st_lsm6dsx_settings st_lsm6dsx_sensor_settings[] = {
>  				.mask = GENMASK(7, 6),
>  			},
>  			.freq_fine = 0x4f,
> +			.ts_sensitivity = 21701,
> +			.ts_trim_coeff = 28212,
>  		},
>  		.shub_settings = {
>  			.page_mux = {
> @@ -2248,20 +2252,13 @@ static int st_lsm6dsx_init_hw_timer(struct st_lsm6dsx_hw *hw)
>  	}
>  
>  	/* 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;
> -- 
> 2.25.1
> 

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