Re: [PATCH v2] iio: imu: inv_icm42600: fix timestamping by limiting FIFO reading

[Jonathan Cameron]

On Mon, 29 Jun 2026 21:51:55 +0200
Jean-Baptiste Maneyrol via B4 Relay <devnull+jean-baptiste.maneyrol.tdk.com@xxxxxxxxxx> wrote:

> From: Jean-Baptiste Maneyrol <jean-baptiste.maneyrol@xxxxxxx>
> 
> Timestamps are made by measuring the chip clock using the watermark
> interrupts. If we read more than watermark samples as done today, we
> are reducing the period between interrupts and distort the time
> measurement. Fix that by reading only watermark samples in the
> interrupt case.
> 
> Fixes: 7f85e42a6c54 ("iio: imu: inv_icm42600: add buffer support in iio devices")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@xxxxxxxxxxxxxx>
> ---
> Signed-off-by: Jean-Baptiste Maneyrol <jean-baptiste.maneyrol@xxxxxxx>

That's not confusing at all :)

I've applied with the invensense one only - shout if you want something else.

Thanks,

Jonathan



> ---
> Changes in v2:
> - Delete watermark computation rework, keep only FIFO read fix.
> - Link to v1: https://patch.msgid.link/20260623-inv-icm42600-fix-watermark-fifo-reading-v1-1-f3f5694a818a@xxxxxxx
> 
> To: Jean-Baptiste Maneyrol <jean-baptiste.maneyrol@xxxxxxx>
> To: Jonathan Cameron <jic23@xxxxxxxxxx>
> To: David Lechner <dlechner@xxxxxxxxxxxx>
> To: Nuno Sá <nuno.sa@xxxxxxxxxx>
> To: Andy Shevchenko <andy@xxxxxxxxxx>
> Cc: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
> Cc: Jean-Baptiste Maneyrol <jmaneyrol@xxxxxxxxxxxxxx>
> Cc: linux-iio@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  drivers/iio/imu/inv_icm42600/inv_icm42600_buffer.c | 9 +++++----
>  drivers/iio/imu/inv_icm42600/inv_icm42600_buffer.h | 1 +
>  2 files changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/iio/imu/inv_icm42600/inv_icm42600_buffer.c b/drivers/iio/imu/inv_icm42600/inv_icm42600_buffer.c
> index 68a395758031..5c3840acf085 100644
> --- a/drivers/iio/imu/inv_icm42600/inv_icm42600_buffer.c
> +++ b/drivers/iio/imu/inv_icm42600/inv_icm42600_buffer.c
> @@ -248,6 +248,7 @@ int inv_icm42600_buffer_update_watermark(struct inv_icm42600_state *st)
>  
>  	/* compute watermark value in bytes */
>  	wm_size = watermark * packet_size;
> +	st->fifo.watermark.value = watermark;
>  
>  	/* changing FIFO watermark requires to turn off watermark interrupt */
>  	ret = regmap_update_bits_check(st->map, INV_ICM42600_REG_INT_SOURCE0,
> @@ -454,11 +455,10 @@ int inv_icm42600_buffer_fifo_read(struct inv_icm42600_state *st,
>  	st->fifo.nb.accel = 0;
>  	st->fifo.nb.total = 0;
>  
> -	/* compute maximum FIFO read size */
> +	/* compute maximum FIFO read size (watermark for max = 0 interrupt case) */
>  	if (max == 0)
> -		max_count = sizeof(st->fifo.data);
> -	else
> -		max_count = max * inv_icm42600_get_packet_size(st->fifo.en);
> +		max = st->fifo.watermark.value;
> +	max_count = max * inv_icm42600_get_packet_size(st->fifo.en);
>  
>  	/* read FIFO count value */
>  	raw_fifo_count = (__be16 *)st->buffer;
> @@ -574,6 +574,7 @@ int inv_icm42600_buffer_init(struct inv_icm42600_state *st)
>  
>  	st->fifo.watermark.eff_gyro = 1;
>  	st->fifo.watermark.eff_accel = 1;
> +	st->fifo.watermark.value = 1;
>  
>  	/*
>  	 * Default FIFO configuration (bits 7 to 5)
> diff --git a/drivers/iio/imu/inv_icm42600/inv_icm42600_buffer.h b/drivers/iio/imu/inv_icm42600/inv_icm42600_buffer.h
> index ffca4da1e249..88b8b9f780af 100644
> --- a/drivers/iio/imu/inv_icm42600/inv_icm42600_buffer.h
> +++ b/drivers/iio/imu/inv_icm42600/inv_icm42600_buffer.h
> @@ -34,6 +34,7 @@ struct inv_icm42600_fifo {
>  		unsigned int accel;
>  		unsigned int eff_gyro;
>  		unsigned int eff_accel;
> +		unsigned int value;
>  	} watermark;
>  	size_t count;
>  	struct {
> 
> ---
> base-commit: cc746297b23e89bd5df9f91f3a0ca209e8991763
> change-id: 20260623-inv-icm42600-fix-watermark-fifo-reading-624caf080754
> 
> Best regards,
> --  
> Jean-Baptiste Maneyrol <jean-baptiste.maneyrol@xxxxxxx>
> 
> 
>