Re: [PATCH 2/2] iio: imu: st_lsm6dsx: Decouple sensor ODR from FIFO batch data rate

Lorenzo Bianconi
2025-10-10T16:22:10Z

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On Oct 10, David Lechner wrote:
> On 10/10/25 8:15 AM, Lorenzo Bianconi wrote:
> > On Oct 10, Francesco Lavra wrote:
> >> On Fri, 2025-10-10 at 10:13 +0200, Lorenzo Bianconi wrote:
> >>>> On Fri, 2025-10-10 at 00:30 +0200, Lorenzo Bianconi wrote:
> >>>>>> The rate at which accelerometer or gyroscope sensor samples are fed
> >>>>>> to the hardware FIFO (batch data rate, or BDR) does not have to
> >>>>>> coincide with the sensor sampling frequency (output data rate, or
> >>>>>> ODR); the only requirement is for the BDR to not be greater than
> >>>>>> the ODR. Having a BDR lower than the ODR is useful in cases where
> >>>>>> an application requires a high sampling rate for accurate detection
> >>>>>> of motion events (e.g. wakeup events), but wants to read sensor
> >>>>>> sample values from the device buffer at a lower data rate.
> >>>>>
> >>>>> can you please provide more details here? Are you using the hw fifo
> >>>>> to
> >>>>> read
> >>>>> data? If we configure the hw fifo according to the BDR (even assuming
> >>>>> the
> >>>>> watermark is set 1) the hw will generate interrupts according to the
> >>>>> BDR
> >>>>> (bdr < odr).
> >>>>
> >>>> Yes, I'm using the hw fifo to read data. The use case is to enable
> >>>> event
> >>>> detection (which works best at high sampling rates) and sensor data
> >>>> streaming at the same time, without requiring the data stream to be at
> >>>> the
> >>>> same rate as the sensor sampling rate. So the amount of I2C (or SPI)
> >>>> traffic (as well as the rate of periodic interrupts) required by the
> >>>> data
> >>>> stream is kept to a minimum without sacrificing the accuracy of event
> >>>> detection.
> >>>
> >>> I guess you can get the same result (reduce sensor data interrupt rate
> >>> keeping high odr value) configuring the hw fifo watermark.
> >>> Does it work for you?
> >>
> >> Setting the hw fifo watermark to a high value reduces the rate of
> >> interrupts, but doesn't do much to reduce the amount of I2C traffic, so the
> >> issue would still be there.
> >
> > ack, now I got the goal of the series. I think the series is mostly fine.
> > I guess hwfifo_odr instead of bdr is more meaningful, what do you think?
> > Naming is always hard.
> >
> > Regards,
> > Lorenzo
>
> In the IIO subsystem, we prefer to include the units in the variable/
> field name as well, e.g. hw_fifo_odr_mHz.

ack, but please avoid camel case :)

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