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

David Lechner
2025-10-10T15:50:45Z

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.