linux-kernel - Re: [PATCH 1/3] iio: imu: st_lsm6dsx: set buffer sampling frequency for accelerometer only

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Message-ID: <bb80803ccd2d3d920f7a53b578ac09093bf29074.camel@baylibre.com>
Date: Fri, 16 Jan 2026 20:06:46 +0100
From: Francesco Lavra <flavra@...libre.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: Lorenzo Bianconi <lorenzo@...nel.org>, David Lechner
 <dlechner@...libre.com>,  Nuno Sá <nuno.sa@...log.com>,
 Andy Shevchenko <andy@...nel.org>,  linux-iio@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] iio: imu: st_lsm6dsx: set buffer sampling frequency
 for accelerometer only

On Fri, 2026-01-16 at 18:03 +0000, Jonathan Cameron wrote:
> On Mon, 12 Jan 2026 18:10:32 +0100
> Francesco Lavra <flavra@...libre.com> wrote:
> 
> > On Sun, 2026-01-11 at 16:18 +0000, Jonathan Cameron wrote:
> > > On Fri,  9 Jan 2026 19:15:26 +0100
> > > Francesco Lavra <flavra@...libre.com> wrote:
> > >   
> > > > The st_lsm6dsx_hwfifo_odr_store() function, which is called when
> > > > userspace
> > > > writes the buffer sampling frequency sysfs attribute, calls
> > > > st_lsm6dsx_check_odr(), which accesses the odr_table array at index
> > > > `sensor->id`; since this array is only 2 entries long, an access
> > > > for
> > > > any
> > > > sensor type other than accelerometer or gyroscope is an out-of-
> > > > bounds
> > > > access.
> > > > 
> > > > To prevent userspace from triggering an out-of-bounds array access,
> > > > and
> > > > to
> > > > support the only use case for which FIFO sampling frequency values
> > > > different from the sensor sampling frequency may be needed (which
> > > > is
> > > > for
> > > > keeping FIFO data rate low while sampling acceleration data at high
> > > > rates
> > > > for accurate event detection), do not create the buffer sampling
> > > > frequency
> > > > attribute for sensor types other than the accelerometer.  
> > > 
> > > I'm not following why we need to drop this attribute for the
> > > gyroscope.
> > > Perhaps lay out what the combinations of controls are and the
> > > attributes
> > > we end up with.  
> > 
> > It's not like we need to drop this attribute, it's just that I don't
> > see a
> > need for it. The only reason I added this attribute was to be able to
> > control (e.g. lower) the rate of data coming from the sensor while
> > maintaining a high accuracy for event detection; and accurate event
> > detection requires a high sampling rate for the accelerometer.
> 
> Ok. So key here is for accelerations we are looking at impacts as a
> typical
> use case, whereas gyroscope tends to be slow orientation change stuff.
> That sounds a bit usecase specific. If someone is using these to detect
> shaft rotation
> issues they are going to care about sampling rates on the gyro as well,
> or is there something inherent in the gyroscope events (i.e. maybe there
> aren't any gyro events?) that makes this not relevant?

All the events supported by this driver (motion detection and tap
detection) use acceleration data only.
Some chip variants (e.g LSM6DSV) have more advanced features such as
configurable finite state machines that can take inputs from both the
accelerometer and the gyroscope and generate event interrupts; but I don't
think these events would map cleanly to standard IIO event types.


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