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

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Message-ID: <20260116203523.07c46e10@jic23-huawei>
Date: Fri, 16 Jan 2026 20:35:23 +0000
From: Jonathan Cameron <jic23@...nel.org>
To: Francesco Lavra <flavra@...libre.com>
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, 16 Jan 2026 20:06:46 +0100
Francesco Lavra <flavra@...libre.com> wrote:

> 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.
> 
Doh! The fact we don't support gyro events was the detail I was completely missing ;(

Please add that to the patch description.

Thanks

Jonathan