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

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Message-ID: <2ba028f8a6239c821da80ff487b9df5536e58169.camel@baylibre.com>
Date: Mon, 12 Jan 2026 18:10:32 +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 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. So the
gyroscope is not involved here, and the attribute is only needed for the
accelerometer.

Before this change, we have:
- accel IIO device with separate samp_freq and buffer/samp_freq
- gyro IIO device with separate samp_freq and buffer/samp_freq
- (optionally) external sensor IIO devices with separate samp_freq and
buffer/samp_freq (and trying to set buffer/samp_freq for these triggers an
out-of-bounds array access)

After this change, we have the accel IIO device with separate samp_freq and
buffer/samp_freq, while the other IIO devices have only a single samp_freq
attribute.

> As you note in the cover letter we can change this now with ABI issues as
> it is just in my tree, so I don't mind the change, just want to
> understand
> it a little better than I currently do!

It's not just in your tree, it has been pulled into Linus's tree for 6.19.


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