Re: [v2 06/10] iio: document bno055 private sysfs attributes

Jonathan Cameron
2021-11-14T16:15:58Z

On Tue, 9 Nov 2021 11:22:27 +0100
Andrea Merello <andrea.merello@gmail.com> wrote:

> Few inline comments; ok for the rest.
>
> Il giorno gio 28 ott 2021 alle ore 12:59 Jonathan Cameron
> <jic23@kernel.org> ha scritto:
> >
> > On Thu, 28 Oct 2021 12:18:36 +0200
> > Andrea Merello <andrea.merello@gmail.com> wrote:
> >
> > > This patch adds ABI documentation for bno055 driver private sysfs
> > > attributes.
> >
> > Hohum. As normal I dislike custom attributes but reality is these
> > don't map to anything 'standard' and I don't want them getting adopted
> > in places where the 'standard' approach works.
> >
> > So thinking a bit more on this, I wonder if we can fit it into standard
> > ABI.
> >
> > We can't use the normal range specification method of
> > _scale because it's internal to the device and the output reading is
> > unaffected. The range specification via _raw_available would let us know
> > the range, but it is not writeable so..
> >
> > A control that changes the internal scaling of the sensor in a fashion
> > that is not visible to the outside world maps to calibscale. Whilst
> > that was intended for little tweaks to the input signal (often front
> > end amplifier gain tweak) it works here. It doesn't map through to
> > anything userspace is expected to apply. That combined with
> > _raw_available to let us know what the result is should work?
> >
> > What do you think of that approach? It's obviously a little more complex
> > to handle in the driver, but it does map to existing ABI and avoids
> > custom attributes etc.
>
> If I read the ABI documentation, then I would say that calibscale has
> nothing to do with this, but I think you have obviously a better
> feeling than me about what calibscale is really for. To be honest I've
> probably not a clear idea about what calibscale is indeed...

Original intent was that it was a tweak for input amplifiers on some sensor
types that you'd set as part of a calibration process. These days, for
many sensors that have this it's handled at factory anyway and these
tweak values are rarely exposed to software.

>
> In general, I would say that is better to stick to standard attributes
> when possible, and of course to avoid having the same thing mapped on
> random custom attributes in each driver, but IMO only up to the extent
> which doesn't force something that is really something different to
> map on a standard thing just because of the sake of having as much
> standard things as possible... But all this is probably quite obvious,
> and it all depends on the above (i.e. is it calibscale fitting well in
> your opinion?) .. Up to you on this one..
>
> BTW I'm missing why this should complicate the driver.. I guess I'll
> find out if I'll implement it :)
Inverse of the range values which is always a mess without floating point.

Ok, I'm persuaded that we have to go with range here even if it is a bit
painful and might cause confusion if we start getting it in lots of drivers.
*fingers crossed we don't*

There is still a units question though. Should we express the ranges
in _processed or _raw units? Or do we make it explicit and call it
rangeprocessed for example? For some devices the range will naturally
be expressed as the range of ADC raw values, so there is definite room
for confusion if we don't make it clear in the name.

I'm open to other suggestions of how we name this to avoid falling into
any heffalump traps.

>
> > >
> > > Signed-off-by: Andrea Merello <andrea.merello@iit.it>
> > > ---
> > > .../ABI/testing/sysfs-bus-iio-bno055 | 84 +++++++++++++++++++
> > > 1 file changed, 84 insertions(+)
> > > create mode 100644 Documentation/ABI/testing/sysfs-bus-iio-bno055
> > >
> > > diff --git a/Documentation/ABI/testing/sysfs-bus-iio-bno055 b/Documentation/ABI/testing/sysfs-bus-iio-bno055
> > > new file mode 100644
> > > index 000000000000..930a70c5a858
> > > --- /dev/null
> > > +++ b/Documentation/ABI/testing/sysfs-bus-iio-bno055
> > > @@ -0,0 +1,84 @@
> > > +What: /sys/bus/iio/devices/iio:deviceX/in_accel_range
> > > +KernelVersion: 5.15
> > > +Contact: linux-iio@vger.kernel.org
> > > +Description:
> > > + Range for acceleration readings in G. Note that this does not
> > > + affects the scale (which should be used when changing the
> > > + maximum and minimum readable value affects also the reading
> > > + scaling factor).
> >
> > Having this in G but the sensor output in m/s^2 seems inconsistent.
> >
> > > +
> > > +What: /sys/bus/iio/devices/iio:deviceX/in_anglvel_range
> > > +KernelVersion: 5.15
> > > +Contact: linux-iio@vger.kernel.org
> > > +Description:
> > > + Range for angular velocity readings in dps. Note that this does
> > > + not affects the scale (which should be used when changing the
> > > + maximum and minimum readable value affects also the reading
> > > + scaling factor).
> >
> > Again, units need to match or this is going to be really confusing.
> >
> > > +
> > > +What: /sys/bus/iio/devices/iio:deviceX/in_accel_range_available
> > > +KernelVersion: 5.15
> > > +Contact: linux-iio@vger.kernel.org
> > > +Description:
> > > + List of allowed values for in_accel_range attribute
> > > +
> > > +What: /sys/bus/iio/devices/iio:deviceX/in_anglvel_range_available
> > > +KernelVersion: 5.15
> > > +Contact: linux-iio@vger.kernel.org
> > > +Description:
> > > + List of allowed values for in_anglvel_range attribute
> > > +
> > > +What: /sys/bus/iio/devices/iio:deviceX/fast_magnetometer_calibration_enable
> > > +KernelVersion: 5.15
> > > +Contact: linux-iio@vger.kernel.org
> > > +Description:
> > > + Can be 1 or 0. Enables/disables the "Fast Magnetometer
> > > + Calibration" HW function.
> >
> > Naming needs to be consistent with the ABI. This is a channel type specific function
> > and to match existing calibration related ABI naming it would be.
> >
> > in_magn_calibration_fast_enable
> >
> > Some of the others need renaming in a similar way.
> >
> > > +
> > > +What: /sys/bus/iio/devices/iio:deviceX/fusion_enable
> > > +KernelVersion: 5.15
> > > +Contact: linux-iio@vger.kernel.org
> > > +Description:
> > > + Can be 1 or 0. Enables/disables the "sensor fusion" (a.k.a.
> > > + NDOF) HW function.
> > > +
> > > +What: /sys/bus/iio/devices/iio:deviceX/in_calibration_data
> > > +KernelVersion: 5.15
> > > +Contact: linux-iio@vger.kernel.org
> > > +Description:
> > > + Reports the binary calibration data blob for the IMU sensors.
> >
> > Why in_ ? What channels does this apply to?
> >
> > > +
> > > +What: /sys/bus/iio/devices/iio:deviceX/in_autocalibration_status_accel
> > > +KernelVersion: 5.15
> > > +Contact: linux-iio@vger.kernel.org
> > > +Description:
> > > + Can be "Idle", "Bad", "Barely enough", "Fair", or "Good".
> > > + Report the autocalibration status for the accelerometer sensor.
> >
> > For interfaces that really don't have any chance of generalising this one is terrible.
> > Any hope at all of mapping this to something numeric?
> >
> > in_accel_calibration_auto_status
> >
> >
> > > +
> > > +What: /sys/bus/iio/devices/iio:deviceX/in_autocalibration_status_gyro
> > > +KernelVersion: 5.15
> > > +Contact: linux-iio@vger.kernel.org
> > > +Description:
> > > + Can be "Idle", "Bad", "Barely enough", "Fair", or "Good".
> > > + Reports the autocalibration status for the gyroscope sensor.
> >
> > in_angvel_calibration_auto_status
> > etc.
> >
> > > +
> > > +What: /sys/bus/iio/devices/iio:deviceX/in_autocalibration_status_magn
> > > +KernelVersion: 5.15
> > > +Contact: linux-iio@vger.kernel.org
> > > +Description:
> > > + Can be "Idle", "Bad", "Barely enough", "Fair", or "Good".
> > > + Reports the autocalibration status for the magnetometer sensor.
> > > +
> > > +What: /sys/bus/iio/devices/iio:deviceX/in_autocalibration_status_sys
> > > +KernelVersion: 5.15
> > > +Contact: linux-iio@vger.kernel.org
> > > +Description:
> > > + Can be "Idle", "Bad", "Barely enough", "Fair", or "Good".
> > > + Reports the status for the IMU overall autocalibration.
> > > +
> > > +What: /sys/bus/iio/devices/iio:deviceX/unique_id
> >
> > Hmm. So normally we just dump these in the kernel log. I guess you need it
> > here to associate a calibration blob with a particular sensor?
>
> Well, it was originally in kernel log, but putting in an attribute was
> one of the changes that has been requested for V2.

Oops. :) Inconsistency is my middle name...

> It is needed by the user who copies the calibration data to the
> calibration file, in order for her/him to be able to properly name it
> (in case of more than 1 sensor on the same setup).

Fair enough that makes complete sense.

>
> > We could put it in label, but that would stop us using that for things like
> > positioning of the sensor. So perhaps this is something that we should add
> > to the main ABI doc. Probably as serial_number rather than unique ID though.
>
> OK, for renaming to "serial_number". I'm not sure they are
> conceptually the same thing, but I think it works anyway.
> Of course I can move its doc to the main file. Do you want a separate
> patch for this?

Separate patch would be great. Thanks,

Jonathan

>
> > > +KernelVersion: 5.15
> > > +Contact: linux-iio@vger.kernel.org
> > > +Description:
> > > + 16-bytes, 2-digits-per-byte, HEX-string representing the sensor
> > > + unique ID number.
> >