Re: [PATCH 2/2] docs: abi: iio: Add event when offset changes

[Gwendal Grignou <gwendal@chromium.org>]

On Sun, Aug 30, 2020 at 6:11 AM Jonathan Cameron <jic23@kernel.org> wrote:
>
> On Fri, 28 Aug 2020 16:31:56 -0700
> Gwendal Grignou <gwendal@chromium.org> wrote:
>
> > Some sensors/sensorhubs can calculate drift or hard iron offsets to
> > apply to raw data to get the true measure data.
> > These offsets are applied by the user space application.
> > When these offsets change, events are raised to tell the application
> > to update the cached offset values.
> >
> > Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
> Hi Gwendal
>
> This strikes me as rather prone to races.  I guess if the updates
> tend to be fairly minor we will just have slightly less accurate data
> until the update gets picked up by userspace.
In case of hard iron for instance, the sensorhub needs several samples
to find out the current offset are now invalid. So it is likely the
measurement of the geomagnetic field was incorrect for a while.
For accelerometer online calibration, we don't expect vast changes
when new offsets are available.
>
> We have had some discussions about how to handle meta data updates
> in the past.  One option was to provide a metadata index channel
> that could be used to indicate there was an update to something
> sat in a separate fifo.
An extra sounds like a waste of space, as it will mostly be 0 most of
the time, and edge to 1 once in a while. An event looks more
appropriate.
>
> > ---
> >  Documentation/ABI/testing/sysfs-bus-iio | 33 +++++++++++++++++++++++++
> >  1 file changed, 33 insertions(+)
> >
> > diff --git a/Documentation/ABI/testing/sysfs-bus-iio b/Documentation/ABI/testing/sysfs-bus-iio
> > index 47df16c87862d..40da602e7a555 100644
> > --- a/Documentation/ABI/testing/sysfs-bus-iio
> > +++ b/Documentation/ABI/testing/sysfs-bus-iio
> > @@ -1716,3 +1716,36 @@ Description:
> >               Mass concentration reading of particulate matter in ug / m3.
> >               pmX consists of particles with aerodynamic diameter less or
> >               equal to X micrometers.
> > +
> > +What:                /sys/bus/iio/devices/iio:deviceX/in_anglvel_x_offset
> > +What:                /sys/bus/iio/devices/iio:deviceX/in_anglvel_y_offset
> > +What:                /sys/bus/iio/devices/iio:deviceX/in_anglvel_z_offset
> > +KernelVersion:       x.y
> > +Contact:     linux-iio@vger.kernel.org
> > +Description:
> > +             Gyroscope drift calculated by the sensor. In addition to factory
> > +             calibration, sensor or sensorhub can
> > +             detect gyroscope drift and report it to userspace.
>
> This looks like standard ABI that should probably already be documented,
> unrelated to the controversial part of this patch. I would split it out into
> it's own patch a I can pick that up much faster.
Done in v2.
>
>
> > +
> > +What:                /sys/bus/iio/devices/iio:deviceX/in_magn_x_offset
> > +What:                /sys/bus/iio/devices/iio:deviceX/in_magn_y_offset
> > +What:                /sys/bus/iio/devices/iio:deviceX/in_magn_z_offset
> > +KernelVersion:       x.y
> > +Contact:     linux-iio@vger.kernel.org
> > +Description:
> > +             Hard Iron bias calculated by the sensor or sensorhub. To be applied by
> > +             user space application to the raw data to obtain the geomagnetic field.
>
> Same as above.
Done in v2.
>
> > +
> > +What:                /sys/.../iio:deviceX/events/in_accel_offset_change_en
> > +What:                /sys/.../iio:deviceX/events/in_magn_offset_change_en
> > +What:                /sys/.../iio:deviceX/events/in_magn_scale_change_en
> > +What:                /sys/.../iio:deviceX/events/in_anglvel_offset_change_en
> > +KernelVersion:       x.y
> > +Contact:     linux-iio@vger.kernel.org
> > +Description:
> > +             Some sensors internally calculate offset to apply to remove bias (for
> > +             instance, hard/soft-iron bias for magnetometer, online calibration bias for
> > +             gyroscope or accelerometer).
> > +             When the sensor computes a new set of offset values, it generates an
> > +             event for the userspace application to refresh the offsets to apply to raw
> > +             data.
>
> I'm not totally sold on this idea, though would like inputs from other people before
> ruling it out.
>
> One minor change I'd make would be to have a single event to indicate that something
> userspace might care about in the way of metadata for this channel has changed.
> I guess something like
Make sense, scale and offset should be checked together if they both exists.
Using vents/in_<type>_metadata_en in v2.
>
> in_accel_metachange_en etc with a single new event code.  For events, it's the event
> code mapping that normally matters more than sysfs binding as that is much more
> tightly restricted.
>
> Jonathan
>