iio: documentation: Document accelerometer label use

Some 2-in-1 laptops / convertibles with 360° (yoga-style) hinges,
have 2 accelerometers, 1 in their base and 1 in their display.

In many cases the kernel can detect the location of each accelerometer
based on e.g. information from the ACPI tables.

It is important for userspace to know the location of the 2 accelerometers.
Rather then adding a new sysfs-attribute for this we can relay this
information to userspace by using standardized label strings for this.
This mirrors how this is done for proximity sensors.

This commit documents 2 new standardized label strings for this purpose:
"accel-base"
"accel-display"

Note the "base" and "display" suffixes were chosen to match the values
used for the systemd/udev hwdb.d/60-sensor.hwdb file's ACCEL_LOCATION
property.

Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Mark Pearson <mpearson@lenovo.com>
Cc: Bastien Nocera <hadess@hadess.net>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20210215191003.698888-2-hdegoede@redhat.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/ABI/testing/sysfs-bus-iio

@@ -72,6 +72,13 @@ Description:
 		calculated by firmware from other sensor readings, rather then
 		being a raw sensor reading.
 
+		For accelerometers used in 2-in-1s with 360° (yoga-style) hinges,
+		which have an accelerometer in both their base and their display,
+		the following standardized labels are used:
+
+		* "accel-base"
+		* "accel-display"
+
 What:		/sys/bus/iio/devices/iio:deviceX/current_timestamp_clock
 KernelVersion:	4.5
 Contact:	linux-iio@vger.kernel.org