Update after playing with this a bit:
 
While the sensors are detected by the mainline rc4 (and rc5) kernel as stated 
above, the rotation does not work correctly. I am not sure if this is still a 
kernel issue or happens elsewhere. Would be great if you could point me in the 
right direction.

According to the output of the monitor-sensor command of iio-proxy-
sensor, it detects the correct orientation in all attempts. However, in
some scenarios something rotates the screen back to normal automatically
and immediately. When this happens, monitor-sensor does NOT output a
switch to "normal". So I suppose the sensor outputs remain correctly
detected and the switching back happens elsewhere.

On the other hand, this may be connected to a hardware quirk in how the 
rotation is implemented on the Dell XPS, which similarly also occurs in Windows 
(https://www.dell.com/community/XPS/Dell-XPS-13-9365-Rotation-lock-unavailable-grayed-out-EXPLAINED/td-p/6075400):
* Laptop mode, i.e. keyboard-to-screen angle BELOW 90 degrees: In Windows, this 
automatically engages the rotation lock, i.e. the screen does not rotate at all.
* Tent/tablet mode, i.e. keyboard-to-screen angle ABOVE 90 degrees: In Windows, 
this releases the rotation lock automatically.

The behavior I see in Ubuntu also depends on the keyboard-screen angle. 
However, the rotation lock in the Gnome panel does NOT automatically engage if 
the angle is below 90 degrees.
This is what happens:

* Angle BELOW 90:
- Orientation "left-up": screen rotates to portrait correctly, but immediately 
switches back to normal.
- Orientation "right-up": screen rotates to portrait correctly and in most 
attempts stays like this, but occasionally it immediately switches back to 
normal.
- Orientation "bottom-up": screen rotates rotates upside-down correctly, but 
immediately switches back to normal.
- Orientation "normal": screen always rotates back correctly (if it didn't 
automatically switch back anyway).

* Angle ABOVE 90:
- Orientation "left-up": screen rotates to portrait correctly and in most 
attempts stays like this, but occasionally it immediately switches back to 
normal.
- Orientation "right-up": screen rotates to portrait correctly and apparently 
randomly either stays like this or immediately switches back to normal.
- Orientation "bottom-up": screen rotates rotates upside down correctly most of 
the time (but occasionally doesn't).
- Orientation "normal": screen always rotates back correctly (if it didn't 
automatically switch back anyway).

As you can see, the "left-up" and "right-up" behaviors seem somewhat
(but not entirely) swapped depending on the keyboard-screen angle.

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https://bugs.launchpad.net/bugs/1792813

Title:
  iio-sensor-proxy says: "Could not find any supported sensors" on Dell
  XPS 15 9575 2-in-1 on Cosmic 18.10

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