Comment # 14 on bug 1089932 from Fabian Vogt

(In reply to Terje J. Hanssen from comment #12)
> Yes, 3x scaling became large enough, even without glasses :)
> Will something in-between also work, i.e 2.5*96=240 ?

It might - give it a try!

I don't think the detection tries to derive fractional scales though - as most
displays actually have ~105 dpi instead of 96 you would get a scale of 1.1
everywhere...

> If EDID is broken I can file a bug to Dell. This XPS13 9370 "developer" is
> pre-installed and supported with Ubuntu 16.04LTS from Dell (to which I've
> added Leap15 in a dualboot setup). Though I haven't discovered similar
> scaling problem there, Unity boots pure grahical up to its login menu.
> Inside the Unity desktop I've selected scale 2.0 for menus and text.
> 
> So to the output here:
> 
> xrandr
> Screen 0: minimum 320 x 200, current 3840 x 2160, maximum 8192 x 8192
> eDP-1 connected primary 3840x2160+0+0 (normal left inverted right x axis y
> axis) 294mm x 165mm

That results in 332dpi x 333dpi.
It should pick 2x scale at least - if not more.

>    3840x2160     60.00*+  59.98    59.97    48.00  
[...]
> 
> terje@linux-hxpp:~> hexdump -C /sys/class/drm/*/edid
[...]

EDID says the same physical size. So no broken EDID.
Which is good as we can fix software more easily!

I'll try to have a look at the detection code.