Comment 5 for bug 1886729

Thank you for the response. I think these might fall into the third category of 'VESA is its own special story' ?

I bought both the adapters from the dell configurator at the time I bought the machine.

This is the part that flakes: https://www.dell.com/en-us/shop/accessories/apd/470-acfc
The other part, that works fine

It is advertised as suitable for 4K 60hz, which as you note exceeds USB3.1, but see the link from the dell site - https://www.usb.org/sites/default/files/D2T1-4%20-%20VESA%20DP%20Alt%20Mode%20over%20USB%20Type-C.pdf - it is not being carried over USB, but over alt-mode DP as far as I can tell, which should have plenty of bandwidth.

The monitors are, from memory DP 1.2, requiring MST to operate - https://downloads.dell.com/manuals/all-products/esuprt_display_projector/esuprt_display/dell-up2414q_user%27s%20guide_en-us.pdf confirms this.

However, the mystery deepens.

There are two USB-C/thunderbolt connectors on the laptop. My initial report had the combined-power-HDMI-DP one in the left connector, and the DP only one in the right connector.

On a lets-be-pedantic hunch I just plugged them in on the opposite sides.

With the DP only adapter plugged in on the left side, it:
 - reports both the MST panels
 - both panels autoconfigure for a combined 4k 60hz
 - yay!

"""
DP-1-8 connected 1920x2160+5760+0 (normal left inverted right x axis y axis) 527mm x 296mm
   1920x2160 59.99*+
DP-1-9 connected 1920x2160+3840+0 (normal left inverted right x axis y axis) 527mm x 296mm
   1920x2160 59.99*+
   1920x1200 59.88
"""

With both adapters plugged in, the DP only one still on the left side, the behaviour that I originally filed the bug because of returns:
"""
xrandr --output DP-3 --auto --output DP-1-8 --auto --output DP-1-9 --auto
xrandr: cannot find crtc for output DP-1-9
"""

If I turn DP-3 off I can run the single DP monitor fine even though the adapter is plugged in.

This behaviour is also consistent if I switch the power/hdmi/dp adapter to use HDMI rather than DP.

What I had hoped to do is to use - ideally - all three outputs, or at worst, two external outputs and have my laptop folded flat and out of the way while I use my clicky-clacky keyboard and real mouse at my desk; being able to use either adapter, one at a time, is a dissapointment.

What I think this shows so far:
- each adapter works independently, with either adapter, from either USB-C port.
- something I don't know enough about yet causes some resource constraint or conflict when both external screens are in use.