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I started having an issue with displayport daisy-chaining around the update to linux kernel 4.15 and can no longer use dual monitors. I will try and explain in enough detail while keeping it brief.
Here is my setup:
dell xps 13 9343 laptop with one mini displayport output (intel integrated graphics)
two dell U2715H monitors (each monitor has a mini dp input, dp input, and dp output)
when using the external monitors I do not use the laptop display so i will ignore it from here on out.
Previously, when I would connect the two monitors, the xrandr output would show:
DP1-8 Connected (first monitor)
DP1-1-8 Connected (second monitor)
Now, when I connect the 2 monitors xrandr only shows one display:
DP1-8 Connected (1st)
DP1-1 Disconnected (2nd)
the very odd thing is that if I switch the input on the second monitor to the setting that is not connected, it shows a second display as connected.
For example
mini DP from laptop --> DP input on first monitor
DP output --> DP Input on second monitor
if I then set the 2nd monitor to input from mini DP, (with no cable connected to mini DP input) xrandr shows 2 displays connected:
DP1-8 Connected
DP1-1 Connected
as soon as I switch the 2nd monitor input back to DP (which is the one connected to the first monitor) xrandr shows only 1 display connected
The same happens in reverse. I connected the second monitor with a with a DP to mini DP cable. When I set the input on the 2nd monitor to miniDP, xrandr only shows 1 display connected. As soon as I switch the input to DP, (with no cable connected to DP input) xrandr shows 2 displays:
DP1-8 Connected
DP1-1 Connected
If anyone can provide some insight on this it would be much appreciated
Last edited by heimer16 (2018-02-11 03:42:43)
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For anyone who is interested, I was able to get things working again. I realized I had DP 1.2 enabled on both monitors. I disabled DP 1.2 on the second monitor (the one daisy-chained to the first monitor) and now it is working. I suppose having it enabled on both monitors was causing some kind of conflict or "daisy-chain loop" or something, and now that I think of it, this is probably the recommended configuration. I'm not sure what changed, but it had worked having DP1.2 enabled on both monitors until recently.
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