External Display limited to 1920x1080 even though display can go higher 2556x1440

[LaughingBubba]

Hi there, I'm trying out dual booting my surface book to Kubuntu 17.04. Everything has been fine so far except when plugging in an external screen via the mini Display Port with an HDMI adapter. The OS recognises the screen and the manufacturer (BenQ GW2765) and also that is connected via HDMI and I suspect this is why the display settings are limited to 1920x1080 as the max resolution even though it's capable of 2556x1440 (and certainly does display at this res when booted into Win10 using the same port and adapter)

I do have the latest relevant NVIDIA ppa drivers installed but noticed that plasma uses sddm instead of xorg. I found a post that where someone had a similar problem on a desk top using the HDMI port and their solution was to use the DVI port instead.

I'd rather not buy yet another dongle on the off chance it will work if a change to the xorg or sddm conf could solve the problem?

Cheers
J

[EDIT]
I tried this solution https://ubuntuforums.org/showthread.php?t=2225304 for a mode of 2560x1440_60.00 which failed with xrandr: Configure crtc 1 failed

[oshunluvr]

Since Display Port and HDMI are both capable of higher resolutions than DVI, the issue lies elsewhere, like the driver or kernel. Not that using a dual-DVI connection can't do high resolution, but DP and HDMI can both do 4K and higher in some cases, so that's not your limitation.

The Surface book is a Microsoft product and I don't know anyone else here using one. You might start by posting some relevant specs and the output of some relevant commands and logs. Start with a review of your /var/log/Xorg.0.log file and the output of xrandr with the external monitor connected. The specs and model number of the Surface book and video chip would be the minimum info needed before anyone could likely help.

[LaughingBubba]

Hi and thanks for the response. Spec wise the machine is an i7-6600U with 16GB RAM, the primary GPU is an NVIDIA GeForce 940M with 1 GB of RAM rendering to a 3000x2000 primary screen. I'm sure it's a driver / kernel config issue as the external monitor does render as 2560x1444 when booted into win10 so it's not a hardware issue.

xrandr output is as follows:
Screen 0: minimum 320 x 200, current 3000 x 2000, maximum 8192 x 8192
eDP-1 connected primary 3000x2000+0+0 (normal left inverted right x axis y axis) 290mm x 190mm
3000x2000 59.99*+
2560x1600 59.99
DP-1 disconnected (normal left inverted right x axis y axis)
HDMI-1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 597mm x 336mm
1920x1080 60.00* 50.00 59.94
1920x1080i 60.00 50.00 59.94
1680x1050 59.88
1600x900 60.00
1280x1024 75.02 60.02
1280x800 59.91
1152x864 75.00
1280x720 60.00 50.00 59.94
1024x768 75.03 60.00
832x624 74.55
800x600 75.00 60.32
720x576 50.00
720x576i 50.00
720x480 60.00 59.94
720x480i 60.00 59.94
640x480 75.00 60.00 59.94
720x400 70.08

I then renamed the log file rebooted and ran the following:
> xrandr --newmode "2560x1440_60.00" 311.83 2560 2744 3024 3488 1440 1441 1444 1490 -HSync +Vsync
> xrandr --addmode HDMI-1 "2560x1440_60.00"
> xrandr --output HDMI-1 -s "2560x1440_60.00"

The resulting Xorg.0.log file is attached.

[LaughingBubba]

I finally found the post that originally got me talking about DVI https://forums.linuxmint.com/viewtopic.php?t=196959 that got me to thinking it could be the cabling. I connected to my other monitor with the adapter cable I use with that (HDMI -> miniDP) and it displayed in the native res of 3840x2160. So I then used that cable to connect to the BenQ GW2765 and that worked fine so the conclusion is that HDMI -> miniDP dongle I was using only supported HDMI 1.0 which is limited to 1920x1080.

I still have a problem in that the monitor now only shows a mirror (2560x1440) of the top left hand corner of the primary display (3000x2000) but at least now its in the res I expect.