On my Dell XPS 9560, and Linus post-5.3 git tree, my Dell U3415W is no longer driven from X. The screen goes into standby, and attempting to use it shows:
$ xrandr --output DP-1-2-8 --above eDP-1 --auto
xrandr: cannot find crtc for output DP-1-2-8
Note that the screen lights up during boot, as it always did. Once lightdm is started, the screen turns off. When I shut the machine down and X terminates, the screen lights back up.
I will be adding attachments shortly.
Created attachment 145467 [details]
dmesg output with DRM debug, covering boot and X turn-up
Created attachment 145468 [details]
Xorg log with the "modeset" driver
Created attachment 145469 [details] [review]
I currently set enable_guc to 2, as I find the graphics behave better with this firmware loaded. I find behaviour is the same with enable_guc set to 0 or 2, and can test in either scenario for you.
The bad commit for me is:
Merge: 3c2edc36a774 945b584c94f8
Author: Linus Torvalds <firstname.lastname@example.org>
Date: Thu Sep 19 16:24:24 2019 -0700
Merge tag 'drm-next-2019-09-18' of git://anongit.freedesktop.org/drm/drm
(In reply to Tony Vroon from comment #4)
> The bad commit for me is:
> commit 574cc4539762561d96b456dbc0544d8898bd4c6e
> Merge: 3c2edc36a774 945b584c94f8
> Author: Linus Torvalds <email@example.com>
> Date: Thu Sep 19 16:24:24 2019 -0700
> Merge tag 'drm-next-2019-09-18' of git://anongit.freedesktop.org/drm/drm
Since you're able to bisect, any chance you could bisect into that merge, please? I hope Manasi and Lyude can look into this, but in the mean time bisecting to the actual regressing non-merge commit might be the quickest route to fixing this.
> Since you're able to bisect, any chance you could bisect into that merge,
I've not had to bisect into a merge before, do you have a quick guide you can link to?
Already fixed with
Author: Ville Syrjälä <firstname.lastname@example.org>
Date: Tue Sep 3 18:40:18 2019 +0300
Revert "drm/i915: Fix DP-MST crtc_mask"
Looks like it hasn't made it into Linus's tree yet.