[Nouveau] [Bug 90351] [Regression, bisected] Backlight of internal eDP display doesn't turn off any more (Dell XPS 17/GeForce GT 555M)

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=90351

Erik van Pienbroek <erik-freedesktop-bugzilla at vanpienbroek.nl> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
 Attachment #115605|0                           |1
        is obsolete|                            |

--- Comment #2 from Erik van Pienbroek <erik-freedesktop-bugzilla at vanpienbroek.nl> ---
Created attachment 128201
  --> https://bugs.freedesktop.org/attachment.cgi?id=128201&action=edit
debug logging k.8.8-300.nvmst.fc25.x86_64

The nouveau atomic modesetting changes have arrived recently in the drm-next
tree. I did some testing with a recent drm-next git checkout and the situation
has improved regarding this bug, but it's not fully solved yet. The findings
were also discussed in downstream bug report
https://bugzilla.redhat.com/show_bug.cgi?id=1123661 but I'll also share them
here.

Initial testing was done with drm-next git rev d8c1abd, dated 20161111. With
this kernel I can confirm that with no external monitors connected the
backlight of the internal laptop monitor now turns off properly when the GNOME
session is locked. So we can consider that part of this bug report resolved
(the patch which was uploaded to this bug report earlier can be considered
obsolete now)

However, when an external monitor is attached (using a mini-DP-to-DP adapter)
and powered on as well then locking the GNOME session results in the behaviour
which was also mentioned in comment 0, that is: the backlight of both the
internal monitor and the external monitor initially turn off properly, but
after 8 seconds the backlight of both monitors suddenly turns back on while the
image on both monitors stays blank. When I press any key or move the mouse the
image comes back normally and I can unlock the GNOME session and resume work.

As suggested by Hans de Goede in the downstream bug report I also did testing
with a custom Fedora kernel which was prepared by Ben Skeggs recently
(containing a backport of the nouveau atomic modesetting changes:
https://copr.fedorainfracloud.org/coprs/bskeggs/nouveau-atomic-mst) but that
kernel also showed the same issue.

I managed to collect debug logging from both nouveau and gnome-settings-daemon
(using kernel boot flags 'log_buf_len=8M nouveau.debug=disp=trace,i2c=trace
drm.debug=0x14' and running gnome-settings-daemon with the '--debug' argument)
and uploaded it to both this bug report.  In this logging these are the related
timestamp events:
nov 21 19:05:53 - Boot of kernel 4.8.8-300.nvmst.fc25.x86_64
nov 21 19:07:00 - Pressed Super+L to lock the GNOME session
(both monitors turns off now including both backlights)
nov 21 19:07:08 - Backlight of both the internal and the external monitors turn
back on automatically showing just a blank screen
nov 21 19:07:29 - Unlock of the GNOME session

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