[Bug 199633] New: Stable kernel regression after 4.16.3: startxfce4 no longer works: X startup seems to hang

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=199633

            Bug ID: 199633
           Summary: Stable kernel regression after 4.16.3: startxfce4 no
                    longer works: X startup seems to hang
           Product: Drivers
           Version: 2.5
    Kernel Version: 4.16.4
          Hardware: x86-64
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: high
          Priority: P1
         Component: Video(DRI - non Intel)
          Assignee: drivers_video-dri@kernel-bugs.osdl.org
          Reporter: klaus.kusche@computerix.info
        Regression: No

On my system, X is not started during boot:
My system boots with DRM framebuffer text consoles only.
After login, a "exec startxfce4" in .bash_profile brings up X and the desktop
for GUI users (not for root etc.) on the login vt (not an unused vt).

This worked fine up to and including 4.16.3
and fails with any later 4.16 kernel:
After login, the screen becomes and stays completely black:
No text, no desktop or background, not even a mouse cursor.

After switching to another virtual terminal, typing something,
and switching back, the X and desktop startup complete normally:
Up to now, I was unable to find any error message or other trace
of the problem afterwards, everything works fine.

Details:

* Just switching vt's forth and back does not suffice
(X still hangs after switching back).
I actually have to type something in the second vt,
but there is no need to login on the second vt to make things going.

* When logging in on the second vt and checking the process list,
the xorg server is up, as are the first few processes of the session startup
(but by far not all, not even a third of them).

* Bug component drivers/video was just a guess, could be anything from video/fb 
to mouse or evdev, could even be unrelated to drivers.
The only thing I know for sure is that it depends on the kernel
and only the kernel: The problem disappears when booting a 4.16.3 or earlier
and reappears when booting a later kernel, with everything else unchanged.

* It is not related to a specific DRM and X driver:
radeon kernel + modesetting X
radeon kernel + radeon X
amdgpu kernel + modesetting X
amdgpu kernel + amdgpu X
all show exactly the same problem.
(radeon + modesetting is my default, on AMD cape verde)

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