The problem seems to be, that not all drivers export the newly queried "rotation" property to user space in contrast to the atomic mode setting propertiers, which are expected to all get exported to my knowledge.

The "rotation" property isn't atomic, so it might make sense to instead of from now on guarding all atomic properties that we just don't use the Property class for rotation and instead introduce a local variable in DrmPlane for the rotation property.

That still leaves an inconsistency where you're guarding the insert in initProps but not guarding access.

If you're gonna do that, you should error on initProps if one of the other props doesn't exist.

Yes, that's a good idea. We could then decide to not use this plane/crtc/connector.

That said, my assumption is that for an atomic mode property this should never happen. But since we would do this check only once at the beginning of runtime I'm fine with it. Or should we assert? I mean if my assumption is correct it would be a critical driver bug. Maybe don't hide it.

I would say all access needs guarding. My interpretation of https://01.org/linuxgraphics/gfx-docs/drm/drm-kms-properties.html is that the properties are not mandatory. For our usage rotation is clearly optional so we don't need to assert on it or report it as an error. We should consider it optional.

I got kwin_wayland to work in openQA again, I had to revert the following commits:

    Revert "[drm] Implement changing of modes"
    This reverts commit 33a4cf405071f2c74977a68c5b332dde4cea5853.

    Revert "[platforms/drm] Fix typo in cleanup of eglSurface"
    This reverts commit 7739dea33c2233b1fbbd49f52fc6263bfc2e2f0c

    Revert "[platforms/drm] Add support for rotation property on the Plane"
    This reverts commit 77b5c3caa9534862a057ea76f26bd094aaaff56e

    Revert "[platforms/drm] Rotate screen if requested from KScreen"
    This reverts commit b2d8bbec8157f9f8fbe70ace473162437dee5676

    Revert "[platforms/drm] Restore previous mode if an atomic test fails"
    This reverts commit 26cdfd317fc87b024ebcd9c2adefc86cfd59504e

    Revert "[platforms/drm] Properly adjust cursor position on a rotated output"
    This reverts commit c06c234778f8c9ad3c5ba708097240a7a88d8a62

The revert of b2d8bbec8157f9f8fbe70ace473162437dee5676 is what fixes the crash originally reported here.
However, even with that revert it did not work:

The behaviour I experienced while trying to figure out what's wrong exactly:
- dbus-launch startplasmacompositor (or just using sddm) resulted in ksplashqml starting, but after a few seconds kwin_wayland crashed
- The crash is not related to the reason that broke it, kwin_wayland exits for an invisible reason (nothing in the log) and crashes in ~DrmOutput.
- Running kwin_wayland with konsole/kwrite/anything else worked fine

So my guess is that kscreen's requests for rotation somehow cause the DRM backend to give up.

As a workaround you could also 
export KWIN_DRM_NO_AMS=1

A data point,

The KWIN_DRM_NO_AMS property is not totally useful, if I export it, the kwin "starts" but it is not functional : see the following qemu screenshot : https://i.imgur.com/Gjy2cy4.jpg

It basically doesn't get screen resolution correctly and I can't interact with the kwin/plasmashell.

Possible patch at: https://phabricator.kde.org/D8752

(In reply to Martin Flöser from comment #8)
> Possible patch at: https://phabricator.kde.org/D8752

I tried this patch and kwin still crashes in QEMU, but at a different place now at least:

#0  KWin::DrmOutput::pixelSize (this=0x0) at /usr/src/debug/kwin-5.11.90git.20171108T174438~9df174483/plugins/platforms/drm/drm_output.cpp:162
#1  0x00007fffdcccfca9 in KWin::DrmCrtc::blank (this=0x555555889a10)
    at /usr/src/debug/kwin-5.11.90git.20171108T174438~9df174483/plugins/platforms/drm/drm_object_crtc.cpp:85
#2  0x00007fffdccd1398 in KWin::DrmOutput::~DrmOutput (this=0x555555843240, __in_chrg=<optimized out>)
    at /usr/src/debug/kwin-5.11.90git.20171108T174438~9df174483/plugins/platforms/drm/drm_output.cpp:68
#3  0x00007fffdccd1699 in KWin::DrmOutput::~DrmOutput (this=0x555555843240, __in_chrg=<optimized out>)
    at /usr/src/debug/kwin-5.11.90git.20171108T174438~9df174483/plugins/platforms/drm/drm_output.cpp:87
#4  0x00007fffdccc9912 in KWin::DrmBackend::updateOutputs (this=this@entry=0x5555557cb500)
    at /usr/src/debug/kwin-5.11.90git.20171108T174438~9df174483/plugins/platforms/drm/drm_backend.cpp:460
#5  0x00007fffdccca81d in KWin::DrmBackend::openDrm (this=0x5555557cb500)
    at /usr/src/debug/kwin-5.11.90git.20171108T174438~9df174483/plugins/platforms/drm/drm_backend.cpp:326
#6  0x00007ffff5a0ffec in QMetaObject::activate(QObject*, int, int, void**) () from /usr/lib64/libQt5Core.so.5
#7  0x00007ffff7b1c9e2 in KWin::LogindIntegration::hasSessionControlChanged (this=<optimized out>, _t1=<optimized out>, _t1@entry=true)
    at /usr/src/debug/kwin-5.11.90git.20171108T174438~9df174483/build/kwin_autogen/EWIEGA46WW/moc_logind.cpp:188

At this point `export KWIN_DRM_NO_AMS=1` helps though and the session starts up. Unfortunately kwin_wayland gives up just after these messages:

kscreen.kwayland: Loading Wayland backend.
kscreen.kded: PowerDevil SuspendSession action not available!
kscreen.kwayland: Loading Wayland backend.
kwin_wayland: Couldn't find current GLX or EGL context.

and crashes somewhere in the destructors of Qt's global objects. That's exactly the same state as Comment 5 though.

Thanks for testing. Looks like init on DrmOutput fails. I'll have a look.

Git commit c601e875cfa7d673f1567c71087036631e42d272 by Martin Flöser.
Committed on 13/11/2017 at 20:19.
Pushed by graesslin into branch 'master'.

[platforms/drm] At safety checks for the properties

Summary:
The AMS code accesses elements in a vector which might not be valid. This
change refactors the code to be more robust, especially the DrmPlane,
which started to crash after adding transformation support.

Reviewers: #kwin, #plasma, fvogt, subdiff

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D8752

M  +14   -5    plugins/platforms/drm/drm_object.cpp
M  +17   -14   plugins/platforms/drm/drm_object.h
M  +2    -2    plugins/platforms/drm/drm_object_connector.cpp
M  +5    -2    plugins/platforms/drm/drm_object_crtc.cpp
M  +24   -9    plugins/platforms/drm/drm_object_plane.cpp
M  +3    -0    plugins/platforms/drm/egl_gbm_backend.cpp

https://commits.kde.org/kwin/c601e875cfa7d673f1567c71087036631e42d272

Can I re-open this bug, the commit above fixes the issue of crash, but right after starting plasma kwin_wayland is hung, and there is no mouse pointer as well, so it's still impossible to interact with Plasma.. or should I report separate bug?

(In reply to Bhushan Shah from comment #12)
> Can I re-open this bug, the commit above fixes the issue of crash, but right
> after starting plasma kwin_wayland is hung, and there is no mouse pointer as
> well, so it's still impossible to interact with Plasma.. or should I report
> separate bug?

Please use a separate bug. It's a different issue now, most likely related to the incorrect request we get from KScreen.