[Bug 198936] Xorg consume 100% with XFCE after latest DRM2 changes

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=198936

            Bug ID: 198936
           Summary: Xorg consume 100% with XFCE after latest DRM2 changes
           Product: Base System
           Version: 11.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: hselasky at FreeBSD.org

Created attachment 154848
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=154848&action=edit
Patch suggestion

Hi,

Without the attached kernel patch(es), Xorg starts consuming alot of CPU and
becomes very unresponsive and unusable.

Using ktrace reveals that X-org is issuing DRM_IOCTL_MODE_GETCONNECTOR over and
over again with no apparent reason. It doesn't happen when using a simple
window manager like blackbox. I was not able to use XFCE4 (9-stable userland)
with 11-current kernel at all, after the latest DRM2 kernel updates. It worked
fine before the update.

I'm not sure what is causing it. Going through the new DRM2 code revealed that
a mode sorting function did not take all parameters like interlaced or not into
account, causing the mode list to be reshuffelled every time a new mode scan
was done. Not sure if Xorg cares about this though.

I can test patches if you have other suggestions.

--HPS

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