Linux 4.x QXL 64-bit kernel modesetting breaks 32-bit X.Org

With Linux 4.x we get KMS for QXL enabled by default. But then, when mixing a 64-bit kernel with 32-bit userspace, X.Org fails to start and says something like: “failed to set DRM interface version 1.4: Inappropriate ioctl for device”.

What works:

• booting with the 32-bit kernel (I’m starting to really want #8183 (closed))
• qxl.modeset=0

I’ve tried a local xserver-xorg-video-qxl 0.1.4-3~bpo8+1 backport, but it did not help.

This affects feature/10298-linux-4.x-aufs (#10298 (closed)) and feature/stretch (KMS is required since X.Org runs as non-root; and maybe #11441 (closed) too).

Related issues

• Related to #11441 (closed)
• Related to #10298 (closed)
• Blocked by #8183 (closed)

Original created by @intrigeri on 11518 (Redmine)