Hangs/freezes introduced between 5.4 and 5.5

While doing tests because of hanging X11 sessions (Hardware name: Acer Aspire ES1-131/Garp_BA, BIOS V1.23 06/22/2016), I noticed that the test i915_suspend of igt-gpu-tools also stopped finishing somewhere between 5.4 and 5.5. Only the console is occupied by an unfinished process i915_suspend. Rebooting with CTRL-ALT-DEL works as well as logins via ssh.

This can be triggered for example by running ./scripts/run-tests.sh -s -t i915_suspend@forcewake-hibernate even without swap space, i.e. the test being skipped. Turning off CONFIG_PM does not help.

I bisected this to commit 16ffe73c186b789 (drm/i915/pmu: Use GT parked for estimating RC6 while asleep).