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5.3.11 regression: No RC6 on Kaby Lake

Submitted by Tomáš Janoušek @liskin

Assigned to Chris Wilson @ickle

Link to original bug (#112315)

Description

"drm/i915/gen8+: Add RC6 CTX corruption WA" (d4360736a7c0a6326e3bbdf7d41181f6ed03d9a6) in 5.3 stable broke RC6 on my Kaby Lake ThinkPad 25 (T470 equiv). This prevents the CPU package from entering package C-states. Happens every time, no suspend/resume needed. The commit message suggests suspend/resume may actually help, but that's not the case here either. No mention of "RC6 context corrupted" in dmesg, but perhaps I need to set drm.debug?

Reverting the commit fixes the issue.

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