i915: disable GEN6_MBCTL write in gen6_init_clock_gating
commitda2b6ca48433d8dbd238de9cf359964e876a0af4
authoremaste <emaste@FreeBSD.org>
Mon, 21 Mar 2016 00:59:30 +0000 (21 00:59 +0000)
committeremaste <emaste@FreeBSD.org>
Mon, 21 Mar 2016 00:59:30 +0000 (21 00:59 +0000)
tree2613a9bdfc2af6216206b0c55dd9e3856e15d25c
parent4e5b50116d60c5136bee585af21791efb8e9eadb
i915: disable GEN6_MBCTL write in gen6_init_clock_gating

This write came from Linux commit b4ae3f22d238 which has been implicated
in Sandy Bridge power consumption issues (albeit under different
conditions on Linux). Disabling it restores normal power consumption on
my Sandy Bridge laptop (Thinkpad X220).

PR: 207889
Reviewed by: cem, dumbbell
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D5665
sys/dev/drm2/i915/intel_pm.c