[Bug 207889] Significant increase in Sandy Bridge laptop power consumption after Linux 3.8 i915 update

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=207889

            Bug ID: 207889
           Summary: Significant increase in Sandy Bridge laptop power
                    consumption after Linux 3.8 i915 update
           Product: Base System
           Version: 11.0-CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: emaste at freebsd.org

As reported on the FreeBSD-x11 mailing list - adding this PR to keep track of
the details.

I updated my regular laptops to HEAD after this commit and have discovered that
the power consumption seems to have increased significantly. This is on a
Thinkpad X220 (Sandy Bridge).

With a r286085 kernel installed on the disk it and reports 11W while idle at a
vt(4) console without the driver loaded. After loading i915kms.ko but still at
the
vt(4) console that increased to 14W.

Booting from a USB stick at r296565 showed 13W initially, rising to 25W after
loading i915kms.ko.

These numbers were obtained from "acpiconf -i0" while running on battery, from
the "Present rate" value near the bottom of the output. In each case I let the
laptop wait in the same state for a couple of minutes and then compared 3 or 4
results over 20 seconds or so to see that the power consumption appeared to
have settled.

There are related reports on Linux although those specifically deal with
increased power consumption after a suspend-resume cycle.

Links:
https://bugs.freedesktop.org/show_bug.cgi?id=54089
https://bugzilla.kernel.org/show_bug.cgi?id=58971
https://patchwork.kernel.org/patch/2827634/ (patch v1)
https://github.com/torvalds/linux/commit/7dcd2677ea912573d9ed4bcd629b0023b2d11505
https://github.com/torvalds/linux/commit/d11a96d0a4927e8948a6e92a581779f961d9745b

I have the two patches referenced there in a code review
(https://reviews.freebsd.org/D5599) although it does not address the increased
consumption immediately after loading i915kms.ko, and after further testing it
seems I see normal power consumption after suspend-resume with and without the
patch.

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