i915 update to Linux 3.8 ready!

[Ed Maste]

On 17 March 2016 at 11:20, Ed Maste <emaste at freebsd.org> wrote:
> On 15 March 2016 at 19:59,  <am_dxer at fastmail.fm> wrote:
>>
>> Thank you both for the suggestions. After suspending and resuming the
>> machine, I find that the power used by the GPU is significantly less.
>
> Thank you for testing, and I see the same behaviour on my Thinkpad
> X220: after loading i915kms.ko only RC6 is nonzero and RC6+ and RC6++
> remain at zero, while after
> boot RC6++ is much higher.

I tried reverting the Sandy Bridge part of the change that was
identified as the source of the similar problem in Linux, and it fixes
the power consumption issue for me.

My partial revert is just removes the write to the GEN6_MBCTL register
in gen6_init_clock_gating(). Of course there's some reason it was
added in Linux, but the driver was functional on Sandy Bridge and
earlier hardware for years prior to the recent FreeBSD i915 update,
without that register write. Thus I think it's reasonable to disable
it until we either fully understand the problem or (more likely) just
update to a later Linux i915 version and verify normal Sandy Bridge
power consumption.

The patch: https://people.freebsd.org/~emaste/D5665.diff

Code review: https://reviews.freebsd.org/D5665