[Intel-gfx] [bisect] regression in suspend with i915 on 82852/855GM

[Philipp Gesang]

···<date: 2015-06-25, Thursday>···<from: Jani Nikula>···

> On Wed, 24 Jun 2015, Philipp Gesang <phg at phi-gamma.net> wrote:
> > Hi,
> >
> > suspend/resume results in the backlight not coming back to life
> > on my X40 laptop with an “Display controller: Intel Corporation
> > 82852/855GM Integrated Graphics Device”.
> >
> > I bisected the issue. Apparently, commit
> > b0cd324faed23d10d66ba6ade66579c681feef6f introduced the problem.
> >
> >     Author: Jani Nikula <jani.nikula at intel.com>
> >     Date:   Wed Nov 12 16:25:43 2014 +0200
> >
> >         drm/i915: don't save/restore backlight hist ctl registers
> >
> > I can confirm that suspend works as expected with a 4.1 kernel if
> > the lines to dump/undump the BLC_HIST_CTL register are
> > reintroduced; see the attached patch. (According to a web search,
> > the same issue seems to have arisen before in 2009, see [1].)
> >
> > Let me know if you’d like me to run further tests on that
> > particular hardware.
> 
> Please run 'intel_reg read 0x61260' after resume both on the patched and
> unpatched kernel to see what the hardware wants. The tool is in the
> intel-gpu-tools package [1].

Nice tool. With a vanilla kernel I get a value of 0x80000000
before suspend and 0x00000000 after resume. With the patched
kernel it’s 0x80000000 before and after.

On the vanilla kernel, issuing ``intel_reg write 0x61260 0x80000000`` 
after resume turns the backlight back on.

Best,
Philipp

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