···<date: 2015-06-25, Thursday>···<from: Jani Nikula>··· > On Wed, 24 Jun 2015, Philipp Gesang <phg@xxxxxxxxxxxxx> 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@xxxxxxxxx> > > 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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