Comment 75 for bug 1169376

Today, I got down the latest source to the Ubuntu kernel and took a look at what was changed that broke the brightness control. On April 8th, they decided to take out a line that applied to Ivy Bridge processors in the file drivers/gpu/drm/i915/intel_display.c. I put the line back in, recompiled the kernel and installed it and now the brightness control is working fine on my laptop (a new XPS 13 Developer's Edition, which has an Ivy Bridge processor).

I'm not qualified to determine what the proper fix is for this, but apparently at least some machines with Ivy Bridge processors require that the 'quirk' be left in.