and the commit from linux:

commit 7aa1e7f06d6ea1bce3b27630d50769d13da28b1a
Author: Daniel Vetter <>
Date:   Wed Jun 27 16:43:36 2012 +0200

    Revert "drm/i915: allow PCH PWM override on IVB"

    This reverts commit f82cfb6bcda164ef3a66b8c3fc549b1f9bdd09ad.

    This breaks the backlight controls on my IVB asus zenbook with an eDP

    I guess the right fix would be to read this bit and use either the pch
    or the cpu register to frob the backlight values. But that is stuff
    for -next.

    Cc: Jesse Barnes <>
    Signed-Off-by: Daniel Vetter <>

On 28 January 2015 at 01:11, Adrian Chadd <> wrote:
> ugh, so because i can't sleep, I read the Linux i915 history for
> backlight changes.
> Turns out if I disable bit 30 in ivb_pch_pwm_override() to register
> BLC_PWM_PCH_CTL1, the acpi video setting works. The keyboard settings
> still don't, but I'll worry about that later.
> Bit 30 in the linux drm code is:
> #define   BLM_PCH_OVERRIDE_ENABLE               (1 << 30)
> The linux dri code doesn't set that for ivybridge - only for broadwell.
> (also, the linux driver has POSTING_READ() which I'm guessing is to
> ensure things made it out to the device and have been flushed before
> doing another IO operation.)
> I have my (via sysctl) panel backlight control working again.
> -adrian
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