linux-kernel - Re: PROBLEM: Intel VGA output busticated on 4.3-rc2 (regression)

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Date:	Wed, 7 Oct 2015 18:32:02 +0300
From:	Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:	Nick Bowler <nbowler@...conx.ca>
Cc:	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	intel-gfx@...ts.freedesktop.org
Subject: Re: PROBLEM: Intel VGA output busticated on 4.3-rc2 (regression)

On Wed, Oct 07, 2015 at 10:29:22AM -0400, Nick Bowler wrote:
> On 10/7/15, Ville Syrjälä <ville.syrjala@...ux.intel.com> wrote:
> > On Tue, Oct 06, 2015 at 11:42:33AM -0400, Nick Bowler wrote:
> >> On 9/24/15, Nick Bowler <nbowler@...conx.ca> wrote:
> >> > Testing out 4.3-rc2, first thing I notice is that the VGA output is
> >> > not working.  Specifically, the display is continuously powering on
> >> > and off -- at no point is any image visible on the screen (I am
> >> > expecting to see the console output).  The display connected to the
> >> > HDMI output is working fine.
> [...]
> >>   b8afb9113c519a8bd742f7df8c424b0af69a75cd is the first bad commit
> >>   commit b8afb9113c519a8bd742f7df8c424b0af69a75cd
> >>   Author: Ville Syrjälä <ville.syrjala@...ux.intel.com>
> >>   Date:   Mon Jun 29 15:25:48 2015 +0300
> >>
> >>       drm/i915: Keep GMCH DPLL VGA mode always disabled
> [...]
> > @@ -1790,13 +1790,13 @@ static void i9xx_disable_pll(struct intel_crtc
> > *crtc)
> >         /* Make sure the pipe isn't still relying on us */
> >         assert_pipe_disabled(dev_priv, pipe);
> >
> > -       I915_WRITE(DPLL(pipe), 0);
> > +       I915_WRITE(DPLL(pipe), DPLL_VGA_MODE_DIS);
> >         POSTING_READ(DPLL(pipe));
> >  }
> >
> >
> > That hunk is the only relevant part for your machine. Can you try to revert
> > just that manually?
> >
> > But I'm really surprised that would have any effect since we only used
> > to enable "VGA mode" when the DPLL is off. And when the DPLL is off,
> > there's nothing on the screen anyway.
> 
> Nevertheless, manually reverting just that hunk seems to fix it.

Hmm. You said VGA has the problem, but HDMI does not. Was the problem
happening even when you have both displays enabled at the same time, or
just when VGA was enabled alone?

I've attached two potential patches that might help. Can you give a try
to just patch 1, and if that alone doesn't help then both patches
together?

-- 
Ville Syrjälä
Intel OTC

View attachment "0001-restore-DPLL-write.patch" of type "text/x-diff" (768 bytes)

View attachment "0002-enable-VGA-mode-before-P1-P2-write.patch" of type "text/x-diff" (894 bytes)