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

[Nick Bowler]

Hi,

This issue is still present in 4.3-rc4.

On 9/24/15, Nick Bowler <nbowler at draconx.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.
>
> Linux 4.2 did not suffer from this problem.
>
> In dmesg I see the following messages, which I do not see on a working
> kernel.  Full dmesg from 4.3-rc2 is attached (gzipped).
>
>   [    0.115339] [drm:drm_calc_timestamping_constants] *ERROR* crtc
> 21: Can't calculate constants, dotclock = 0!
>   [    0.117582] [drm:intel_opregion_init] *ERROR* No ACPI video bus found
>
> This is an older machine with Intel G45 graphics.

I was able to identify the commit which fixed my boot crashes, so I
cherry-picked 80aa93128653 ("drm/i915: disable_shared_pll doesn't
work on pre-gen5") on top of all otherwise untestable commits.  This
allowed bisection to proceed:

  b8afb9113c519a8bd742f7df8c424b0af69a75cd is the first bad commit
  commit b8afb9113c519a8bd742f7df8c424b0af69a75cd
  Author: Ville Syrjälä <ville.syrjala at linux.intel.com>
  Date:   Mon Jun 29 15:25:48 2015 +0300

      drm/i915: Keep GMCH DPLL VGA mode always disabled

      We disable the DPLL VGA mode when enabling the DPLL, but we enaable it
      again when disabling the DPLL. Having VGA mode enabled even in unused
      DPLLs can cause problems for CHV, so it seems wiser to always keep it
      disabled. And let's just do that on all GMCH platforms to keep things
      as similar as possible between them.

      Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
      Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani at intel.com>
      Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>

  :040000 040000 7797d596e73ecf75723375028decd25fbe332ee0
9f90a92eec483919853d68563bbb09a71a305532 M	drivers

Unfortunately it does not revert cleanly on master.

Regards,
  Nick