Issue after drm/i915: prefer wide & slow to fast & narrow in DP configs.

[Michal Srb <msrb@suse.com>]

Hi,

I am debugging an issue with IBM POS machine (4852-570, Truman), with 
8086:0102 Intel Sandybridge graphic card and internal monitor connected over 
display port.

The issue is that sporadically, after mode change, the monitor remains blank.
There is no difference in drm.debug=0xe level log or content of <debugfs>/dri 
between the state when monitor displays image and the state when it remains 
blank.

(The monitor supports only 1024x768, so every mode change is into this 
resolution. It happens when the intel driver switches resolution during boot, 
when X starts, X ends, modetest starts or modetest ends.)

I've bisected the cause to this commit:

  commit 2514bc510d0c3aadcc5204056bb440fa36845147
  Author: Jesse Barnes <jbarnes@virtuousgeek.org>
  Date:   Thu Jun 21 15:13:50 2012 -0700

      drm/i915: prefer wide & slow to fast & narrow in DP configs
    
      High frequency link configurations have the potential to cause trouble
      with long and/or cheap cables, so prefer slow and wide configurations
      instead.  This patch has the potential to cause trouble for eDP
      configurations that lie about available lanes, so if we run into that we
      can make it conditional on eDP.


The machine reports to have 2 lanes available and that seems to be correct.
One lane was used before the commit and the screen used to show picture after 
every mode change:

  [drm:intel_dp_mode_fixup], DP ink computation with max lane count 2 max bw 
0a pixel clock 65000KHz
  [drm:intel_dp_mode_fixup], DP link bw 0a lane count 1 clock 270000 bpp 24
  [drm:intel_dp_mode_fixup], DP link bw required 156000 available 216000

Two lanes are used after the commit and the screen occasionally remains blank 
after mode change:

  [drm:intel_dp_mode_fixup], DP link computation with max lane count 2 max bw 
0a pixel clock 65000KHz
  [drm:intel_dp_mode_fixup], DP link bw 06 lane count 2 clock 162000 bpp 24
  [drm:intel_dp_mode_fixup], DP link bw required 156000 available 259200


Could you advise me where to look further? Could this be a hardware bug? 
Reverting the commit works as a workaround, but doesn't look like correct way 
to fix this...

Thank you,
Michal Srb