3.8.2->3.8.3 i915 regression: GMBUS [i915 gmbus dpb] timed out, falling back to bit banging on pin 5

[Arkadiusz Miskiewicz <a.miskiewicz@gmail.com>]

Hello.

After upgrading from 3.8.2 to 3.8.3 I'm getting regression :

diff:
  [drm] Supports vblank timestamp caching Rev 1 (10.10.2010).
  [drm] Driver supports precise vblank timestamp query.
  vgaarb: device changed decodes: PCI:0000:00:02.0,olddecodes=io+mem,decodes=io+mem:owns=io+mem
+ [drm] GMBUS [i915 gmbus dpb] timed out, falling back to bit banging on pin 5
  fbcon: inteldrmfb (fb0) is primary device
- Console: switching to colour frame buffer device 180x56
+ Console: switching to colour frame buffer device 128x48
  i915 0000:00:02.0: fb0: inteldrmfb frame buffer device
  i915 0000:00:02.0: registered panic notifier
  acpi device:01: registered as cooling_device0

console becomes weird, ends up in 2/3 of screen, X when starts gets tiny fonts etc.

Machine is Thinkpad T400 with gm45 GPU
00:02.0 VGA compatible controller [0300]: Intel Corporation Mobile 4 Series Chipset Integrated Graphics Controller [8086:2a42] (rev 07) (prog-if 00 [VGA controller])
        Subsystem: Lenovo Device [17aa:2112]

Reverting bisected commit from 3.8.3 fixes the problem

2a9810441fcc26cf3f006f015f8a62094fe57a90 is the first bad commit
commit 2a9810441fcc26cf3f006f015f8a62094fe57a90
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sat Dec 1 21:03:22 2012 +0100

    drm/i915: reorder setup sequence to have irqs for output setup
    
    commit 52d7ecedac3f96fb562cb482c139015372728638 upstream.
    
    Otherwise the new&shiny irq-driven gmbus and dp aux code won't work that
    well. Noticed since the dp aux code doesn't have an automatic fallback
    with a timeout (since the hw provides for that already).
    
    v2: Simple move drm_irq_install before intel_modeset_gem_init, as
    suggested by Ben Widawsky.
    
    v3: Now that interrupts are enabled before all connectors are fully
    set up, we might fall over serving a HPD interrupt while things are
    still being set up. Instead of jumping through massive hoops and
    complicating the code with a separate hpd irq enable step, simply
    block out the hotplug work item from doing anything until things are
    in place.
    
    v4: Actually, we can enable hotplug processing only after the fbdev is
    fully set up, since we call down into the fbdev from the hotplug work
    functions. So stick the hpd enabling right next to the poll helper
    initialization.
    
    v5: We need to enable irqs before intel_modeset_init, since that
    function sets up the outputs.
    
    v6: Fixup cleanup sequence, too.
    
    Reviewed-by: Imre Deak <imre.deak@intel.com>
    Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

:040000 040000 26e83b14e8d49da8c451dc2c837646a337a79085 fa2cbfcf159808ce188675115888242245e4e69d M      drivers


relevant dmesg:

 [drm] Initialized drm 1.1.0 20060810
 [drm] radeon defaulting to userspace modesetting.
 i915 0000:00:02.0: setting latency timer to 64
 i915 0000:00:02.0: irq 47 for MSI/MSI-X
 [drm] Supports vblank timestamp caching Rev 1 (10.10.2010).
 [drm] Driver supports precise vblank timestamp query.
 [drm] GMBUS [i915 gmbus dpb] timed out, falling back to bit banging on pin 5
 fbcon: inteldrmfb (fb0) is primary device
 i915 0000:00:02.0: fb0: inteldrmfb frame buffer device
 i915 0000:00:02.0: registered panic notifier
 [drm] Initialized i915 1.6.0 20080730 for 0000:00:02.0 on minor 0

-- 
Arkadiusz Miśkiewicz, arekm / maven.pl