i915 completely unusable in 2.6.38.x

[Melchior FRANZ]

* Melchior FRANZ -- Tuesday 26 April 2011:
> [https://bugzilla.kernel.org/show_bug.cgi?id=31522]

I've replied to this error message, but here again for this audience:
The commit that broke all 2.6.38.* releases for my machine is this:
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=ba3820ade317ee36e496b9b40d2ec3987dd4aef0
(Takashi Iwai: "drm/i915: Revive combination mode for backlight
control").

In 'is_backlight_combination_mode()' INTEL_INFO(dev)->gen
equals 4, and the function returns 0x40000000 in "combination mode".
In 'intel_panel_get_backlight()' this happens:


  val = I915_READ(BLC_PWM_CTL) & BACKLIGHT_DUTY_CYCLE_MASK; // val = 0x0b4a
  if (IS_PINEVIEW(dev))                                     // false
          val >>= 1;

  if (is_backlight_combination_mode(dev)){
          u8 lbpc;

          val &= ~1;                                        // val = 0x0b4a
          pci_read_config_byte(dev->pdev, PCI_LBPC, &lbpc); // lbpc = 0
          val *= lbpc;                                      // val = 0
  }


The backlight remains off and does also not react to the "brighter" key
event.

Reverting the patch fixes my system, obviously.

m.