commit cf0a6584aa6d382f802f2c3cacac23ccbccde0cd
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Feb 6 11:24:41 2013 +0100

    drm/i915: write backlight harder
    
    770c12312ad617172b1a65b911d3e6564fc5aca8 is the first bad commit
    commit 770c12312ad617172b1a65b911d3e6564fc5aca8
    Author: Takashi Iwai <tiwai@suse.de>
    Date:   Sat Aug 11 08:56:42 2012 +0200
    
        drm/i915: Fix blank panel at reopening lid
    
    changed the register write sequence for restoring the backlight, which
    helped prevent non-working backlights on some machines. Turns out that
    the original sequence was the right thing to do for a different set of
    machines. Worse, setting the backlight level _after_ enabling it seems
    to reset it somehow. So we need to make that one conditional upon the
    backlight having been reset to zero, and add the old one back.
    
    Cargo-culting at it's best, but it seems to work.
    
    Cc: stable@vger.kernel.org
    Cc: Takashi Iwai <tiwai@suse.de>
    Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=47941
    Reviewed-by: Jani Nikula <jani.nikula@intel.com>
    Acked-by: Takashi Iwai <tiwai@suse.de>
    Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
index bee8cb6..a3730e0 100644
--- a/drivers/gpu/drm/i915/intel_panel.c
+++ b/drivers/gpu/drm/i915/intel_panel.c
@@ -321,6 +321,9 @@ void intel_panel_enable_backlight(struct drm_device *dev,
 	if (dev_priv->backlight_level == 0)
 		dev_priv->backlight_level = intel_panel_get_max_backlight(dev);

+	dev_priv->backlight_enabled = true;
+	intel_panel_actually_set_backlight(dev, dev_priv->backlight_level);
+
 	if (INTEL_INFO(dev)->gen >= 4) {
 		uint32_t reg, tmp;

@@ -356,12 +359,12 @@ void intel_panel_enable_backlight(struct drm_device *dev,
 	}

 set_level:
-	/* Call below after setting BLC_PWM_CPU_CTL2 and BLC_PWM_PCH_CTL1.
-	 * BLC_PWM_CPU_CTL may be cleared to zero automatically when these
-	 * registers are set.
+	/* Check the current backlight level and try to set again if it's zero.
+	 * On some machines, BLC_PWM_CPU_CTL is cleared to zero automatically
+	 * when BLC_PWM_CPU_CTL2 and BLC_PWM_PCH_CTL1 are written.
 	 */
-	dev_priv->backlight_enabled = true;
-	intel_panel_actually_set_backlight(dev, dev_priv->backlight_level);
+	if (!intel_panel_get_backlight(dev))
+		intel_panel_actually_set_backlight(dev, dev_priv->backlight_level);
 }

 static void intel_panel_init_backlight(struct drm_device *dev)