Strange effect with i915 backlight controller

[Takashi Iwai]

At Thu, 10 Nov 2011 16:11:29 +0100,
Daniel Mack wrote:
> 
> On 11/08/2011 01:57 AM, Daniel Mack wrote:
> > Didn't get any response yet, hence copying LKML for a broader audience.
> 
> Nobody, really?
> 
> This is a rather annoying regression, as touching the brightness keys
> appearantly switches off the whole machine. I'm sure this is trivial to
> fix, I just don't have the insight of this driver and the chipset.

I vaguely remember that the bit 0 is invalid on some old chips.
Maybe 915GM is one of them, as it's gen3?  If so, the patch like below
may work.


Takashi

---
diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
index 499d4c0..be952d1 100644
--- a/drivers/gpu/drm/i915/intel_panel.c
+++ b/drivers/gpu/drm/i915/intel_panel.c
@@ -249,8 +249,11 @@ static void intel_panel_actually_set_backlight(struct drm_device *dev, u32 level
 	if (IS_PINEVIEW(dev)) {
 		tmp &= ~(BACKLIGHT_DUTY_CYCLE_MASK - 1);
 		level <<= 1;
-	} else
+	} else {
 		tmp &= ~BACKLIGHT_DUTY_CYCLE_MASK;
+		if (INTEL_INFO(dev)->gen < 4)
+			tmp &= ~1;
+	}
 	I915_WRITE(BLC_PWM_CTL, tmp | level);
 }