drm/i915/panel: Restore saved value of BLC_PWM_CTL

After a GPU reset, the backlight controller registers may be also reset
to 0. In that case we should restore those to the original values
programmed by the BIOS. Note that we still lack the code to handle the
case where the BIOS failed to program those registers at all...
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_panel.c

@@ -125,15 +125,55 @@ static int is_backlight_combination_mode(struct drm_device *dev)
 	return 0;
 }
 
+static u32 i915_read_blc_pwm_ctl(struct drm_i915_private *dev_priv)
+{
+	u32 val;
+
+	/* Restore the CTL value if it lost, e.g. GPU reset */
+
+	if (HAS_PCH_SPLIT(dev_priv->dev)) {
+		val = I915_READ(BLC_PWM_PCH_CTL2);
+		if (dev_priv->saveBLC_PWM_CTL2 == 0) {
+			dev_priv->saveBLC_PWM_CTL2 = val;
+		} else if (val == 0) {
+			I915_WRITE(BLC_PWM_PCH_CTL2,
+				   dev_priv->saveBLC_PWM_CTL);
+			val = dev_priv->saveBLC_PWM_CTL;
+		}
+	} else {
+		val = I915_READ(BLC_PWM_CTL);
+		if (dev_priv->saveBLC_PWM_CTL == 0) {
+			dev_priv->saveBLC_PWM_CTL = val;
+			dev_priv->saveBLC_PWM_CTL2 = I915_READ(BLC_PWM_CTL2);
+		} else if (val == 0) {
+			I915_WRITE(BLC_PWM_CTL,
+				   dev_priv->saveBLC_PWM_CTL);
+			I915_WRITE(BLC_PWM_CTL2,
+				   dev_priv->saveBLC_PWM_CTL2);
+			val = dev_priv->saveBLC_PWM_CTL;
+		}
+	}
+
+	return val;
+}
+
 u32 intel_panel_get_max_backlight(struct drm_device *dev)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	u32 max;
 
+	max = i915_read_blc_pwm_ctl(dev_priv);
+	if (max == 0) {
+		/* XXX add code here to query mode clock or hardware clock
+		 * and program max PWM appropriately.
+		 */
+		printk_once(KERN_WARNING "fixme: max PWM is zero.\n");
+		return 1;
+	}
+
 	if (HAS_PCH_SPLIT(dev)) {
-		max = I915_READ(BLC_PWM_PCH_CTL2) >> 16;
+		max >>= 16;
 	} else {
-		max = I915_READ(BLC_PWM_CTL);
 		if (IS_PINEVIEW(dev)) {
 			max >>= 17;
 		} else {
@@ -146,14 +186,6 @@ u32 intel_panel_get_max_backlight(struct drm_device *dev)
 			max *= 0xff;
 	}
 
-	if (max == 0) {
-		/* XXX add code here to query mode clock or hardware clock
-		 * and program max PWM appropriately.
-		 */
-		DRM_ERROR("fixme: max PWM is zero.\n");
-		max = 1;
-	}
-
 	DRM_DEBUG_DRIVER("max backlight PWM = %d\n", max);
 	return max;
 }