1 files changed, 15 insertions, 2 deletions

diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
index 0e018cb49147..41b3be217493 100644
--- a/drivers/gpu/drm/i915/intel_panel.c
+++ b/drivers/gpu/drm/i915/intel_panel.c
@@ -1098,12 +1098,25 @@ static u32 get_backlight_min_vbt(struct intel_connector *connector)
         struct drm_device *dev = connector->base.dev;
         struct drm_i915_private *dev_priv = dev->dev_private;
         struct intel_panel *panel = &connector->panel;
+        int min;
 
         WARN_ON(panel->backlight.max == 0);
 
+        /*
+         * XXX: If the vbt value is 255, it makes min equal to max, which leads
+         * to problems. There are such machines out there. Either our
+         * interpretation is wrong or the vbt has bogus data. Or both. Safeguard
+         * against this by letting the minimum be at most (arbitrarily chosen)
+         * 25% of the max.
+         */
+        min = clamp_t(int, dev_priv->vbt.backlight.min_brightness, 0, 64);
+        if (min != dev_priv->vbt.backlight.min_brightness) {
+                DRM_DEBUG_KMS("clamping VBT min backlight %d/255 to %d/255\n",
+                              dev_priv->vbt.backlight.min_brightness, min);
+        }
+
         /* vbt value is a coefficient in range [0..255] */
-        return scale(dev_priv->vbt.backlight.min_brightness, 0, 255,
-                     0, panel->backlight.max);
+        return scale(min, 0, 255, 0, panel->backlight.max);
 }
 
 static int bdw_setup_backlight(struct intel_connector *connector)