drm/i915: Addin-offset is an unreliable indicator of LVDS presence (v2)

My Samsung N210 has a VBT with DEVICE_TYPE_INT_LFP with a zero
addin-offset. With the check in place, the panel was declared absent.

v2: Only trust BIOS writers that have graduated to writing OpRegions.
(We are all doomed.)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Zhao Yakui <yakui.zhao@intel.com>
Cc: Adam Jackson <ajax@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>

diff --git a/drivers/gpu/drm/i915/intel_lvds.c b/drivers/gpu/drm/i915/intel_lvds.c
index fe79c5a..047bd95 100644 (file)
--- a/drivers/gpu/drm/i915/intel_lvds.c
+++ b/drivers/gpu/drm/i915/intel_lvds.c
@@ -777,38 +777,44 @@ static void intel_find_lvds_downclock(struct drm_device *dev,
  * If it is present, return 1.
  * If it is not present, return false.
  * If no child dev is parsed from VBT, it assumes that the LVDS is present.
- * Note: The addin_offset should also be checked for LVDS panel.
- * Only when it is non-zero, it is assumed that it is present.
  */
-static int lvds_is_present_in_vbt(struct drm_device *dev)
+static bool lvds_is_present_in_vbt(struct drm_device *dev)
 {
        struct drm_i915_private *dev_priv = dev->dev_private;
-       struct child_device_config *p_child;
-       int i, ret;
+       int i;
 
        if (!dev_priv->child_dev_num)
-               return 1;
+               return true;
 
-       ret = 0;
        for (i = 0; i < dev_priv->child_dev_num; i++) {
-               p_child = dev_priv->child_dev + i;
-               /*
-                * If the device type is not LFP, continue.
-                * If the device type is 0x22, it is also regarded as LFP.
+               struct child_device_config *child = dev_priv->child_dev + i;
+
+               /* If the device type is not LFP, continue.
+                * We have to check both the new identifiers as well as the
+                * old for compatibility with some BIOSes.
                 */
-               if (p_child->device_type != DEVICE_TYPE_INT_LFP &&
-                       p_child->device_type != DEVICE_TYPE_LFP)
+               if (child->device_type != DEVICE_TYPE_INT_LFP &&
+                   child->device_type != DEVICE_TYPE_LFP)
                        continue;
 
-               /* The addin_offset should be checked. Only when it is
-                * non-zero, it is regarded as present.
+               /* However, we cannot trust the BIOS writers to populate
+                * the VBT correctly.  Since LVDS requires additional
+                * information from AIM blocks, a non-zero addin offset is
+                * a good indicator that the LVDS is actually present.
                 */
-               if (p_child->addin_offset) {
-                       ret = 1;
-                       break;
-               }
+               if (child->addin_offset)
+                       return true;
+
+               /* But even then some BIOS writers perform some black magic
+                * and instantiate the device without reference to any
+                * additional data.  Trust that if the VBT was written into
+                * the OpRegion then they have validated the LVDS's existence.
+                */
+               if (dev_priv->opregion.vbt)
+                       return true;
        }
-       return ret;
+
+       return false;
 }
 
 /**