drm/i915: Fix the VBT child device parsing for BSW

Recent BSW VBT has a VBT child device size 37 bytes instead of the 33
bytes our code assumes. This means we fail to parse the VBT and thus
fail to detect eDP ports properly and just register them as DP ports
instead.

Fix it up by using the reported child device size from the VBT instead
of assuming it matches out struct defintions.

The latest spec I have shows that the child device size should be 36
bytes for rev >= 195, however on my BSW the size is actually 37 bytes.
And our current struct definition is 33 bytes.

Feels like the entire VBT parses would need to be rewritten to handle
changes in the layout better, but for now I've decided to do just the
bare minimum to get my eDP port back.

Cc: Vijay Purushothaman <vijay.a.purushothaman@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_bios.c

@@ -447,6 +447,12 @@ parse_general_definitions(struct drm_i915_private *dev_priv,
 	}
 }
 
+static union child_device_config *
+child_device_ptr(struct bdb_general_definitions *p_defs, int i)
+{
+	return (void *) &p_defs->devices[i * p_defs->child_dev_size];
+}
+
 static void
 parse_sdvo_device_mapping(struct drm_i915_private *dev_priv,
 			  struct bdb_header *bdb)
@@ -476,10 +482,10 @@ parse_sdvo_device_mapping(struct drm_i915_private *dev_priv,
 	block_size = get_blocksize(p_defs);
 	/* get the number of child device */
 	child_device_num = (block_size - sizeof(*p_defs)) /
-				sizeof(*p_child);
+		p_defs->child_dev_size;
 	count = 0;
 	for (i = 0; i < child_device_num; i++) {
-		p_child = &(p_defs->devices[i]);
+		p_child = child_device_ptr(p_defs, i);
 		if (!p_child->old.device_type) {
 			/* skip the device block if device type is invalid */
 			continue;
@@ -1067,25 +1073,19 @@ parse_device_mapping(struct drm_i915_private *dev_priv,
 		DRM_DEBUG_KMS("No general definition block is found, no devices defined.\n");
 		return;
 	}
-	/* judge whether the size of child device meets the requirements.
-	 * If the child device size obtained from general definition block
-	 * is different with sizeof(struct child_device_config), skip the
-	 * parsing of sdvo device info
-	 */
-	if (p_defs->child_dev_size != sizeof(*p_child)) {
-		/* different child dev size . Ignore it */
-		DRM_DEBUG_KMS("different child size is found. Invalid.\n");
+	if (p_defs->child_dev_size < sizeof(*p_child)) {
+		DRM_ERROR("General definiton block child device size is too small.\n");
 		return;
 	}
 	/* get the block size of general definitions */
 	block_size = get_blocksize(p_defs);
 	/* get the number of child device */
 	child_device_num = (block_size - sizeof(*p_defs)) /
-				sizeof(*p_child);
+				p_defs->child_dev_size;
 	count = 0;
 	/* get the number of child device that is present */
 	for (i = 0; i < child_device_num; i++) {
-		p_child = &(p_defs->devices[i]);
+		p_child = child_device_ptr(p_defs, i);
 		if (!p_child->common.device_type) {
 			/* skip the device block if device type is invalid */
 			continue;
@@ -1105,7 +1105,7 @@ parse_device_mapping(struct drm_i915_private *dev_priv,
 	dev_priv->vbt.child_dev_num = count;
 	count = 0;
 	for (i = 0; i < child_device_num; i++) {
-		p_child = &(p_defs->devices[i]);
+		p_child = child_device_ptr(p_defs, i);
 		if (!p_child->common.device_type) {
 			/* skip the device block if device type is invalid */
 			continue;

drivers/gpu/drm/i915/intel_bios.h

@@ -277,9 +277,9 @@ struct bdb_general_definitions {
 	 * And the device num is related with the size of general definition
 	 * block. It is obtained by using the following formula:
 	 * number = (block_size - sizeof(bdb_general_definitions))/
-	 *	     sizeof(child_device_config);
+	 *	     defs->child_dev_size;
 	 */
-	union child_device_config devices[0];
+	uint8_t devices[0];
 } __packed;
 
 /* Mask for DRRS / Panel Channel / SSC / BLT control bits extraction */