drm: Separate EDID Header Check from EDID Block Check

    Provides function drm_edid_header_is_valid() for EDID header check
    and replaces EDID header check part of function drm_edid_block_valid()
    by a call of drm_edid_header_is_valid().
    This is a prerequisite to extend DDC probing, e. g. in function
    radeon_ddc_probe() for Radeon devices, by a central EDID header check.

    Tested for kernel 2.6.35, 2.6.38 and 3.0

Cc: <stable@kernel.org>
Signed-off-by: Thomas Reim <reimth@gmail.com>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Acked-by: Stephen Michaels <Stephen.Micheals@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_edid.c

@@ -127,6 +127,23 @@ static const u8 edid_header[] = {
 	0x00, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x00
 };
 
+ /*
+ * Sanity check the header of the base EDID block.  Return 8 if the header
+ * is perfect, down to 0 if it's totally wrong.
+ */
+int drm_edid_header_is_valid(const u8 *raw_edid)
+{
+	int i, score = 0;
+
+	for (i = 0; i < sizeof(edid_header); i++)
+		if (raw_edid[i] == edid_header[i])
+			score++;
+
+	return score;
+}
+EXPORT_SYMBOL(drm_edid_header_is_valid);
+
+
 /*
  * Sanity check the EDID block (base or extension).  Return 0 if the block
  * doesn't check out, or 1 if it's valid.
@@ -139,12 +156,7 @@ drm_edid_block_valid(u8 *raw_edid)
 	struct edid *edid = (struct edid *)raw_edid;
 
 	if (raw_edid[0] == 0x00) {
-		int score = 0;
-
-		for (i = 0; i < sizeof(edid_header); i++)
-			if (raw_edid[i] == edid_header[i])
-				score++;
-
+		int score = drm_edid_header_is_valid(raw_edid);
 		if (score == 8) ;
 		else if (score >= 6) {
 			DRM_DEBUG("Fixing EDID header, your hardware may be failing\n");

include/drm/drm_crtc.h

@@ -804,6 +804,7 @@ extern struct drm_display_mode *drm_gtf_mode_complex(struct drm_device *dev,
 extern int drm_add_modes_noedid(struct drm_connector *connector,
 				int hdisplay, int vdisplay);
 
+extern int drm_edid_header_is_valid(const u8 *raw_edid);
 extern bool drm_edid_is_valid(struct edid *edid);
 struct drm_display_mode *drm_mode_find_dmt(struct drm_device *dev,
 					   int hsize, int vsize, int fresh);