drm/i915: Pick the first mode from EDID as the fixed mode when there is no preferred mode

Some monitors apparently forget to mark any mode as preferred in the
EDID. In this particular case we have a very generic looking ID
"PNP Model 0 Serial Number 4" / "LVDS 800x600" so a specific quirk
doesn't seem particularly wise. Also the quirk we have
(EDID_QUIRK_FIRST_DETAILED_PREFERRED) is actually defunct so we'd
have to fix it first.

When there is no preferred mode we currently fall back to the VBT.
That approach fails us here as the VBT mode is 1024x768 whereas
the panel resolution is 800x600. So instead of falling back to the
VBT when there is no preferred mode let's just pick the first
probed mode. Only if the EDID provided no modes we fall back to
the VBT.

For this machine the VBIOS would appear to select the 800x600
60Hz EST mode rather than the first detailed mode (which is
the new fallback will pick). The two modes differ only by
having opposite sync polarities, which does not seem to matter
to the panel in question.

v2: Make sure the probed_modes list is not empty

Cc: Adam Jackson <ajax@redhat.com>
Cc: Roberto Viola <cagnulein@gmail.com>
Tested-by: Roberto Viola <cagnulein@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109780Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190321132446.22394-2-ville.syrjala@linux.intel.comReviewed-by: Adam Jackson <ajax@redhat.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_panel.c

@@ -104,11 +104,13 @@ intel_panel_edid_fixed_mode(struct intel_connector *connector)
 {
 	struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
 	const struct drm_display_mode *scan;
+	struct drm_display_mode *fixed_mode;
+
+	if (list_empty(&connector->base.probed_modes))
+		return NULL;
 
 	/* prefer fixed mode from EDID if available */
 	list_for_each_entry(scan, &connector->base.probed_modes, head) {
-		struct drm_display_mode *fixed_mode;
-
 		if ((scan->type & DRM_MODE_TYPE_PREFERRED) == 0)
 			continue;
 
@@ -123,7 +125,20 @@ intel_panel_edid_fixed_mode(struct intel_connector *connector)
 		return fixed_mode;
 	}
 
-	return NULL;
+	scan = list_first_entry(&connector->base.probed_modes,
+				typeof(*scan), head);
+
+	fixed_mode = drm_mode_duplicate(&dev_priv->drm, scan);
+	if (!fixed_mode)
+		return NULL;
+
+	fixed_mode->type |= DRM_MODE_TYPE_PREFERRED;
+
+	DRM_DEBUG_KMS("[CONNECTOR:%d:%s] using first mode from EDID: ",
+		      connector->base.base.id, connector->base.name);
+	drm_mode_debug_printmodeline(fixed_mode);
+
+	return fixed_mode;
 }
 
 /* adjusted_mode has been preset to be the panel's fixed mode */