drm/probe-helper: Default to 640x480 if no EDID on DP

If we're unable to read the EDID for a display because it's corrupt /
bogus / invalid then we'll add a set of standard modes for the
display. Since we have no true information about the connected
display, these modes are essentially guesses but better than nothing.
At the moment, none of the modes returned is marked as preferred, but
the modes are sorted such that the higher resolution modes are listed
first.

When userspace sees these modes presented by the kernel it needs to
figure out which one to pick. At least one userspace, ChromeOS [1]
seems to use the rules (which seem pretty reasonable):
1. Try to pick the first mode marked as preferred.
2. Try to pick the mode which matches the first detailed timing
   descriptor in the EDID.
3. If no modes were marked as preferred then pick the first mode.

Unfortunately, userspace's rules combined with what the kernel is
doing causes us to fail section 4.2.2.6 (EDID Corruption Detection) of
the DP 1.4a Link CTS. That test case says that, while it's OK to allow
some implementation-specific fall-back modes if the EDID is bad that
userspace should _default_ to 640x480.

Let's fix this by marking 640x480 as default for DP in the no-EDID
case.

NOTES:
- In the discussion around v3 of this patch [2] there was talk about
  solving this in userspace and I even implemented a patch that would
  have solved this for ChromeOS, but then the discussion turned back
  to solving this in the kernel.
- Also in the discussion of v3 [2] it was requested to limit this
  change to just DP since folks were worried that it would break some
  subtle corner case on VGA or HDMI.

[1] https://source.chromium.org/chromium/chromium/src/+/a051f741d0a15caff2251301efe081c30e0f4a96:ui/ozone/platform/drm/common/drm_util.cc;l=488
[2] https://lore.kernel.org/r/20220513130533.v3.1.I31ec454f8d4ffce51a7708a8092f8a6f9c929092@changeidSigned-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220601112302.v4.1.I31ec454f8d4ffce51a7708a8092f8a6f9c929092@changeid

drivers/gpu/drm/drm_probe_helper.c

@@ -569,8 +569,17 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector,
 		count = drm_add_override_edid_modes(connector);
 
 	if (count == 0 && (connector->status == connector_status_connected ||
-			   connector->status == connector_status_unknown))
+			   connector->status == connector_status_unknown)) {
 		count = drm_add_modes_noedid(connector, 1024, 768);
+
+		/*
+		 * Section 4.2.2.6 (EDID Corruption Detection) of the DP 1.4a
+		 * Link CTS specifies that 640x480 (the official "failsafe"
+		 * mode) needs to be the default if there's no EDID.
+		 */
+		if (connector->connector_type == DRM_MODE_CONNECTOR_DisplayPort)
+			drm_set_preferred_mode(connector, 640, 480);
+	}
 	count += drm_helper_probe_add_cmdline_mode(connector);
 	if (count != 0) {
 		ret = __drm_helper_update_and_validate(connector, maxX, maxY, &ctx);