[PATCH AUTOSEL 5.15 13/69] drm/probe-helper: Default to 640x480 if no EDID on DP

[Sasha Levin]

From: Douglas Anderson <dianders at chromium.org>

[ Upstream commit fae7d186403ee5a9375ec75938e0de99718e066a ]

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@changeid

Signed-off-by: Douglas Anderson <dianders at chromium.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk at quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov at linaro.org>
Acked-by: Jani Nikula <jani.nikula at intel.com>
Reviewed-by: Sean Paul <seanpaul at chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220601112302.v4.1.I31ec454f8d4ffce51a7708a8092f8a6f9c929092@changeid
Signed-off-by: Sasha Levin <sashal at kernel.org>
---
 drivers/gpu/drm/drm_probe_helper.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_probe_helper.c b/drivers/gpu/drm/drm_probe_helper.c
index 5606bca3caa8..86a5be9a5b17 100644
--- a/drivers/gpu/drm/drm_probe_helper.c
+++ b/drivers/gpu/drm/drm_probe_helper.c
@@ -516,8 +516,17 @@ 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)
 		goto prune;
-- 
2.35.1