From 981c6da0cb32e655fce43d4b73328f5aacc7fe01 Mon Sep 17 00:00:00 2001
From: Douglas Anderson <dianders@chromium.org>
Date: Wed, 1 Jun 2022 11:23:24 -0700
Subject: drm/probe-helper: Default to 640x480 if no EDID on DP
Git-commit: fae7d186403ee5a9375ec75938e0de99718e066a
Patch-mainline: v6.0-rc1
References: jsc#PED-1166 jsc#PED-1168 jsc#PED-1170 jsc#PED-1218 jsc#PED-1220 jsc#PED-1222 jsc#PED-1223 jsc#PED-1225 jsc#PED-2849
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@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
Acked-by: Patrik Jakobsson <pjakobsson@suse.de>
---
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 425f56280d51..75a71649b64d 100644
--- a/drivers/gpu/drm/drm_probe_helper.c
+++ b/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);
--
2.38.1