From: Douglas Anderson dianders@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/+/a051f741d0a15caff2251301... [2] https://lore.kernel.org/r/20220513130533.v3.1.I31ec454f8d4ffce51a7708a8092f8...
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.I31ec454f8... Signed-off-by: Sasha Levin sashal@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;