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. None of the modes returned is marked as preferred, but the modes are currently 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: 1. Try to pick the first mode marked as preferred. 2. If no modes were marked as preferred then pick the first mode.
The rules above seem pretty reasonable, but they have unfortunate side effect that when we have no EDID present we'll default to the highest resolution (least likely to work) mode.
Let's change things slightly. In the case of a failed EDID read we still won't mark anything preferred but we _won't_ sort the modes at the end of drm_helper_probe_single_connector_modes(). The drm_add_modes_noedid() adds 640x480 first and so by skipping the call to drm_mode_sort() it will stay first. That will be a hint to userspace to default to 640x480.
This change makes userspace that behaves like ChromeOS does compliant with section 4.2.2.6 (EDID Corruption Detection) of the DP 1.4a Link CTS. That section indicates that, at least on DP, if we have a corrupt EDID userspace may allow other modes to be tried but should default to 640x480 in the absence of more information. Note that if drm_add_modes_noedid() ever changes to _not_ list 640x480 first we might need to do more here, but that seems unlikely and, in any case, it would be caught by a future run of DP compliance testing.
Note: this change could pave the way to further improvement to drm_helper_probe_single_connector_modes(). Specifically, in the case of no EDID we could add additional "standard" modes that are riskier than 1024x768 (the current max we add). Now that we're giving userspace the hint that it should default to 640x480 perhaps it would be OK to offer the options of the higher resolution modes just in case they work. This further improvement is left as an exercise to the reader.
[1] https://source.chromium.org/chromium/chromium/src/+/a051f741d0a15caff2251301...
Signed-off-by: Douglas Anderson dianders@chromium.org --- Note that this is the second of two related and similar-sounding but different patches. See also ("drm/probe-helper: For DP, add 640x480 if all other modes are bad") [2]. I'm hoping to land _both_ of the patches since they address different issues. This patch addresses the case of a corrupt EDID and having 640x480 be the default in the "guessed" modes. The other patch handles the case where the EDID _isn't_ corrupt but all the modes listed can't be made with the existing situations. The two patches can land in either order.
Also note that I didn't carry any Tested-by / Reviewed-by tags since the patch is now quite different (yet again for v2 to v3).
[2] https://lore.kernel.org/r/20220510131309.v2.2.I4ac7f55aa446699f8c200a23c1046...
Changes in v3: - Don't set preferred, just disable the sort.
Changes in v2: - Don't modify drm_add_modes_noedid() 'cause that'll break others - Set 640x480 as preferred in drm_helper_probe_single_connector_modes()
drivers/gpu/drm/drm_probe_helper.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-)
diff --git a/drivers/gpu/drm/drm_probe_helper.c b/drivers/gpu/drm/drm_probe_helper.c index 682359512996..21dd60f30cc7 100644 --- a/drivers/gpu/drm/drm_probe_helper.c +++ b/drivers/gpu/drm/drm_probe_helper.c @@ -425,6 +425,7 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector, bool verbose_prune = true; enum drm_connector_status old_status; struct drm_modeset_acquire_ctx ctx; + bool sort_list = true;
WARN_ON(!mutex_is_locked(&dev->mode_config.mutex));
@@ -516,8 +517,16 @@ 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); + /* + * Want lower res modes, like 640x480, first. That indicates + * to userspace that these are "better" modes. Since we have + * no EDID the modes are a guess anyway, so guess the safer + * mode first. + */ + sort_list = false; + } count += drm_helper_probe_add_cmdline_mode(connector); if (count == 0) goto prune; @@ -576,7 +585,8 @@ int drm_helper_probe_single_connector_modes(struct drm_connector *connector, if (list_empty(&connector->modes)) return 0;
- drm_mode_sort(&connector->modes); + if (sort_list) + drm_mode_sort(&connector->modes);
DRM_DEBUG_KMS("[CONNECTOR:%d:%s] probed modes :\n", connector->base.id, connector->name);