Next version of https://patchwork.freedesktop.org/series/122850/ v4: Another blunder. I uploaded the patches from my ChromeiumOS kernel dev repo instead of drm-tip/drm-tip. Apologies for the noise :( v3: Still learning the ropes of upstream workflow. Apologies for mucking up v2. This is just a re-upload. v2: Reorganize into: 1) Add for final failure state for SST and MST link training fallback. 2) Add a DRM helper for setting downstream MST ports' link-status state. 3) Make handling SST and MST connectors simpler via intel_dp. 4) Update link-status for downstream MST ports. 5) Emit a uevent with the "link-status" trigger property. v1: Currently, when link training fails after all fallback values have been exhausted, the i915 driver seizes to send uevents to userspace. This leave userspace thinking that the last passing atomic commit was successful, and that all connectors (displays) are connected and operational, when in fact, the last link failed to train and the displays remain dark. This manifests as "zombie" displays in userspace, in which users observe the displays appear in their display settings page, but they are dark and unresponsive. Since, at the time of writing, MST link training fallback is not implemented, failing MST link training is a significantly more common case then a complete SST link training failure. And with users using MST hubs more than ever to connect multiple displays via their USB-C ports we observe this case often. This patchset series suggest a solution, in which a final failure state is defined. In this final state, the connector's bit rate capabilities, namely max_link_rate and max_link_lane_count, are set to 0. This effectively set the connector's bandwidth to 0Gbps, thus causing all its modes to be pruned in the following connector probing. Next, with this state defined, we emit a link-status=Bad uevent. The next time userspace probes the connector, it should recognize that the connector has no modes and ignore it since it is in a bad state. I am aware that always sending a uevent and never stopping may result in some userspaces having their expectations broken and enter an infinite loop of modesets and link-training attempts. However, per DRM link-status spec: ``` * link-status: * Connector link-status property to indicate the status of link. The * default value of link-status is "GOOD". If something fails during or * after modeset, the kernel driver may set this to "BAD" and issue a * hotplug uevent. Drivers should update this value using * drm_connector_set_link_status_property(). * * When user-space receives the hotplug uevent and detects a "BAD" * link-status, the sink doesn't receive pixels anymore (e.g. the screen * becomes completely black). The list of available modes may have * changed. User-space is expected to pick a new mode if the current one * has disappeared and perform a new modeset with link-status set to * "GOOD" to re-enable the connector. ``` (form drivers/gpu/drm/drm_connector.c - DOC: standard connector properties) it seems reasonable to assume that the suggested state is an extension of the spec's guidelines, in which the next new mode userspace picks for a connector with no modes is - none, thus breaking the cycle of failed link-training attempts. I suspect that, maybe, zeroing out the bit rate capabilities is not the right way to go, and perhaps marking the connector as disconnected instead may be a better solution. However, if marking a connector disconnected is the way to go, We will have to iterate over all MST ports in the MST case and mark the spawned connectors as disconnected as well. As a final note I should add that this approach was tested with ChromeOS as userspace, and we observed that the zombie displays stop showing up once the connectors are pruned of all their modes and are ignored by userspace. For your consideration and guidance. Thanks, Gil Dekel (6): drm/i915/dp_link_training: Add a final failing state to link training fallback drm/i915/dp_link_training: Add a final failing state to link training fallback for MST drm/dp_mst: Add drm_dp_set_mst_topology_link_status() drm/i915: Move DP modeset_retry_work into intel_dp drm/i915/dp_link_training: Set all downstream MST ports to BAD before retrying drm/i915/dp_link_training: Emit a link-status=Bad uevent with trigger property drivers/gpu/drm/display/drm_dp_mst_topology.c | 39 ++++++++++ drivers/gpu/drm/i915/display/intel_display.c | 14 +++- .../drm/i915/display/intel_display_types.h | 6 +- drivers/gpu/drm/i915/display/intel_dp.c | 75 ++++++++++++------- drivers/gpu/drm/i915/display/intel_dp.h | 2 +- .../drm/i915/display/intel_dp_link_training.c | 11 ++- include/drm/display/drm_dp_mst_helper.h | 3 + 7 files changed, 110 insertions(+), 40 deletions(-) -- Gil Dekel, Software Engineer, Google / ChromeOS Display and Graphics
