Other then the name typo (s/Pual/Paul): Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx> (just since I co-authored things~) Reviewed-by: Lyude Paul <lyude@xxxxxxxxxx> I think we definitely want to make sure we get intel's opinions on this though, especially regarding the usage of link-status. I think we're close enough to link-status's intended purpose, but I definitely would like to know what others think about that since userspace will definitely have to handle situations like this a bit differently than with SST. Also - definitely make sure you take a look at Imre's patch series that's currently on the list (I just finished reviewing it), since it adds some things to the helpers that might end up being useful here :) https://patchwork.freedesktop.org/series/122589/ On Thu, 2023-08-24 at 16:50 -0400, Gil Dekel wrote: > 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. I -think- this is probably fine, that's likely how I'd > > 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 > -- Cheers, Lyude Paul (she/her) Software Engineer at Red Hat
