[PATCH v4 0/6] drm/i915/dp_link_training: Define a final failure state when link training fails

[Lyude Paul]

Other then the name typo (s/Pual/Paul):

Signed-off-by: Lyude Paul <lyude at redhat.com> (just since I co-authored
things~)
Reviewed-by: Lyude Paul <lyude at redhat.com>

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