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

[Rodrigo Vivi <rodrigo.vivi@intel.com>]

On Wed, Aug 30, 2023 at 05:41:37PM -0400, Lyude Paul wrote:
> Other then the name typo (s/Pual/Paul):
> 
> Signed-off-by: Lyude Paul <lyude@redhat.com> (just since I co-authored
> things~)

I believe having the Co-developed-by: in the patches that you helped
out would be nice.

> Reviewed-by: Lyude Paul <lyude@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
>