Quoting Lyude Paul (2019-01-29 19:09:59)
> When resuming, we check whether or not any previously connected
> MST topologies are still present and if so, attempt to resume them. If
> this fails, we disable said MST topologies and fire off a hotplug event
> so that userspace knows to reprobe.
>
> However, sending a hotplug event involves calling
> drm_fb_helper_hotplug_event(), which in turn results in fbcon doing a
> connector reprobe in the caller's thread - something we can't do at the
> point in which i915 calls drm_dp_mst_topology_mgr_resume() since
> hotplugging hasn't been fully initialized yet.
>
> This currently causes some rather subtle but fatal issues. For example,
> on my T480s the laptop dock connected to it usually disappears during a
> suspend cycle, and comes back up a short while after the system has been
> resumed. This guarantees pretty much every suspend and resume cycle,
> drm_dp_mst_topology_mgr_set_mst(mgr, false); will be caused and in turn,
> a connector hotplug will occur. Now it's Rute Goldberg time: when the
> connector hotplug occurs, i915 reprobes /all/ of the connectors,
> including eDP. However, eDP probing requires that we power on the panel
> VDD which in turn, grabs a wakeref to the appropriate power domain on
> the GPU (on my T480s, this is the PORT_DDI_A_IO domain). This is where
> things start breaking, since this all happens before
> intel_power_domains_enable() is called we end up leaking the wakeref
> that was acquired and never releasing it later. Come next suspend/resume
> cycle, this causes us to fail to shut down the GPU properly, which
> causes it not to resume properly and die a horrible complicated death.
>
> (as a note: this only happens when there's both an eDP panel and MST
> topology connected which is removed mid-suspend. One or the other seems
> to always be OK).
>
> We could try to fix the VDD wakeref leak, but this doesn't seem like
> it's worth it at all since we aren't able to handle hotplug detection
> while resuming anyway. So, let's go with a more robust solution inspired
> by nouveau: block fbdev from handling hotplug events until we resume
> fbdev. This allows us to still send sysfs hotplug events to be handled
> later by user space while we're resuming, while also preventing us from
> actually processing any hotplug events we receive until it's safe.
>
> This fixes the wakeref leak observed on the T480s and as such, also
> fixes suspend/resume with MST topologies connected on this machine.
>
> Changes since v2:
> * Don't call drm_fb_helper_hotplug_event() under lock, do it after lock
> (Chris Wilson)
> * Don't call drm_fb_helper_hotplug_event() in
> intel_fbdev_output_poll_changed() under lock (Chris Wilson)
> * Always set ifbdev->hpd_waiting (Chris Wilson)
>
> Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> Fixes: 0e32b39ceed6 ("drm/i915: add DP 1.2 MST support (v0.7)")
> Cc: Todd Previte <tprevite@xxxxxxxxx>
> Cc: Dave Airlie <airlied@xxxxxxxxxx>
> Cc: Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx>
> Cc: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> Cc: Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx>
> Cc: Imre Deak <imre.deak@xxxxxxxxx>
> Cc: intel-gfx@xxxxxxxxxxxxxxxxxxxxx
> Cc: <stable@xxxxxxxxxxxxxxx> # v3.17+
I suspect the locking is overkill, but certainly easier to reason than
trying to remember all the ins and outs of fbdev with its dubious async
initialisation.
Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
-Chris
