[Bug 108687] ThinkPad T450 - Broadwell-U: [drm:intel_dp_start_link_train [i915]] *ERROR* failed to enable link training

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=108687

--- Comment #5 from Ville Syrjala <ville.syrjala at linux.intel.com> ---
[  678.059456] [drm:intel_hpd_irq_handler [i915]] Received HPD interrupt on PIN
5 - cnt: 0
[  678.059619] [drm:intel_dp_hpd_pulse [i915]] got hpd irq on port B - long
[  678.059723] [drm:i915_hotplug_work_func [i915]] running encoder hotplug
functions
[  678.059818] [drm:i915_hotplug_work_func [i915]] Connector DP-1 (pin 5)
received hotplug event.
[  678.059904] [drm:intel_dp_detect [i915]] [CONNECTOR:72:DP-1]
[  678.059991] [drm:intel_encoder_hotplug [i915]] [CONNECTOR:72:DP-1] status
updated from connected to disconnected

So here the display seems to disappear. Not sure if it was actually
disconnected?

Then we suspend before userspace has had a chance to turn off the display.
[  678.258481] PM: suspend entry (deep)

During resume we try to light up the display again (since it was still enabled
when we suspended):
[  679.515110] [drm:intel_dp_start_link_train [i915]] *ERROR* failed to get
link status
[  679.515134] [drm:intel_dp_start_link_train [i915]] [CONNECTOR:72:DP-1] Link
Training failed at link rate = 162000, lane count = 4

So the display hasn't re-appeared, or somehow the communication with the
display wasn't yet possible for whatever reason.

Then we see:
[  679.528207] [drm:intel_dp_detect [i915]] [CONNECTOR:72:DP-1]
[  679.528216] [drm:drm_helper_hpd_irq_event [drm_kms_helper]]
[CONNECTOR:72:DP-1] status updated from disconnected to disconnected

So clearly the display doesn't seem to be there.

A bit later the display seems to come to life:
[  680.113265] [drm:i915_hotplug_work_func [i915]] Connector DP-1 (pin 5)
received hotplug event.
[  680.113329] [drm:intel_dp_detect [i915]] [CONNECTOR:72:DP-1]
[  680.113844] [drm:intel_dp_read_dpcd [i915]] DPCD: 12 14 c4 01 01 01 01 81 02
02 06 00 00 00 00
[  680.115697] [drm:intel_dp_print_rates [i915]] source rates: 162000, 270000,
540000
[  680.115756] [drm:intel_dp_print_rates [i915]] sink rates: 162000, 270000,
540000
[  680.115813] [drm:intel_dp_print_rates [i915]] common rates: 162000, 270000,
540000
[  680.116236] [drm:drm_dp_read_desc [drm_kms_helper]] DP branch: OUI 00-00-00
dev-ID  HW-rev 0.0 SW-rev 0.0 quirks 0x0000
[  680.116603] [drm:intel_dp_detect [i915]] Sink is not MST capable
[  680.123057] [drm:drm_detect_monitor_audio [drm]] Monitor has basic audio
support
[  680.123444] [drm:intel_encoder_hotplug [i915]] [CONNECTOR:72:DP-1] status
updated from disconnected to connected

And now we are able to link train as well.

[  680.127754] [drm:intel_dp_start_link_train [i915]] [CONNECTOR:72:DP-1] Link
Training Passed at Link Rate = 162000, Lane count = 4

If the display was connected the whole time while suspended, then it seems to
be  some kind of platform issue where the AUX channel doesn't quite work right
during resume. Long ago I saw something like that on one IVB but then the
problem disappeared for whatever reason.

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