[Bug 107281] New: Some DP MST hubs cause the driver to get stuck in link training clock recovery loop on hot-plugging

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=107281

            Bug ID: 107281
           Summary: Some DP MST hubs cause the driver to get stuck in link
                    training clock recovery loop on hot-plugging
           Product: DRI
           Version: DRI git
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/Intel
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: nathan.d.ciobanu at linux.intel.com
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org

Created attachment 140705
  --> https://bugs.freedesktop.org/attachment.cgi?id=140705&action=edit
Instrumented driver dmesg

The i915 driver can get stuck in the link training clock recovery infinite loop
when a USB-C DP MST hub with two DP displays is hot-plugged. 

Hardware:
- USB-C DP 1.2 MST hub with 2xDP connectors, 2xUSB2.0 ports, 1xEthernet port, 1
USB-C PD (pass-through power)
- Dell P2715Qt 3840x2160 DP display
- HP ZR24w 1920x1200 DP display
- KBL-Y Chromebook (running ChromeOS but tested with drm-tip kernel)

Steps to reproduce:
1. Connect the two displays to the MST hub: Dell in the first port, HP in the
second port
1.5. The ports may not be labeled with numbers, but check the display order in
the UI settings of the laptop, the order should be eDP - DP 3840x2160 - DP
1920x1200, order is important otherwise the bug will not be reproducible
2. Plug the MST Hubs USB-C connector into one of the USB-C DP ports of the
laptop
3. Allow time for the driver to train the eDP and the two external displays
4. Verify the order in step 1.5 in the laptop display settings UI
5. Unplug the USB-C MST hub
6. Give the system time to recover and the eDP to be retrained
7. Plug the USB-C MST hub as described in step 2

Observed behavior:
1. The two external displays remain blank
2. The display settings UI on the laptop shows that 2 external monitors are
connected
3. Subsequently hot-plugging the MST hub doesn't improve things even if the MST
hub is plugged into a different USB-C port on the laptop
4. The eDP continues to work
5. Rebooting the laptop is the only solution to get the two external displays
working again

Frequency:
Always (100%)

Workarounds:
- Change the order the two displays are connected on the MST hub
- Reboot laptop and do not hot-plug

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