[Bug 87319] Dock with builtin DP/VGA "dongle" fails cycling through modes

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=87319

--- Comment #8 from Chih-Hsyuan Ho <chih.ho at canonical.com> ---
Created attachment 110893
  --> https://bugs.freedesktop.org/attachment.cgi?id=110893&action=edit
drm debug messages while doing hot-plugging with the VGA port in the docking
station

   On the VGA port on the Dock, there is no hotplug event after the DP-2 port
output state is set to NOCRTC.

Investigation:
  After we compare the drm debug message of the VGA ports (the VGA port on the
machine and on the dock).
 1. The VGA port on the machine:
      After we turn off the DP-2 (output state was set to NOCRTC.), the drm
driver receives the hot plug event from DP-2 port.
     The drm driver will probe the port.
     At 2032.729660 second in the dmesg_vga_port_on_the_machine.log in the
attached compressed file, DP-2 output state is set to NOCRTC.
    At 2032.755454 second, drm driver receive a hot-plug event.

 2. The VGA port on the dock:
      After we turn off the DP-2 (output state was set to NOCRTC.), there is no
the hot plug event from DP-2 port.
      At 1721.038377 second in the dmesg_vga_port_on_the_dock.log in the
attached compressed file, DP-2 output state is set to NOCRTC.
      After that, there is no hot plug event.

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