[Bug 64178] New: [Intel GM45] xrandr --output <output> --mode <prev_mode> is not able to light up an output that has been disabled with xrandr --off.

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=64178

          Priority: medium
            Bug ID: 64178
          Assignee: intel-gfx-bugs at lists.freedesktop.org
           Summary: [Intel GM45] xrandr --output <output> --mode
                    <prev_mode>  is not able to light up an output that
                    has been disabled with xrandr --off.
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
          Severity: normal
    Classification: Unclassified
                OS: Linux (All)
          Reporter: eich at pdx.freedesktop.org
          Hardware: x86-64 (AMD64)
            Status: NEW
           Version: unspecified
         Component: DRM/Intel
           Product: DRI

Created attachment 78802
  --> https://bugs.freedesktop.org/attachment.cgi?id=78802&action=edit
Proposed Fix.

This issue has been found on a Intel GM45 (0x8086 0x2a42) (among others):

1. Start an Xserver on a system with multiple outputs attached.
2. Ensure all outputs are lit.
3. Disable one output with 'xrandr --output <output_ID> --off
4. Observe this output become blank and enter power saving state.
5. Issue xrandr --output <output_ID> --mode <previous_mode> 

=> One will observe that this output will remain blank.

Investigation results: 

The problem is an asymetric behavior in intel_crtc_set_config(): If a full mode
set is required any encoders in power save state are activated as a side
effect. Otherwise they are simply left in their provious state.
This asymetry in behavior can cause a surprising and unexpected behavior.

The attached patch will fix this problem. There are possibly other ways to fix
the issue - to me this seemed to be the simplest.

Ticket opened to put the patch up for discussion.

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