As I understand it, the situation is this: the 945GM hardware has two
'output drivers' ("CRTCs") for driving displays. When xrandr runs, it
asks the driver for information on all connected displays, so the driver
has to disconnect one of its CRTCs from your external monitor, and
temporarily hook it up to the other available outputs in turn to see if
there's a display connected to any of them. Once it's done, it hooks the
CRTC back up to your external monitor and your display returns.
I'm using a MacBook with a 945GM chip, and I have four available outputs
- LVDS (the internal panel), TMDS-1 (external DVI), VGA (external VGA)
and TV. Of those, I'm always using LVDS and TMDS-1, and the VGA
connector can detect that it's disconnected without having to start up a
CRTC, therefore when I run xrandr on a stock install, my screen flashes
black while it probes the TV output.
I don't have a TV connected, and I never intend to, so what I did was
modify my xorg.conf to tell it to pretend there's no TV output at all.
First, I added a new top-level section:
Section "Monitor"
Identifier "DisabledMonitor"
Option "Ignore" "true"
EndSection
The "Ignore" option means that any output connected to this monitor will
be ignored.
Then, I added an option to the "Device" section:
Option "Monitor-TV" "DisabledMonitor"
This connects the TV output to the output-disabling monitor section
above.
After making this change and logging out, running xrandr no longer
causes black flashes (incidentally, black flashes caused by programs
such as Totem that used XVideo are also gone).
--
[i945gm] querying driver for supported resolutions makes screen flash
https://bugs.launchpad.net/bugs/268387
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