RandR questions

Alex Deucher alexdeucher at gmail.com
Tue Nov 2 05:43:28 PDT 2010

On Tue, Nov 2, 2010 at 3:01 AM, Pedro DeKeratry <pdekeratry at gmail.com> wrote:
> First let me describe the behavior that prompted my questions. This is
> on a laptop running Ubuntu Desktop 10.10 and the xorg.conf is
> configured to run a mutli-display using the external HDMI and external
> VGA ports, thus the laptop screen is blank/off. If I unplug the HDMI
> connection the system does some display switching and my laptop screen
> turns on. When I plug the HDMI connection back in nothing happens. A
> couple of xrandr commands later and I can get the HDMI output
> displaying how it was originally. Suppose though that instead of
> issuing the xrandr commands to bring the external HDMI connection back
> up after plugging it in, I reboot the machine instead. Since my
> /etc/xorg.conf is unchanged I would expect that both my external
> monitors come up, however, the laptop screen comes on instead of my
> HDMI external connection which is now shown as disconnected. To get
> things back the way they were I can either used xrandr like previously
> or the ATI gfx menu options. Note that this only happens with regards
> to my HDMI connection because I think the laptop screen and the HDMI
> share the TMDS graphics hardware ( Assuming my understanding of these
> things is correct ; ) .) Unplugging the VGA doesn't create any auto
> switching response.

Your laptop screen and hdmi port are likely using separate encoders,
but you only have 2 display controllers so you can only use two
displays at a time.  Digital connectors (DVI, HDMI, DP) have a hot
plug pin that can generate an interrupt when the monitor is connected
or disconnected, but older analog monitors (VGA, TV) do not.

> So, with that said:
> Is is xrandr that does the auto switching from ext. HDMI to laptop
> automatically when HDMI monitor signal is lost? Or is that the gfx
> drivers or some other X program? ( I'd like to disable it if possible
> )

When a connect/disconnect interrupt is generated the drm sends an
event to userspace which can then do something with the event.  In
your case I think it just runs 'xrandr --auto' when it receives the
event, but you can have it do whatever you want.

> Is it xrandr that is saving some kind of persistent configuration
> settings somewhere that overrides my xorg.conf file at the next
> reboot? I couldn't find any sort of conf file anywhere related to
> this. Googling xrandr info doesn't show much except same man pages.

randr does not save any persistent state.  if you want to force a
particular setup, you need to specify it in your xorg.conf or via
xrandr commands in your desktop startup scripts.

> Is xrandr scheduled to replace xorg.conf altogether? I've noticed that
> my xorg.conf really is pretty much as minimal as you can get. In
> previous Linux systems I've had much more intricate xorg.conf files
> with a lot more details filled in. Other than loading the driver for
> the gfx card, it seems like everything else can be pretty much done
> through xrandr. Am I understanding correctly where xrandr is headed in
> the Linux/X world?

xrandr is just a utility to dynamically reconfigure your displays.
xorg.conf is for specifying specific settings.  See this page for info
an using xrandr and specifying display settings in your xorg.conf:


> --Pedro
> On Sat, Oct 30, 2010 at 4:03 AM, Jeremy Huddleston
> <jeremyhu at freedesktop.org> wrote:
>> This would be a good place...
>> On Oct 29, 2010, at 21:07, Pedro DeKeratry wrote:
>>> Hi everyone,
>>> Is this the appropriate place to ask questions about the xrandr
>>> command line utility in order to understand how it interacts with my
>>> system environment at large or is such a question better suited to a
>>> distro specific mailing list?
>>> --Pedro
>>> _______________________________________________
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