XRandR failing to expose two monitors on single RV280 as separate heads (was: dual-DPU XRandR almost working, DVI-0 stays blank)

Alex Deucher alexdeucher at gmail.com
Tue Feb 23 00:20:44 PST 2010

On Tue, Feb 23, 2010 at 3:06 AM, martin f krafft <madduck at madduck.net> wrote:
> also sprach Corbin Simpson <mostawesomedude at gmail.com> [2010.02.22.2354 +0100]:
>> You'll have to Zaphod the head. Good luck. :3
> I am not sure I understand. Zaphod is multiple Screen entries in the
> ServerLayout, right?
>  Section "ServerLayout"
>      Identifier      "Dual-Head"
>      Screen        0 "ScreenLeft"
>      Screen        1 "ScreenMiddleRight" RightOf "ScreenLeft"
>  EndSection
> If so, then I am already using that to combine two screens for the
> two graphics cards in the system. I previously had three screens
> like this, and each referenced a specific output of the graphics
> card (using Screen lines in the Device sections), but this stopped
> working with KMS[0].
> 0. http://lists.freedesktop.org/archives/xorg/2010-February/049355.html
> ScreenMiddleRight in the above references a single Radeon 9200 Device with
> two monitors connected to its DVI and VGA ports. I can use XRandR to
> place one of them --left-of the other, but the result is a single
> head across both monitors.
> This is curious, because when I connect my laptop (Intel GPU) to an
> external display, and I place LVDS --right-of VGA-1, then the window
> manager sees two heads, not one spread across both displays.

You are mistaken.  When you use xrandr to create a dualhead layout,
it's still only one X screen.  The size of the desktop just changes.

> It seems to me that I need to tell the radeon driver that it should
> present the two displays as separate heads. Apparently, this is not
> needed for the intel driver.

The two drivers behave the same way in a single card, multiple output
setup.  The radeon driver will behave the same way if you run it on
one card with all the zaphod stuff removed.  If you enable the zaphod
stuff, you can't use xrandr to configure things across screens.


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