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Old 11-27-11, 05:08 AM   #1
gregfr
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Default How does twinview split the 2 screens?

I'm running into a weird problem. I was running 4 displays using xinerama, but it was too slow, so here is what I did:

1 - I configured 2 screens using twinview and disabled the other 2

Works ok: ie as you would expect, if I expand a window it grows to fill ONE screen. xrandr however sees only 1 large screen spanning both physical screens.

2 - to this config I add a 3rd screen, as "separate"

It works BUT the first 2 screens are now merged into one, ie a window expands to fill BOTH. xrandr sees exactly the same.

How comes in the first config works ok and the second doesn't? Nothing changes in the xorg.conf (except of course enable the 3rd monitor).
How can I fix the expanding behavior in the second config?

TIA

greg
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Old 11-27-11, 06:45 PM   #2
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Default Re: How does twinview split the 2 screens?

I might be wrong here but IIRC I've read on forums infos about the described behavior being "feature" instead of a "bug". TwinView - as the name suggests - initially was targeted at supporting dual-monitor setups and thus xinerama info it reports to X clients is limited to 2 monitors only (hint: it's only my guess). You should try to use "override xinerama info" driver feature and manually fill-in your monitors config into xorg - there were reports that using this trick people had been able to use quad monitor setups with dual-head videocards with help of a special DVI->2xDVI splitters.
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Old 11-28-11, 02:23 AM   #3
AaronP
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Default Re: How does twinview split the 2 screens?

lexa2 is on the right track, but the situation is actually slightly different: the Xinerama protocol extension can only describe a single logical X screen to clients. When TwinView is enabled, it sends its own Xinerama protocol information to clients when there's a single X screen, telling them where the two displays are located within the single protocol X screen. However, when you add the third physical display, now you have two logical X screens (one for the TwinView pair of displays, and one for the third display). Since Xinerama can't handle that situation, the driver has to simply tell clients that Xinerama is disabled and that no layout information is available.
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Old 11-28-11, 06:48 AM   #4
gregfr
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Default Re: How does twinview split the 2 screens?

I didn't activate xinerama, because it was my understanding that xinerama was incompatible with compositing and xrandr. I have the following options in my /etc/X11/xorg.conf:

Option "Xinerama" "0"

in server layout, and later in screen:

Option "TwinViewXineramaInfoOrder" "DFP-0"

is it possible that the driver activates xinerama nevertheless? and in a compiz-compatible mode? if so, is xinerama necessary to "split" the twinview logical screen? is there any other way to inform the clients so they behave like expected?

I gave up the xinerama-based 4 screens configuration because it was making the computer uselessly slow, I was hoping "2 twinviews" configuration would work better.

Thanks for your answers

greg
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Old 12-09-11, 03:34 AM   #5
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Default Re: How does twinview split the 2 screens?

no one ?
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Old 12-09-11, 09:05 PM   #6
AaronP
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Default Re: How does twinview split the 2 screens?

There are two separate things called Xinerama that are easily confused:
1. Code in the X server to make separate X screens appear as a single logical X screen to clients.
2. An X11 protocol extension for communicating the layout of the physical displays to X clients.

When there's only one logical X screen, TwinView uses #2 to tell X clients where the displays are, similarly to how the server would if #1 were enabled. The problem is with #2: the protocol itself simply has no way to tell clients about more than one logical X screen. The option in xorg.conf only controls #1.
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Old 12-10-11, 06:05 PM   #7
gregfr
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Default Re: How does twinview split the 2 screens?

Thanks for the clarification, it makes more sense now!
Knowing that, is there any way to do one of these:

- either not use xinerama, have 2 twinview screens, and make the windows still magnify correctly?
- or have 2 sessions, one on each twinview screen (having a root session open could be usefull for me)?

TIA

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Old 12-11-11, 11:52 AM   #8
AaronP
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Default Re: How does twinview split the 2 screens?

I'm afraid not. The Xinerama extension really needs to be extended to support more than one X screen, but no one has gotten around to writing such an extension.
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