I'm trying to get my laptop set up with a single desktop spread over three monitors, the laptop screen, plus two external DVI monitors, but the drivers won't let me. I know the hardware can do it because the open-source driver can do it, but then the open-source driver has other issues ...

"lshw" reports the display device as "Whistler XT [AMD Radeon HD 6700M Series]".

When I use the Catalyst Control Centre to configure the layout I want, all appears well, but only two of the three screens actually come on. I can configure any two I want, but not all three at once.

If I set the desired resolution and reboot, I get a message up that says it didn't work because the requested desktop size is larger than the configured maximum (sorry, I didn't write the exact message down). The requested size is 5440x1200, but the maximum is 4???x1200 (again I can't remember the exact size off the top of my head).

I looked into the xorg.conf file, and the smaller size is indeed there. I edited it to what I want, rebooted, and this time I get a different failure message containing the word "CRTC", I think (again, I did not write down the exact message).

Is there some magic setting I should be selecting? Is this a bug in the driver? A limitation?

Any help would be greatly appreciated! 

Oh, I should say, I also tried setting up the monitors with smaller resolutions, in order to reduce the desktop-size, and still no joy.

I went back, tried again, and wrote down the error message.

When I set the config I want, and reboot, the dialog that comes up on the log-in screen says:

"Could not switch the monitor configuration

could not set the configuration for CRTC 149"

When I then login, and  go back into the Catalyst Control Center, and switch to the "Display Manager" view, a dialog pop up that says:

"AMD Catalyst Control Center has automatically configured your system for maximum desktop size.
Press Apply in Catalyst Control Center to accept this configuration or Cancel to revert changes."

When I dismiss the dialog, I see the settings have automatically had the third monitor disabled, and the desktop size reduced. 

It seems like some logic in CCC knows that something is up, but it won't say what?

If I then re-edit the settings back to how I want them, it tries to do it (no reboot required this time), fails, and gives another, different error message:

"Invalid Settings

The current settings cannot be applied. Possible issues may include:
  - Display(s) cannot be enabled.
  - Setting(s) cannot be applied due to insufficient video memory." 

I don't understand this: like I said, the open-source "radeon" driver can drive three monitors, so why can't this driver do it?

Any help much appreciated!

BTW, is there anywhere better to ask this question?

More fiddling shows that I get the same "Possible" issues message even if I try to clone the display too many times.

I think that rules out the possibility of insufficient video memory, no?