How do I extend my desktop to my 2nd monitor rather than just cloning it?

Question

I have just installed Debian Wheezy for the first time and have had mixed success getting my system up and running. Currently my main issue is that I am unable to extend my desktop to my second screen.

I have an ATI radeon HD 7700 series graphics card connected to 2 displays. Running lspci results in this line, among others:

04:00.0 VGA compatible controller: Advanced Micro Devices [AMD] nee ATI Cape Verde [Radeon HD 7700 Series]

Currently, both are cloned. I initially tried to follow the instructions for installing proprietary ATI drivers, which resulted in the 2nd display being detected but I was unable to extend the desktop rather than clone it (something about my virtual screen not being big enough).

I gathered that support might be better for the free version so I followed these instructions for removing said drivers, followed by these instructions to install the free ones.

As per the troubleshooting step in that page I ran this command:

dmesg | grep -E 'drm|radeon' | grep -iE 'firmware|microcode'

which produced this output:

[    4.925773] [drm] Loading VERDE Microcode  
[    4.990158] platform radeon_cp.0: firmware: agent loaded radeon/VERDE_pfp.bin into memory 
[    5.152647] platform radeon_cp.0: firmware: agent loaded radeon/VERDE_me.bin into memory 
[    5.236165] platform radeon_cp.0: firmware: agent loaded radeon/VERDE_ce.bin into memory [    5.260082] platform radeon_cp.0: firmware: agent loaded radeon/VERDE_rlc.bin into memory 
[    5.376566] platform radeon_cp.0: firmware: agent loaded radeon/VERDE_mc.bin into memory

That's different to the outputs on that page but then they don't really give anymore information so I just assumed everything had worked.

Anywho, now when I go to System Tools -> Preferences -> System Settings -> Display I just see a single display called Unknown, which clones across both of my monitors.

running xrandr -q produces this output:

xrandr: Failed to get size of gamma for output default Screen 0: minimum 1680 x 1050, current 1680 x 1050, maximum 1680 x 1050 default connected 1680x1050+0+0 0mm x 0mm 1680x1050 0.0*

FYI 1680x1050 is the native resolution of both monitors. I do not have a /etc/X11/xorg.conf file, and my /etc/X11/xorg.conf.d/ directory seems to be empty.

This is my first time running a linux system so I'm totally confused and would appreciate kind words and idiot-proof guidance.

Answer 1

I found this answer on AU in a Q&A titled: How can I set up dual monitor display with ATI driver?.

excerpt

1. Open a terminal and type:

     $ gksudo gedit /etc/X11/xorg.conf
   

2. In the sub-section "display" add this code or modify if already exist:

     virtual 2880 1024
   

3. Where 2880 and 1024 are the value returned by the error: required virtual size does not fit available size: requested=(2880, 1024), minimum=(320, 200), maximum=(1600, 1600).

4. Restart the computer.

5. Then you will be able to extend your desktop without issue.

In the OP's configuration he opted to use this:

virtual 3360 1050