Comment 89 for bug 542660

Posted by Peter, on this page : https://bugs.launchpad.net/ubuntu/+source/linux/+bug/597070 hope it can helps

Now using ubuntu natty 11.04 Alpha3 (was using maverick 10.10).

I have established that my main physical screen (iMac display) is detected by the OS radeon driver as output eDP-0, name 'Color LCD', and it is connected but not lit. The external physical screen is detected as output DisplayPort-0, name 'L1710B', and is connected *and* is lit.
Both these outputs are connected to logical (X) screen 0 and they both fit into the viewport of 1920x1080.

If I let KMS set up the graphics it sets DisplayPort-0 to have resolution 1920x1080 (it should be 1280x1024). The kernel modesetting seems to get confused between the two display ports, using the resolution of eDP-0 and assigning it to DisplayPort-0, but then disabling the eDP-0 for some reason.

If I use 'nomodeset' at boot then DisplayPort-0 gets resolution 1280x1024 (correct). This is probably the result of radeon driver probe.

Since it is possible to show terminal output on the iMac display before X starts I suspect that X (or the radeon driver) switches off this output as not being usable.

To me the question is why does the eDP-0 output get switched off? The logs do not offer much except

...
(EE) Radeon(0): channel eq failed: 5 tries
(EE) Radeon(0): channel eq failed
(EE) Radeon(0): clock recovery reached max voltage
(EE) Radeon(0): clock recovery failed
....

perhaps the radeon driver fails to establish the clock frequency for the monitor (iMac display) and switches it off as a precaution against damage?

Earlier I used the ati proprietary driver on ubuntu 10.10 and that worked when driving the iMac display, so it is possible, just not with the OS radeon driver (yet).