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
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).
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).