For a couple days I’ve been looking at a bug in RHEL 5 where the VESA driver is unable to drive a Dell LCD panel connected to a Dell Optiplex 745 using a rather new ATI X1300 card. I was getting bad resolutions, X starting once and then failing to restart until the machine is rebooted, and lots of errors with “no screens found.” On my 745 in my office running in x86_64 mode I could not duplicate the problem even when using the identical monitor and configuration.
Turns out there is a bug that effects how the X server gets the DDC inforamtion about the monitor. This is overerly well documented as #10238 in Freedesktop.org’s Bugzill and as #236416 in Red Hat’s Bugzilla. All non-i386 arches and virtual machines (Xen) use emulation to get the DDC information. Turns out using the same emulation works in this case for i386 RHEL 5 machines.
To work around the bug edit your xorg.conf file and add the following to the Server Layout section:
Option "Int10Backend" "x86emu"