I keep running into this annoying bug where the X server completely locks up if I'm running mergedfb on an r200 card, a GL xscreensaver hack starts up on the 2nd head, and the total resolution of the two heads exceeds 2048. It doesn't happen on r300, doesn't happen if the GL hack is on the primary head, and doesn't happen if the width of both heads is 2048 or less. I'm currently using Xorg 7.0, with DRI from Mesa CVS (as of last week), though I seem to remember seeing this bug over a year ago, as well. Previously, it would lock up the entire machine, but now I can at least ssh in and reboot the machine.
*** Bug 6875 has been marked as a duplicate of this bug. ***
This was fixed somewhere along the way :-)
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