Installed Kernel 4.0-rc3, and the problem with the Radeon R600 driver is now fixed.   Brief details of changes since -rc2 are here: http://lkml.iu.edu/hypermail/linux/kernel/1503.1/00218.html

As with the previous -rc versions, the patched NVIDIA (346.47) and VMware (11.1) drivers compile and load/run OK..

# uname -a
Linux rg6830l 4.0.0-rc3 #1 SMP Mon Mar 9 10:57:55 PDT 2015 x86_64 x86_64 x86_64 GNU/Linux

Robert Gadsdon.  March 9, 2015.