X won't start: radeon

[Steven Bellovin <smb%cs.columbia.edu@localhost>]

Using today's -current, I can't start X on a Dell Dimension 8400.  If  
I run

        X -configure

it prints a list of drivers and then says "Number of created screens  
does not match number of detected devices".    Another machine of the  
same model, using -current from June 24, works properly, with the  
radeon driver.  I saw another thread that described the need for

        PCI_ALLOW_MMAP

in the kernel, albeit to fix different symptoms.  Is that what I  
should try?