Hi, on the following:
Advanced Micro Devices, Inc. [AMD/ATI] RV770/M98L [Mobility Radeon HD 4850]
 on a 2009 imac running kubuntu 18.04 and kde 16.04 and kernel:
4.15.0-29-generic
I'm having no luck obtaining anything but the vesa driver.
The addition of nomodeset to the kernel parameters (booting from refind with no
grub) is necessary to get a graphical desktop.  If I leave modesetting on, the
screen blacks out as the mode is changed.  If I turn it off I get the vesa but
an error is reported that there is no ums available in the radeon module.
This has occurred on kde neon clean install and kubuntu.

Many thanks.