I successfully installed Bionic Beaver, working around the KMS problem with "nomodeset" appended to the GRUB2 options when installing. However, or perhaps as a consequence, Ubuntu installed the graphics system as this, without 3D acceleration:
glxinfo | grep OpenGL gives:
OpenGL renderer string: llvmpipe (LLVM 6.0, 128 bits)
lspci | grep -E 'VGA|Display' gives:
01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] RV730/M96-XT [Mobility Radeon HD 4670]
...which according to other info available here is well supported by the "radeon" driver built into the kernel.
If I look in the Gnome system for additional drivers it says "no additional drivers available", which is fine, since the kernel should handle this anyway. But how do I switch to the "radeon" driver? Of course I should not fall back into the black screen problem caused by unspported KMS...
Some articles address parts of this problem, but most are too different, addressing issues in Unity and older versions of Ubuntu.