Radeon HD 7850 in Kubuntu 16.10

[Mr. Wishes]

I've upgraded from Kubuntu 12.04 (about time!) but I no longer have hardware acceleration working for my Radeon HD 7850 video card. The Xorg log (complete log here) reports "[KMS] drm report modesetting isn't supported.", which may be relevant.

Also:

>cat /proc/cmdline 
BOOT_IMAGE=/boot/vmlinuz-4.8.0-32-generic root=UUID=251248cd-4f73-40b9-a4e8-b44dfab43a60 ro quiet splash vt.handoff=7

so I am not preventing modesetting with cmdline options.

This also might be related:

sudo modprobe radeon
modprobe: ERROR: ../libkmod/libkmod-module.c:832 kmod_module_insert_module() could not find module by name='off'
modprobe: ERROR: could not insert 'off': Unknown symbol in module, or unknown parameter (see dmesg)

And more information:

inxi -G
Graphics:  Card: Advanced Micro Devices [AMD/ATI] Pitcairn PRO [Radeon HD 7850 / R7 265 / R9 270 1024SP]
           Display Server: X.Org 1.18.4 drivers: fbdev,ati (unloaded: vesa,radeon) Resolution: 1920x1200@77.00hz
           GLX Renderer: Gallium 0.4 on llvmpipe (LLVM 3.8, 128 bits) GLX Version: 3.0 Mesa 12.0.3

so it's using the 'ati' driver, whatever that is? And it has abysmal performance and no dual-screen support (it's mirrored across the two monitors).

Does anyone have any suggestions? I've been using kubuntu for about 10 years, but I still get mystified by the low-level interactions between kernel and drivers.

[Mr. Wishes]

I've now booted up a live-USB of 16.10, and the hardware acceleration is fine. Notably, the xorg log has no issues with Kernel Mode Setting. Unfortunately the command line options for the kernel are strange, probably because it's a live-usb, so that doesn't tell me what I should be doing there:

>cat /proc/cmdline                                                                                                                                                                                      
file=/cdrom/preseed/kubuntu.seed boot=casper maybe-ubiquity initrd=/casper/initrd.lz quiet splash --- maybe-ubiquity

However, 'modprobe radeon' doesn't cause any errors, and we have

>inxi -G
Graphics:  Card: Advanced Micro Devices [AMD/ATI] Pitcairn PRO [Radeon HD 7850 / R7 265 / R9 270 1024SP]                                                                                                                         
           Display Server: X.Org 1.18.4 drivers: ati,radeon (unloaded: fbdev,vesa)                                                                                                                                               
           Resolution: 1920x1200@59.95hz, 1920x1200@59.95hz                                                                                                                                                                      
           GLX Renderer: Gallium 0.4 on AMD PITCAIRN (DRM 2.46.0 / 4.8.0-22-generic, LLVM 3.8.1)                                                                                                                                 
           GLX Version: 3.0 Mesa 12.0.3

lsmod now contains a reference to radeon, which I remember it didn't last time.

I'm posting this mostly as a note to myself so I can see what things should be, but I'd rather prefer to fix my existing installation than reinstall.

Could someone please post the output of "cat /proc/cmdline" on their functional, non-livecd, preferably Radeon computer?

[Mr. Wishes]

Turns out there was an old fglrx package still installed - it's not part of the new repositories, but it was still present and wasn't uninstalled by the upgrade. Removing it with apt-get remove made everything work again.