• Trisquel 6.0
• Linux-libre kernel 3.2.0-49-generic, 3.10.5-gnu
• xserver 1.11.4
• mesa 9.1.6
• radeon 7.1.0
• vesa 2.3.2
• No proprietary firmware used.

Radeon driver would not work without the proprietary firmwares from normal Linux kernel. Ubuntu 13.04 worked out-of-the-box, while free distributions like Trisquel automatically fallback to VESA driver. As a result, everything 2D sort of worked but at a very slow pace. Screen resolution could not be correctly detected, limited to 4:3 ratio ones only. 3D acceleration did not work.

lspci -vmmnn

Slot:	05:00.0
Class:	VGA compatible controller [0300]
Vendor:	Advanced Micro Devices [AMD] nee ATI [1002]
Device:	Turks [FirePro V4900] [6749]
SVendor:	Advanced Micro Devices [AMD] nee ATI [1002]
SDevice:	Device [2b06]
PhySlot:	1

lspci -nnk

05:00.0 VGA compatible controller [0300]: Advanced Micro Devices [AMD] nee ATI Turks [FirePro V4900] [1002:6749]
	Subsystem: Advanced Micro Devices [AMD] nee ATI Device [1002:2b06]
	Kernel modules: radeon