[Remarks]

So, there are a couple of Radeon monitoring/tweaking tools available for Linux (aticonfig, AMDOverdriveCtrl, glakkeclock). Unfortunately neither of them supports displaying VRM temperatures or core voltage modification (the later can also be achieved by editing your Radeon's bios using a Windows application, but that's not really convenient now, is it?).

As such, I started hacking and came up with a utility of my own. It displays VRM voltages, average current and allows you view and modify the GPU core voltage.

I've tested it on my (single) ATI card, an Asus HD5850 (reference).

Remarks
- Should work on all reference HD5850 cards with a similar Volterra VT1165 VRM setup.
- It should also support multiple cards, but I haven't been able to test it myself.
- Accesses the Radeon i2c bus by mapping the Radeon i2c controller registers via /dev/mem, thus root is required (anyone have a better idea here?).
- Comes without any warranty, use at your own risk, make sure you know what you're doing, etc.
- May even burn your house down. Probably not, though.

Download
Source code
Github

Compiling
Depending on your distro, you may need to install the pciutils development package (Ubuntu/Debian: apt-get install libpci-dev).

Code:

wget https://github.com/ius/radeonvolt/tarball/master -O - | tar xz
cd ius-radeonvolt*
make

Usage examples

Code:

$ sudo ./radeonvolt

Device [1]: Cypress [Radeon HD 5800 Series]
Current core voltage: 1.0875 V
Presets: 1.0000 / 1.0375 / 1.0875 / 0.9500 V
Core power draw: 62.71 A (68.20 W)
VRM temperatures: 100 / 99 / 98 C
 info[/b]

Before attempting to modify the vcore, make sure the values for the 'current voltage' as well as 'presets' look sane.

Code:

$ sudo ./radeonvolt --vcore 1.1000 --device 1
Setting vddc of device 1 to 1.1000 V (0x34)

Device [1]: Cypress [Radeon HD 5800 Series]
Current core voltage: 1.1000 V
Presets: 1.0000 / 1.0375 / 1.1000 / 0.9500 V
Core power draw: 61.84 A (68.02 W)
VRM temperatures: 100 / 99 / 98 C

Please let me know if it works for you (especially non-5850 or multiple cards). If it doesn't, include the output of lspci -vd1002.

If it does work, feel free to send any spare coins to 19kdfgW1KXQgV7SCLEPAojtHxN9xotGkGH.