What is the best way of monitoring GPU temperature in Xubuntu 14.04 when using the OpenSource driver?

I had a severe problem with GPU temperature in 12.04 and even later, and even if that seems gone in 14.04, I want to keep an eye on my GPU.

Is there a program that would display that, in the xfce-panel or otherwise?

EDIT:

After installing lm-sensors and the Xfce panel Sensors Plugin, I can see some temperatures

but not the GPU. I use the Ubuntu radeon driver.

Also, I have installed psensor: but no GPU

Also, running sensors in Terminal that info is not available:

EDIT2:

At this Psensor page it reads: When the OpenSource ATI driver is used, the monitoring information is available throw lm-sensors ....

I have installed lm-sensors and following this advice I ran sensors-detect and answered yes to all questions there. In Psensor window now there is a new entry, and it's for radeon, but it's always zero.

Editing the names so as to display the chip name, it looks like so:

sensors command gives the same info as above.

EDIT3

Also:

~$ sudo cat /sys/kernel/debug/vgaswitcheroo/switch
[sudo] password for cip: 
0:IGD:+:Pwr:0000:00:02.0
1:DIS: :DynOff:0000:01:00.0

EDIT4:

~$ xrandr --setprovideroffloadsink 0x6c 0x44 X Error of failed request:  BadValue (integer parameter out of range for operation)
  Major opcode of failed request:  140 (RANDR)
  Minor opcode of failed request:  34 ()
  Value in failed request:  0x6c
  Serial number of failed request:  17
  Current serial number in output stream:  18

I use xfce4-sensors-plugin:

It needs lm-sensors installed and configured, and it is in the standard (universe) repository.

The ATI sensor (when used with the radeon open source driver) should be called something like radeon-pci-0100 --- Notice however that with recent kernels the card is kept off when not used, so the sensor is not read in that conditions --- see https://askubuntu.com/a/469439/16395

You can check if the sensors are working activating the offloading as explained in this post: http://xpressrazor.wordpress.com/2013/10/08/enable-and-use-open-source-radeon-drivers-in-a-muxless-hybrid-graphics-intelamd-setup/:

1. List the graphic providers:

   xrandr --listproviders
   

   make a note of the id hexadecimal codes

2. Activate them:

   xrandr --setprovideroffloadsink 0x53 0x79 
   

   (you have to substitute the codes of the Intel and ATI providers, in that order)

3. Open a terminal window and run

   DRI_PRIME=1 glxgears -info 
   

Now using sensors in another window you should see the temperature reading of the radeon card going up.

After doing all those actions that are presented in the question and also after updating Psensor to version 1.1.2 the Radeon temperature appeared there after reboot.

sudo add-apt-repository ppa:jfi/psensor-unstable
sudo apt-get update
sudo apt-get install psensor

More here: Psensor Updated With Option To Display Temperature Sensors On The Panel.

UPDATE:

The radeon info appeared in Psensor only after reboot if set to launch on startup. Also, it is odd that if Psensor is closed and restarted without reboot, the radeon entry is absent again.

In fact the ATI card is not working at that moment as this answer explains: New kernel 3.13 has an advanced dynamic power management of the ATI card with the radeon driver. In normal conditions the ATI discrete is off.

Both Psensor and Xfce Sensor plugin will display the radeon temperature at startup if they are open but if they are restarted the entry will be missing in Psensor, will be zero in the Xfce panel plugin and will be N/A when running sensors.

To test this, and start the ATI card, that answer says to run glxgears:

DRI_PRIME=1 glxgears -info

What happens then is that the Xfce panel plugin will show the GPU temperature, while psensor will not until restarted. sensors command will display the temperature of the ATI card.

If glxgears is shut down, the discrete card will stop. Then, Psensor will continue to show the last GPU temperature until restarted. The panel plugin too, but not for long. In its properties, radeon temp is already zero, uncheck and recheck to refresh.