AMD's Alex Deucher has just published a patch that now exports the internal thermal monitoring sensor through the sysfs interface. This patch makes it possible to monitor the internal GPU temperature on R600 (Radeon HD 2000/3000), R700 (Radeon HD 4000), and Evergreen (Radeon HD 5000) series graphics cards. This has been a feature we have been waiting on for quite a while to see how the thermal performance compares between the open-source ATI Linux graphics driver stack and the proprietary Catalyst driver, but it should be useful for normal users too in ensuring their graphics processors are not overheating especially for those with passively-cooled graphics cards.
This patch, however, will not work for 100% of the R600/R700/Evergreen GPUs as not all OEMs support the actual internal sensor. Most of the Mobility Radeon ASICs also implement a separate ACPI-based thermal monitoring mechanism.
The patch that provides the R600/R700/Evergreen thermal monitoring support to the Radeon DRM KMS code can be found on the DRI development list.