"Last week we reported that the open-source ATI Linux driver had
picked up improved power management in the form of dynamic power
management and power management profiles that can be defined by the
end-user. With the ATI Linux power management finally coming to
fruition within the Linux kernel for its kernel mode-setting / DRM
driver, we have decided to take a close look at how this power
management support is working in the real world.
"Read last week's news post for more details on the work that
has gone into the open-source ATI Linux power management support up
to this point, as it's been a long-time in the making. To
summarize, the patches posted by Alex Deucher of AMD last week
clean-up the in-kernel ATI Radeon power management support for all
ASICs and introduces these new power control options that are
currently exposed to the end-user through a sysfs interface.
"The dynamic power management option introduced (via writing
"dynpm" to the sysfs power_method node) will dynamically adjust the
GPU's clock depending upon the graphics processor's load. This load
determination is being done by looking at the number of pending GPU
fences for the R500 (Radeon X1000) series and older. A fence on the
graphics processor signals when the GPU has executed all
instructions before it in the FIFO queue. In other words, the more
fences that are pending, the more work the GPU still has to finish.
For the Radeon HD 2000 (R600) series and newer there is a GUI idle
IRQ interrupt support, but that is not working reliably for the
R500 and older ASICs."