On the Intel driver we've developed and reviewed some patches which enable GPU frequency settings in sysfs. Similar interfaces have existed for some time in our debugfs, but now we're getting interest from users like PowerTOP and have decided to formalize it. Most likely other DRM drivers have similar features; and perhaps similar proposals have gone out in the past.
Boiling down what we've exposed in our patches: read current GPU frequency read/write min frequency read/write max frequency read HW support lowest frequency (we call this RPn) read HW supported highest frequency (we call this RP0) read HW recommended low frequnecy (we call this RP1)
If no interest is perceived we will move forward with these as Intel only sysfs interfaces.
The Intel specific patches which are ready to go are here: http://lists.freedesktop.org/archives/intel-gfx/2012-September/020342.html
Thanks.