https://bugs.freedesktop.org/show_bug.cgi?id=32931
--- Comment #7 from Alex Deucher agd5f@yahoo.com 2011-01-09 13:11:03 PST --- (In reply to comment #6)
ok so why does the low power profile work fine on rv790 but not on barts? on barts, low seems to be "too low", e.g. it takes about 3 seconds to expand and draw a dolphin window in kde. clicking on a tab in firefox seems to take about 3/4 of a second to appear. it takes 1-1.5 seconds to show the kde menu. sometimes I reclick on things because I think nothing happened. the desktop response is very jerky, reminds me of graphics from over 10 years ago.
The power modes are specified by oems and vary from card to card. Your rv790 likely has higher clocks in it's low power mode. The number of active displays also plays a factor in the clocks that are selected; e.g., if you have 2 displays the clocks are generally higher than with one display.