After changing VTs to and from the xserver VBlanks are left enabled even if there is not dri app running. Starting and stopping a dri app will disable them again. It seems that RADEONDRISetVBlankInterrupt (pScrn, TRUE) is always called in RADEONEnterVT.
Created attachment 13049 [details] [review]
Testing on my hardware showed (Radeon Mobility M7 LW AGP) showed that enabling vblanks is not necessary, if a dri app was open during a vt switch it will call RADEONDRISetVBlankInterrupt when it is restored after the VT switch anyways. Furthermore, I found that after returning to xserver, even if RADEONDRISetVBlankInterrupt had never been called that vblanks were enabled as radeon@pci:0000:01:00.0 was generating 60 interrupts / second. It seems that RADEONDRISetVBlankInterrupt (pScrn, FALSE) can successfully be called in RADEONEnterVT, disabling vblanks and saving power while not effecting dri apps that may have been open, but of course I have no idea if this is true on other setups.
*** Bug 13613 has been marked as a duplicate of this bug. ***
fixed in commit: 6ccf5b33d27218ae1c45ab93c122438ed536d8ba