DRM_*MEMORYBARRIER we had were related to an MMIO space. This means
arch-specific code on the BSDs, unfortunately. Also add
DRM_MEMORYBARRIER() and change the DRM_READMEMORYBARRIER()s that used
to be read/write barriers to it.
use correct address for ring read pointer writeback (yes, we seem to have
been running with bogus values for the ring read pointer, which
'worked' because the return value of radeon_wait_ring() is never
checked and the ring usually never fills up)
pageflipping after a mode switch
take current page into account in AdjustFrame(); writing the CRTC offset
via the CP was probably a bad idea as this can happen asynchronously,
reverted
take frame offset into account when flipping pages
handle CRTC2 as well for pageflipping (untested)
preserve GEN_INT_CNTL on mode switches to prevent interrupts from getting
disabled