Tried to launch vdpau-accelerated mpv with fglrx driver and mmiotrace.
Interesting, the CG_UPLL_FUNC_CNTL always has flags 0x100 | 0x600

R 4 456.556542 1 0xf0000634 0x707 0x0 0
W 4 456.556547 1 0xf0000634 0x705 0x0 0
R 4 456.556553 1 0xf0000634 0x705 0x0 0
W 4 456.556576 1 0xf0000634 0x705 0x0 0
R 4 456.556688 1 0xf0000634 0x705 0x0 0
W 4 456.556693 1 0xf0000634 0x705 0x0 0
R 4 456.556709 1 0xf0000634 0x705 0x0 0
R 4 456.556725 1 0xf0000634 0x705 0x0 0
W 4 456.556730 1 0xf0000634 0x705 0x0 0
R 4 456.556771 1 0xf0000634 0x705 0x0 0
W 4 456.556776 1 0xf0000634 0x704 0x0 0
R 4 456.556837 1 0xf0000634 0x704 0x0 0
W 4 456.556842 1 0xf0000634 0x700 0x0 0
W 4 456.556847 1 0xf0000634 0x708 0x0 0

Any ideas what can 0x100 mask be for?