More specifically...
PPLL_REF_DIV holds the reference divider for the pixel clock PLL
(though the register has a weird format on >= r300)
PPLL_DIV_{0..3} holds the forward and feedback dividers for that PLL,
(feedback divider in bits 0..10 and post div in bits 16..18)
the CLK_CNTL_INDEX bits 8..9 hold the selector for which PPPL_DIV_{0..3}
register holds the current value to use (also called ppll_div_sel)
Ben.