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Apparently writing the DPLL register P1/P2 divider fields won't trigger
an actual change in the DPLL output unless VGA mode is enabled for
prior to the register write that changes the P1/P2 dividers. The write
with the new P1/P2 divider can itself disable VGA mode again without
problems.
I tested the behaviour on my 946GZ, and when manually frobbing the
register with the display on, the behaviour is very clear. However I
can't explain why this machine actually works. The P1/P2 divider
changes caused by normal modesets do seem to make it through to the
hardware somehow since I get a stable picture on the monitor with
any resolution. Maybe it's the "three times for luck" stuff that
somehow masks the problem, or something.
But apparently there are machines (eg. Nick Bowler's G45) where that
isn't the case and we fail to get the correct clock from the DPLL.
Things used to work because we enabled VGA mode for disabled DPLLs,
so when re-enabling the DPLL VGA mode was enabled just prior to the
first register write, and hence the P1/P2 change went through without
a hitch. That got changed in
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| Makefile | ||