[Bug 48880] Set mode has different timings than requested on VGA

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=48880

--- Comment #15 from Luc Verhaegen <libv at skynet.be> 2012-04-19 06:45:19 PDT ---
Alex, i thi(In reply to comment #14)
> 
> As I said before some monitors are really picky about the clocks they get.  The
> pixel clock is generated from a reference clock and a set of dividers.  In some
> cases it's not possible to get exactly the pixel clock of the mode, so the
> driver gets as close as possible.  The two clocks produced are 148.43 Mhz and
> 148.57 Mhz.  Both are within 0.07 Mhz of the actual clock.  Some monitors don't
> like clocks that are slightly lower, others don't like clocks that are slightly
> higher, others don't care.  In some cases you even have monitors that don't
> like certain divider combinations that produce the exact same pixel clock.  As
> you can see from comment 11, even your own monitor is happy and not happy with
> the same pixel clock depending on the connection.
> 
> I'd be leery of changing the pll flags without a lot of thorough testing since
> these change may break certain modes on other monitors.  Did you try
> RADEON_PLL_USE_FRAC_FB_DIV?  It should help the driver get closer to the actual
> pixel clock by allowing a finer grained fb divider, but once again, some
> monitors are picky about certain divider combinations.  I'd be more inclined to
> add this flag than the MINM_OVER_MAXP flag however.

Err, Alex, i think that it is the display engine, for a particular version and
process, that has issues with certain divider combinations which should
theoretically produce the same pixel clock, not the monitor.

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