[Bug 76564] [AMD Fusion E-350] HDMI refresh rates doesn't match expectations

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=76564

--- Comment #28 from jeroen <jeroenk61 at hotmail.com> ---
(In reply to comment #27)
> (In reply to comment #26)
> > (In reply to comment #25)
> > > (In reply to comment #24)
> > > > (In reply to comment #23)
> > > > The problem is that the frequencys are exact enough so that the display
> > > > device (Monitor/TV/Whatever) accepts them, but not 100% precise.
> > > > 
> > > > E.g. for the 50Hz mode we wanted 148.5MHz pixel clock, but got 148.75Mhz
> > > > instead. And for the 24Hz mode we wanted 74.2MHz but got 74.0625Mhz instead.
> > > > 
> > > > So as Alex said somebody would need to dig into that and try to improve the
> > > > numbers without toasting the hardware.
> > > 
> > > So that would mean for example using fb=29.7   Ref=2   post=10?
> > > 
> > > Or would that fry the hardware?
> > 
> > That should work.  You aren't likely to fry the hw.  You just don't want to
> > set a 400 Mhz clock as you monitor properly won't like it.  The hard part is
> > adjusting the algorithm to reliably calculate a good value for a wide range
> > of clocks.
> 
> I'm not sure if those values would work. A post divider of 10 might result
> in a to high VCO and that could indeed damage the hardware (even if that's
> rather unlikely).
> 
> Essentially the target clock multiplied with the post divider must be in a
> certain range. I think between pll->pll_out_max and pll->pll_out_min.
> 
> I think the problem is that we don't try to choose a good value to match the
> target frequency as close as possible in avivo_get_post_div, but just a
> value that either matches the maximum or minimum VCO frequency.

Perhaps before somebody is going to modify the algorithm, it is a good idea to
verify that this is indeed the problem.
If this is the problem why don't more people have these problems?

If I know which values for fb,ref and post to use for 23.976fps I can hard code
these In a patch and test if it indeed works.

So which values are save to get a clock of 74.2MHz?

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