[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 #26 from Alex Deucher <agd5f at yahoo.com> ---
(In reply to comment #25)
> (In reply to comment #24)
> > (In reply to comment #23)
> > > (In reply to comment #21)
> > > > (In reply to comment #20)
> > > > >  
> > > > > When I look at the xrandr output I wonder if the reference frequency is not
> > > > > 75MHz for fgrlx? Can the reference even change is this not fixed by the
> > > > > hardware?
> > > > 
> > > > As far as I know, it's fixed.  I'm not really sure what fglrx is doing. 
> > > > Anyway, it's probably easier to just fix the open source driver.
> > > > 
> > > > the modes are:
> > > >   1920x1080 (0x55)  148.5MHz +HSync +VSync *current +preferred
> > > >         h: width  1920 start 2448 end 2492 total 2640 skew    0 clock  
> > > > 56.2KHz
> > > >         v: height 1080 start 1084 end 1089 total 1125           clock  
> > > > 50.0Hz
> > > > 
> > > >   1920x1080 (0x5a)   74.2MHz +HSync +VSync
> > > >         h: width  1920 start 2558 end 2602 total 2750 skew    0 clock  
> > > > 27.0KHz
> > > >         v: height 1080 start 1084 end 1089 total 1125           clock  
> > > > 24.0Hz
> > > > 
> > > > and the driver ends up calculating the dividers as such:
> > > > 
> > > > for 148.5MHz target clock:
> > > > (100Mhz * 23.8) / (2 * 8) = 148.75Mhz
> > > > 
> > > > for 74.2MHz target clock:
> > > > (100Mhz * 23.7) / (2 * 16) = 74.0625Mhz
> > > > 
> > > > One would need to tweak radeon_compute_pll_avivo() in radeon_display.c to
> > > > try and get dividers that are closer to the target clock.
> > > 
> > > Isn't that what the OSS driver is currently doing? If you look in the post
> > > history those are the exact values that are currently being used
> > 
> > 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.

> Why must it exactly match?

You want to the clock to accurately match what userspace expects.  So if
userspace expects 148.5MHz and the clock is actually 148.75MHz the actual and
expected frame rate will be slightly off.

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