Inconsistent refresh rates in RandR and xvidtune

[Eric Anholt]

On Tue, 2008-10-14 at 21:37 +0200, Tino Keitel wrote:
> Hi,
> 
> a while ago I noticed that RandR and xvidtune can disagree about the
> current refresh rate.
> 
> The display in my notebook offers several refresh rates:
> 
>    1024x768       50.0 +   60.0*    40.0
> 
> First, I'll set it to 60 Hz for sure:
> 
> $ xrandr --rate 60 ; xrandr --rate 60
> 
> xvidtune shows a mode that also shows 60 Hz when started with the GUI.
> On the command line, it looks like this:
> 
> $ xvidtune -show
> "1024x768"     65.00   1024 1048 1184 1344    768  771  777  806 -hsync
> -vsync
> 
> Now I set it to 40 Hz using xrandr:
> 
> $ xrandr --rate 40
> (screen becomes black for a short moment)
> $ xvidtune -show
> "1024x768"     65.00   1024 1048 1184 1344    768  771  777  806 -hsync
> -vsync
> 
> As you can see, xvidtune still shows the old mode, and the xvidtune GUI
> still shows 60 Hz, but I switched to 40 Hz using xrandr.
> 
> Now it gets strange:
> 
> $ xrandr -q | grep "1024x768 "
>    1024x768       50.0 +   60.0     40.0*
> $ xvidtune -show
> "1024x768"     43.33   1024 1048 1184 1344    768  771  777  806 -hsync
> -vsync
> 
> Just running a query changes the xvidtune output.
> 
> For me, this has the following impact:
> 
> When switching to 60 Hz using xrandr --rate 60, I get stuttering video
> in my TV application (MythTV) because the deinterlacer seems to use the
> wrong refresh rate.
> 
> Other tools that use RandR to change the refresh rate, like the Xfce
> display settings, seem to be broken in the same way:
> 
> - changing the refresh rate leaves the xvidtune output at the old mode
>   and MythTV has stuttering video
> 
> - starting the display settings again (which involes a query, to get
>   the current mode I guess) leads to xvidtune output for the new mode
>   and MythTV has smooth video
> 
> I noticed this on a ThinkPad X61s with a i965GM chipset using the Intel
> driver version 2.4 and the current 2.5 branch. I don't know if this is
> related to the Intel driver or if it is a more generic issue.

Regardless of the video mode selected through RandR, your panel gets
driven at a fixed video mode.  The only information we take out of the
mode you select is the h/v active pixels.  So, choosing anything but the
native mode just lies to applications currently, which doesn't sound
useful.

However, panels can be driven at different rates. It would probably be
useful for someone to get the selected video mode's refresh rate applied
to the panel fixed mode.  You might be able to steal code from mjg59's
dynamic panel downclocking patch to help.

-- 
Eric Anholt
eric at anholt.net                         eric.anholt at intel.com


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