How to get current field polarity of interlaced video output?

Thomas Hilber xorg-driver-ati at
Wed Jul 30 13:20:53 PDT 2008

Hi list,

sorry if this has been answered elsewhere but I haven't found a solution yet.
In my project

after starting the Xserver VBLANK interrupts are generated every 20ms. 
This is because I use a 50Hz interlaced modeline. 

To yield synchronicity it's important to know what field (even or odd) is 
currently active on the CRT controller before RADEONPutImage() gets called.

I tried already to use the mechanism behind 


registers but couldn't solve my problem. I yet haven't found code in 
Radeon DDX or in directFB gfxdrivers to answer my question:

Is there some Radeon register or any other means which lets me
determine the current field polarity? It would be enough for me to get this
information only once after the Xserver start because after this I
automatically count fields by VBLANK interrupts.

Maybe you could give me some hints. Thanks!


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