accessing legacy(?) VGA input status register 1 on 945GM/xf86-video-intel

Jesse Barnes jbarnes at
Fri Sep 26 09:10:37 PDT 2008

On Friday, September 26, 2008 1:36 am Theo Veenker wrote:
> Jesse Barnes wrote:
> > On Thursday, September 25, 2008 6:00 am Theo Veenker wrote:
> >> Hi,
> >>
> >> I have an application that presents audio-visual stimuli to subjects. To
> >> be able to precisely synchronize the audio and graphics the application
> >> needs to know when a vsync occurs. My application (from 1994) doesn't
> >> yet use libdrm. I'm using a real-time module which (among other things)
> >> monitors the vretrace bit in the VGA input status register 1 at 0x3DA
> >> and signals the application on each vsync event. It works fine on most
> >> graphics hardware.
> >>
> >> Now I need to make this application work on a laptop with an Intel
> >> 945GM. The vretrace bit at IO address 0x3DA doesn't work (under X)
> >> unless I connect an external VGA display. Then it works, but it reflects
> >> the retrace of the external monitor and not that of the laptop's LCD
> >> screen.
> >>
> >> Since also drmWaitVBlank() didn't work for me on this system, I applied
> >> the change hinted in
> >> to the
> >> xf86-video-intel driver (2.4.2). That makes drmWaitVBlank() work (but
> >> only after I briefly run a GL application like glxgears first).
> >
> > There's some code in the xf86-video-intel driver to disable vblank
> > interrupts when no 3D client is running (look for want_vblank_interrupts
> > in i830_dri.c, you can either remove the code from
> > I830DRISetVBlankInterrupt or make that field unconditionally true). 
> > Maybe that's what you already did.
> Yes that's what I did. With the stock driver drm vblank would only work
> while running, for instance, glxgears. After the 'fix' I just need to run
> glxgears once and after that it works. I can live with that.
> >> I understand the LCD screen is on pipe B and the VGA screen on pipe A.
> >> Can I somehow swap current behaviour so that when I monitor IO address
> >> 0x3DA I can detect vretraces for pipe B instead of for pipe A? That
> >> would save me the trouble of hacking DRM into this legacy aplication.
> >
> > I think the status bit in 0x3da will correspond to the pipe VGA is
> > assigned to in VGACNTRL (the headers should have the info you need, if
> > not check out the docs at
> Thanks for the info. Here is what I did. In i830_driver.c I830PreInit()
> below RestoreHWState() I added this:
>    xf86DrvMsg(pScrn->scrnIndex, X_WARNING, "HACK: setting
> Unfortunately the VGA input status register 1 bit 3 still reflects the
> retraces for the external VGA monitor, instead of the laptop's display
> panel. So this is problably not correct or I missed something.
> In Xorg.0.log I read "Output VGA is connected to pipe A" and "Output LVDS
> is connected to pipe B". Is there a way to swap them, how? And if so would
> it make a differrence; I mean is it actually possible to have the VGA input
> status register 1 connected to the LVDS?

I think so, but given that we disable VGA mode during normal operation, it may 
be that you need to have pipe B selected at some point *while* VGA mode is 
active.  OTOH there may be other bits you need to set as well.  You could try 
searching the docs for the string "VGA" :)

But if you just want to poll, there's also a bit in the PIPE*STAT regs that 
indicates whether vblank is active, iirc.


Jesse Barnes, Intel Open Source Technology Center

More information about the xorg mailing list