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

Theo Veenker Theo.Veenker at
Mon Sep 29 04:52:31 PDT 2008

Jesse Barnes wrote:
> 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" :)

I'm afraid I'm clueless here as I am not comfortable with the internals of
the driver. Do you mean I need to be running in VGA mode all the time, or do
the VGA_PIPE_B_SELECT while temporarily in VGA mode?
How would one achive that?

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

Thanks. I'm going to check that as well. From kernel space do I need to call
mmap or pci_request_regions etc to gain access to these registers, or can
I just access them at the MMIO base address + register offset?


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