[PATCH] ugly patch for DRM based XSync VBLANK support

[Benjamin Herrenschmidt]

On Sat, 2006-03-11 at 14:11 -0800, Jesse Barnes wrote:
> Here's an ugly patch that attempts to add an XSync system counter called 
> "VBLANK" to the server if the underlying drm driver that was just 
> opened supports vblank signals.  It doesn't work very well for a couple 
> of reasons:
>   o drmOpen is working ok (I get back a real fd) but drmWaitVBlank gives
>     me EPERM instead of EINVAL like it should (my test machine doesn't
>     have a drm driver with vblank support yet), so VBLANK support is
>     disabled (IOW I haven't actually tested it :)
>   o it's totally multihead ignorant
> 
> Hopefully someone can clue me in about the first problem, and the second 
> needs a bit more thought.  It seems like there should be an XSync 
> VBLANK counter per-screen, not per-drm and obviously not a single 
> global one.  It looks like the drm drivers could handle this, but it 
> would make things harder for the clients since they'd have to dig 
> through the system counters to find the VBLANK counter corresponding to 
> their head (I guess via strmp?) which means a well defined namespace 
> for the VBLANK XSync counters is necessary.  Thoughts?
> 
> Also, I wasn't sure if this stuff belonged in xf86drm.c, I'm guessing 
> probably not.  Any ideas about a better place?
> 
> Other comments and review appreciated.

Whith also leads to the question is does XSync make any sense in a
MergedFB environment and if not, should we maybe invent some alternative
and drop it in XFixes or other grabbag ? :)

Ben.