From: James Simmons <jsimmons@edgeglobal.com>
  To  : ggi-develop@eskimo.com
  Date: Fri, 6 Aug 1999 15:19:25 -0400 (EDT)

Re: Matrox GGI accellerator

> > Even if the card only allows a z buffer the size of the
> > mode /dev/gfx will only mmap the window area of the zbuffer. 
> 
> No. Due to page granularity you will almost always have to map more.

The trick is to know ahead of time and resize the window to the right
size just in case of a mmap. 

> > This way
> > each window area can belong only to a specific process. The problem I'm
> > have is with window overlapping. 
> 
> That is one point, where the SGI HW helps ... Individual windows have tags
> in the graphics RAM that can be regarded as a kind of "stencil-mask" for
> such clipping purposes.

That would be the hardest part to work. Like I said this is far down the
road. 

> If the graphics engine can be _seriously_ (system locks up, graphics get
> unuseable, ...) confused by simultaneous access, we should use forced
> locking using mmap trickery.
> 
> Otherwise we should rely on advisory locking that will only access the
> framebuffer after having acquired it thus flushing and halting the 
> accel engine.

I agree.