Comment 23 for bug 273329

(In reply to comment #17)
> Anyway, surfaces are not magic as you seem to imply. If you put one in front of
> a piece of memory, it applies the tiling function, whether the buffer is tiled
> or linear (in which case you scramble the contents). Here is what I think is
> the problem :
> - the (low end) card doesn't tile the zbuffer
> - we still setup the tile registers over the zbuffer
> - reads are scrambled by the use of a surface. If the surface wasn't there,
> zbuffer reads would work
>

Oh, I didn't want to imply surfaces are magic. But I think only non-tcl r100-class chips don't use tiling for the z buffer.
But if that's indeed some bogus surface setup, you can verify that with mesa's progs/zreaddraw demo, which wouldn't work in this case.