[Beignet] [Intel-gfx] Preventing zero GPU virtual address allocation

[Song, Ruiling]

> Yeah my big concern was with not making this opt-in like the old patch or
> adding an interface which does a lot more than what we need right now
> (Chris' patch). Just a bitflag to ask for this seems best and is fine with me.
> 
> And for the implementation I think we should reuse the PIN_BIAS logic since
> that'll work in all places where it's possible. One open from my side is how
> we should handle failures to move buffers (in case they ended up at 0
> somehow) - we can either silently fail or return an error to userspace.
> 
> Note that this is only possible if you render to an elg image from ocl, and if
> that egl image is a pinned frontbuffer and if we don't have full ppgtt support.
> I don't know what the spec requires us to do here, or whether we should
> care at all.
So the situation you mentioned only comes up when a pinned buffer under global gtt?
Under global gtt, a buffer would rarely binded at offset 0, in fact the most often cases are under ppgtt.
So, I think silent ignore moving pinned buffer under gtt is acceptable. I will add the check for the binded at zero case in beignet.

Ruiling
> -Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
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