Questions about libdrm_intel and way to share physical memory between CPU and GPU

Alan Cox alan at
Thu Jun 2 11:42:03 PDT 2011

On Sat, 28 May 2011 09:54:01 +0100
Chris Wilson <chris at> wrote:

> On Fri, 27 May 2011 14:37:45 -0700, "Segovia, Benjamin" <benjamin.segovia at> wrote:
> > Hello gurus,
> > 
> > I have two question mostly regarding libdrm_intel
> > 
> > 1/ What is the difference between drm_intel_bo_map and drm_intel_gem_bo_map_gtt ?
> bo_map uses the CPU domain, and so is CPU linear (needs sw detiling).
> bo_gtt_map uses the uncached [WC] GTT domain, and so is GPU linear
> (detiling is performed by the hardware using a fence).
> > 2/ Will it be possible (or is it already possible) to directly share a regularly allocated piece of physical memory? Typical use case is the following one using OpenCL API:
> Yes. I've proposed a vmap interface to bind user-pages into the GTT,
> similar to a completely unused bit of TTM functionality.

It seems to me that stolen memory and other things could all be sorted
out somewhat if the GEM layer and GEM as shmemfs backing were split apart
a bit. A 'privately backed' GEM object wouldn't be able to support
flink() but I can't find much else that would break ?

Wondering about this for things like the GMA500, and also to get back all
that memory the i9xx driver burns on a PC.

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