[Intel-gfx] [PATCH] drm/i915: revert vga arbiter changes

Alex Williamson alex.williamson at redhat.com
Tue Oct 8 18:18:54 CEST 2013

On Tue, 2013-10-08 at 17:08 +0100, Chris Wilson wrote:
> On Tue, Oct 08, 2013 at 05:27:28PM +0200, Daniel Vetter wrote:
> > Adding drm/i915 into the vga arbiter chain means that X (in a piece of
> > well-meant paranoia) will do a get/put on the vga decoding around
> > _every_ accel call down into the ddx. Which results in some nice
> > performance disasters [1].
> And disables DRI for the system, interferring with the use of OpenGL
> compositors, playing games, PRIME, etc. This effect is not limited to
> users of i915, but any system with a dGPU and an igfx enabled Core CPU.
> I still think we are downplaying the complete and utter disaster caused
> by X and the patch, and why the revert is required.

I'm failing to understand the disaster myself, can you explain?  i915 is
already attempting to make use of the VGA arbiter, but the code hadn't
been updated so what it was doing was ineffective.  The external
difference I see is that before i915 incorrectly reported that it was
not involved in VGA arbitration (even though it still claims VGA
transactions).  With the fixes, it opts-out of MMIO arbitration, but
requires I/O port arbitration.  I'm OK with reverting these and going
back to the drawing board for v3.13, but what i915 is doing is still
incorrect and precludes making use of a guest assigned VGA device (or
separate display server requiring VGA), because we can't use the VGA
arbiter for what it's designed to do, re-route VGA accesses.  Thanks,


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