[Intel-gfx] [PATCH 19/21] drm/i915/gt: Use indices for writing into relative timelines

[Chris Wilson]

Quoting Matthew Brost (2020-12-10 19:16:44)
> On Thu, Dec 10, 2020 at 08:02:38AM +0000, Chris Wilson wrote:
> > Relative timelines are relative to either the global or per-process
> > HWSP, and so we can replace the absolute addressing with store-index
> > variants for position invariance.
> > 
> 
> Can you explain the benifit of relative addressing? Why can't we also
> use absolute? If we can always use absolute, I don't see the point
> complicating the breadcrumb code.

It basically allows a third party to move the contexts between hosts
with far less patching of global state. They want us to avoid all fixed
GGTT addressing.

The breadcrumbs themselves do not notice at all, it's just the timeline
setup and decision to take advantage of the relative commands. The
breadcrumb patches in this series are some outstanding fixes from ~6
months ago.
-Chris