[Intel-gfx] [PATCH 3/7] drm/i915: Add per context timelines to fence object

[Chris Wilson]

On Fri, Jan 08, 2016 at 06:47:24PM +0000, John.C.Harrison at Intel.com wrote:
> From: John Harrison <John.C.Harrison at Intel.com>
> 
> The fence object used inside the request structure requires a sequence
> number. Although this is not used by the i915 driver itself, it could
> potentially be used by non-i915 code if the fence is passed outside of
> the driver. This is the intention as it allows external kernel drivers
> and user applications to wait on batch buffer completion
> asynchronously via the dma-buff fence API.

That doesn't make any sense as they are not limited by a single
timeline.

> To ensure that such external users are not confused by strange things
> happening with the seqno, this patch adds in a per context timeline
> that can provide a guaranteed in-order seqno value for the fence. This
> is safe because the scheduler will not re-order batch buffers within a
> context - they are considered to be mutually dependent.

You haven't added per-context breadcrumbs. What we need for being able
to execute requests from parallel timelines, but with requests within a
timeline being ordered, is a per-context page where we can emit the
per-context issued breadcrumb. Then instead of looking up the current
HW seqno in a global page, the request just looks at the current context
HW seqno in the context seq, just
i915_seqno_passed(*req->p_context_seqno, req->seqno).

The retirment ordered requests lists are moved from the engine to the
context and retirement cleanup are restricted to the context.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre