On 11/01/2016 22:58, Chris Wilson wrote:
On Mon, Jan 11, 2016 at 02:47:33PM -0800, Jesse Barnes wrote:
On 01/11/2016 11:03 AM, John Harrison wrote:
On 08/01/2016 22:05, Chris Wilson wrote:
On Fri, Jan 08, 2016 at 06:47:24PM +0000, john.c.harri...@intel.com wrote:
From: John Harrison <john.c.harri...@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.
I don't understand what you mean. Who is not limited by a single timeline? The
point is that the current seqno values cannot be used as there is no guarantee
that they will increment globally once things like a scheduler and pre-emption
arrive. Whereas, the fence internal implementation makes various assumptions
about the linearity of the timeline. External users do not want to care about
timelines or seqnos at all, they just want the fence API to work as documented.
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).
This patch is not attempting to implement per context seqno values. That can be
done as future work. This patch is doing the simplest, least invasive
implementation in order to make external fences work.
Right. I think we want to move to per-context seqnos, but we don't have to do
it before this work lands. It should be easier to do it after the rest of
these bits land in fact, since seqno handling will be well encapsulated aiui.
This patch is irrelevent then. I think it is actually worse because it
is encapsulating a design detail that is fundamentally wrong.
-Chris
Some kind of per-context timeline is required for the external use of
i915 fences. Seqnos cannot be used without a lot of rework because they
dance around with scheduler re-ordering and pre-emption - a low priority
request could go through many different seqnos if it keeps getting
pre-empted. We need to be able to use fences externally on Android at
least, and with SVM it becomes vital for linux too. Therefore we need
some solution. And this is much simpler and than re-writing the whole of
the driver's seqno management.
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