On 27/06/2019 13:43, Chris Wilson wrote:
Quoting Lionel Landwerlin (2019-06-27 11:39:24)
On 27/06/2019 13:21, Chris Wilson wrote:
Quoting Lionel Landwerlin (2019-06-27 09:03:37)
Hi,
This revision gets rid of the synchronous wait. We now implement the
synchronous wait as part of the userspace driver. A thread is spawned
for each engine and waits for availability of the syncobjs before
calling into execbuffer.
Why would you do that? It's not like the kernel already has the ability
to serialises execution asynchronously...
-Chris
Maybe I didn't express myself well.
There is a requirement from the Vulkan spec that we should be able to
queue a workload depending on fences that haven't materialized yet.
The last revision implemented that in the i915 by blocking in the
execbuffer until the input fences had all materialized.
My point was that the syncobj seqno itself could be expressed as a proxy
fence that was replaced with the real fence later. (And that we
previously had code to do exactly that :( i915 would listen to the
proxy fence for its scheduling and so be signaled by the right fence
when it was known. That will be a few orders of magnitude lower latency,
more if we can optimise away the irq by knowing the proxy fence ahead of
time.
-Chris
Agreed, but there is a problem here with the dma_fence_chain implementation.
With timeline semaphores, you could be waiting on for point 6 and nobody
is ever going to signal that point.
Instead point 8 is going to be signaled and we need to signal point 6
when that happens.
I don't think the current dma_fence_chain implementation allows to do
that at the moment.
It would need to be more complex with likely more locking.
I have thought a little bit about what you're proposing and found some
issues with other features we need to support (like import/export).
You could end up building a arbitrary long chain of proxy fences firing
one another in cascade and blowing up the stack in a kernel task
signaling one of them.
-Lionel
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