Re: [Intel-gfx] [RFC v3 1/3] drm/doc/rfc: VM_BIND feature design document

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On 02/06/2022 00:18, Matthew Brost wrote:
On Wed, Jun 01, 2022 at 05:25:49PM +0300, Lionel Landwerlin wrote:
On 17/05/2022 21:32, Niranjana Vishwanathapura wrote:
+VM_BIND/UNBIND ioctl will immediately start binding/unbinding the mapping in an
+async worker. The binding and unbinding will work like a special GPU engine.
+The binding and unbinding operations are serialized and will wait on specified
+input fences before the operation and will signal the output fences upon the
+completion of the operation. Due to serialization, completion of an operation
+will also indicate that all previous operations are also complete.
I guess we should avoid saying "will immediately start binding/unbinding" if
there are fences involved.

And the fact that it's happening in an async worker seem to imply it's not
immediate.


I have a question on the behavior of the bind operation when no input fence
is provided. Let say I do :

VM_BIND (out_fence=fence1)

VM_BIND (out_fence=fence2)

VM_BIND (out_fence=fence3)


In what order are the fences going to be signaled?

In the order of VM_BIND ioctls? Or out of order?

Because you wrote "serialized I assume it's : in order


One thing I didn't realize is that because we only get one "VM_BIND" engine,
there is a disconnect from the Vulkan specification.

In Vulkan VM_BIND operations are serialized but per engine.

So you could have something like this :

VM_BIND (engine=rcs0, in_fence=fence1, out_fence=fence2)

VM_BIND (engine=ccs0, in_fence=fence3, out_fence=fence4)

Question - let's say this done after the above operations:

EXEC (engine=ccs0, in_fence=NULL, out_fence=NULL)

Is the exec ordered with respected to bind (i.e. would fence3 & 4 be
signaled before the exec starts)?

Matt


Hi Matt,

From the vulkan point of view, everything is serialized within an engine (we map that to a VkQueue).

So with :

EXEC (engine=ccs0, in_fence=NULL, out_fence=NULL)
VM_BIND (engine=ccs0, in_fence=fence3, out_fence=fence4)

EXEC completes first then VM_BIND executes.


To be even clearer :

EXEC (engine=ccs0, in_fence=fence2, out_fence=NULL)
VM_BIND (engine=ccs0, in_fence=fence3, out_fence=fence4)


EXEC will wait until fence2 is signaled.
Once fence2 is signaled, EXEC proceeds, finishes and only after it is done, VM_BIND executes.

It would kind of like having the VM_BIND operation be another batch executed from the ringbuffer buffer.

-Lionel



fence1 is not signaled

fence3 is signaled

So the second VM_BIND will proceed before the first VM_BIND.


I guess we can deal with that scenario in userspace by doing the wait
ourselves in one thread per engines.

But then it makes the VM_BIND input fences useless.


Daniel : what do you think? Should be rework this or just deal with wait
fences in userspace?


Sorry I noticed this late.


-Lionel






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