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

[Lionel Landwerlin]

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
>>
>>