[Intel-gfx] [PATCH 11/66] drm/i915: Preallocate stashes for vma page-directories

[Chris Wilson]

Quoting Thomas Hellström (Intel) (2020-07-30 13:28:19)
> 
> On 7/28/20 4:50 PM, Chris Wilson wrote:
> >
> > It's in the user critical path (the shortest path to perform their
> > sequence of operations), but it's before the dma-fence itself. I say
> > that's a particularly nasty false claim that it is not on the critical
> > path, but being where it is circumvents the whole argument.
> >   
> 
> Couldn't the following situation happen?
> 
> 1. CS spawns userptr pinning work.
> 2. CS creates and publishes a DMA-fence that depends on that pinning work.

There's a break before 2 in that we do not publish a dma-fence on pending
userptr work. There's no async wait on the userptr, if the pages are not
available at the point of acquire, we hit an -EAGAIN, and take the
flush_workqueue path until we stop hitting -EAGAIN.

That is as painful as it sounds, and I claim that sitting and spinning in
a user path is no better in terms of critical path than having it inside
the dma-fence section. However, with this pretense we do not violate that
rule.
-Chris