[Linaro-mm-sig] [PATCH 04/18] dma-fence: prime lockdep annotations

[Jason Gunthorpe]

On Fri, Jun 19, 2020 at 06:19:41PM +0200, Daniel Vetter wrote:

> The madness is only that device B's mmu notifier might need to wait
> for fence_B so that the dma operation finishes. Which in turn has to
> wait for device A to finish first.

So, it sound, fundamentally you've got this graph of operations across
an unknown set of drivers and the kernel cannot insert itself in
dma_fence hand offs to re-validate any of the buffers involved?
Buffers which by definition cannot be touched by the hardware yet.

That really is a pretty horrible place to end up..

Pinning really is right answer for this kind of work flow. I think
converting pinning to notifers should not be done unless notifier
invalidation is relatively bounded. 

I know people like notifiers because they give a bit nicer performance
in some happy cases, but this cripples all the bad cases..

If pinning doesn't work for some reason maybe we should address that?

> Full disclosure: We are aware that we've designed ourselves into an
> impressive corner here, and there's lots of talks going on about
> untangling the dma synchronization from the memory management
> completely. But

I think the documenting is really important: only GPU should be using
this stuff and driving notifiers this way. Complete NO for any
totally-not-a-GPU things in drivers/accel for sure.

Jason