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
