[Linaro-mm-sig] [PATCH 1/2] dma-buf.rst: Document why indefinite fences are a bad idea

[Dave Airlie]

On Tue, 21 Jul 2020 at 18:47, Thomas Hellström (Intel)
<thomas_os at shipmail.org> wrote:
>
>
> On 7/21/20 9:45 AM, Christian König wrote:
> > Am 21.07.20 um 09:41 schrieb Daniel Vetter:
> >> On Mon, Jul 20, 2020 at 01:15:17PM +0200, Thomas Hellström (Intel)
> >> wrote:
> >>> Hi,
> >>>
> >>> On 7/9/20 2:33 PM, Daniel Vetter wrote:
> >>>> Comes up every few years, gets somewhat tedious to discuss, let's
> >>>> write this down once and for all.
> >>>>
> >>>> What I'm not sure about is whether the text should be more explicit in
> >>>> flat out mandating the amdkfd eviction fences for long running compute
> >>>> workloads or workloads where userspace fencing is allowed.
> >>> Although (in my humble opinion) it might be possible to completely
> >>> untangle
> >>> kernel-introduced fences for resource management and dma-fences used
> >>> for
> >>> completion- and dependency tracking and lift a lot of restrictions
> >>> for the
> >>> dma-fences, including prohibiting infinite ones, I think this makes
> >>> sense
> >>> describing the current state.
> >> Yeah I think a future patch needs to type up how we want to make that
> >> happen (for some cross driver consistency) and what needs to be
> >> considered. Some of the necessary parts are already there (with like the
> >> preemption fences amdkfd has as an example), but I think some clear docs
> >> on what's required from both hw, drivers and userspace would be really
> >> good.
> >
> > I'm currently writing that up, but probably still need a few days for
> > this.
>
> Great! I put down some (very) initial thoughts a couple of weeks ago
> building on eviction fences for various hardware complexity levels here:
>
> https://gitlab.freedesktop.org/thomash/docs/-/blob/master/Untangling%20dma-fence%20and%20memory%20allocation.odt

We are seeing HW that has recoverable GPU page faults but only for
compute tasks, and scheduler without semaphores hw for graphics.

So a single driver may have to expose both models to userspace and
also introduces the problem of how to interoperate between the two
models on one card.

Dave.