Re: [Intel-gfx] [PATCH 3/3] misc/habalabs: don't set default fence_ops->wait

[Daniel Vetter]

On Tue, May 12, 2020 at 4:14 AM Dave Airlie <airlied@xxxxxxxxx> wrote:
>
> On Mon, 11 May 2020 at 19:37, Oded Gabbay <oded.gabbay@xxxxxxxxx> wrote:
> >
> > On Mon, May 11, 2020 at 12:11 PM Daniel Vetter <daniel.vetter@xxxxxxxx> wrote:
> > >
> > > It's the default.
> > Thanks for catching that.
> >
> > >
> > > Also so much for "we're not going to tell the graphics people how to
> > > review their code", dma_fence is a pretty core piece of gpu driver
> > > infrastructure. And it's very much uapi relevant, including piles of
> > > corresponding userspace protocols and libraries for how to pass these
> > > around.
> > >
> > > Would be great if habanalabs would not use this (from a quick look
> > > it's not needed at all), since open source the userspace and playing
> > > by the usual rules isn't on the table. If that's not possible (because
> > > it's actually using the uapi part of dma_fence to interact with gpu
> > > drivers) then we have exactly what everyone promised we'd want to
> > > avoid.
> >
> > We don't use the uapi parts, we currently only using the fencing and
> > signaling ability of this module inside our kernel code. But maybe I
> > didn't understand what you request. You want us *not* to use this
> > well-written piece of kernel code because it is only used by graphics
> > drivers ?
> > I'm sorry but I don't get this argument, if this is indeed what you meant.
>
> We would rather drivers using a feature that has requirements on
> correct userspace implementations of the feature have a userspace that
> is open source and auditable.
>
> Fencing is tricky, cross-device fencing is really tricky, and having
> the ability for a closed userspace component to mess up other people's
> drivers, think i915 shared with closed habana userspace and shared
> fences, decreases ability to debug things.
>
> Ideally we wouldn't offer users known untested/broken scenarios, so
> yes we'd prefer that drivers that intend to expose a userspace fencing
> api around dma-fence would adhere to the rules of the gpu drivers.
>
> I'm not say you have to drop using dma-fence, but if you move towards
> cross-device stuff I believe other drivers would be correct in
> refusing to interact with fences from here.

The flip side is if you only used dma-fence.c "because it's there",
and not because it comes with an uapi attached and a cross-driver
kernel internal contract for how to interact with gpu drivers, then
there's really not much point in using it. It's a custom-rolled
wait_queue/event thing, that's all. Without the gpu uapi and gpu
cross-driver contract it would be much cleaner to just use wait_queue
directly, and that's a construct all kernel developers understand, not
just gpu folks. From a quick look at least habanalabs doesn't use any
of these uapi/cross-driver/gpu bits.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch