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Date:   Tue, 12 May 2020 12:14:20 +1000
From:   Dave Airlie <airlied@...il.com>
To:     Oded Gabbay <oded.gabbay@...il.com>
Cc:     Daniel Vetter <daniel.vetter@...ll.ch>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>, Olof Johansson <olof@...om.net>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>
Subject: Re: [Intel-gfx] [PATCH 3/3] misc/habalabs: don't set default fence_ops->wait

On Mon, 11 May 2020 at 19:37, Oded Gabbay <oded.gabbay@...il.com> wrote:
>
> On Mon, May 11, 2020 at 12:11 PM Daniel Vetter <daniel.vetter@...ll.ch> 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.

Dave.

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