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Date:   Wed, 20 May 2020 20:04:56 +0200
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Oded Gabbay <oded.gabbay@gmail.com>
Cc:     Daniel Vetter <daniel.vetter@ffwll.ch>,
        Dave Airlie <airlied@gmail.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@lists.linaro.org>, Olof Johansson <olof@lixom.net>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Linux Media Mailing List <linux-media@vger.kernel.org>
Subject: Re: [Intel-gfx] [PATCH 3/3] misc/habalabs: don't set default
 fence_ops->wait
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On Thu, May 14, 2020 at 02:38:38PM +0300, Oded Gabbay wrote:
> On Tue, May 12, 2020 at 9:12 AM Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> >
> > On Tue, May 12, 2020 at 4:14 AM Dave Airlie <airlied@gmail.com> wrote:
> > >
> > > On Mon, 11 May 2020 at 19:37, Oded Gabbay <oded.gabbay@gmail.com> wrote:
> > > >
> > > > On Mon, May 11, 2020 at 12:11 PM Daniel Vetter <daniel.vetter@ffwll.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.
> >
> > 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
> 
> Hi Daniel,
> I want to say explicitly that we don't use the dma-buf uapi parts, nor
> we intend to use them to communicate with any GPU device. We only use
> it as simple completion mechanism as it was convenient to use.
> I do understand I can exchange that mechanism with a simpler one, and
> I will add an internal task to do it (albeit not in a very high
> priority) and upstream it, its just that it is part of our data path
> so we need to thoroughly validate it first.

Sounds good.

Wrt merging this patch here, can you include that in one of your next
pulls? Or should I toss it entirely, waiting for you to remove dma_fence
outright?

Thanks, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch