Re: [RFC PATCH 2/4] rust: sync: Add dma_fence abstractions

[Philipp Stanner <phasta@mailbox.org>]

On Tue, 2026-02-10 at 15:06 +0100, Christian König wrote:
> On 2/10/26 15:00, Philipp Stanner wrote:
> > On Tue, 2026-02-10 at 14:56 +0100, Christian König wrote:
> > > On 2/10/26 14:49, Alice Ryhl wrote:
> > > > On Tue, Feb 10, 2026 at 02:26:31PM +0100, Boris Brezillon wrote:
> > > > > On Tue, 10 Feb 2026 13:15:31 +0000
> > > > > Alice Ryhl <aliceryhl@google.com> wrote:
> > > > > 
> > > > > 
> > > > > 
> > 
> > […]
> > 
> > > > > I mean, the timeout handler should also be considered a DMA-signalling
> > > > > path, and the same rules should apply to it.
> > > > 
> > > > I guess that's fair. Even with a timeout you want both to be signalling
> > > > path.
> > > > 
> > > > I guess more generally, if a fence is signalled by mechanism A or B,
> > > > whichever happens first, you have the choice between:
> > > 
> > > That doesn't happen in practice.
> > > 
> > > For each fence you only have one signaling path you need to guarantee forward progress for.
> > > 
> > > All other signaling paths are just opportunistically optimizations which *can* signal the fence, but there is no guarantee that they will.
> > 
> > Are you now referring to the fast-path callbacks like fence->ops-
> > > is_signaled()? Or are you talking about different reference holders
> > which might want to signal?
> 
> Yes, I'm referring to the is_signaled() callback.
> 
> When you have multiple reference holders which can all signal the fence by calling dma_fence_signal then there is clearly something going wrong.

From our previous discussions it always seemed to me that there is
already something wrong when is_signaled() fastpaths are being
utilized. Remember the mess we have in Nouveau because of that?

I agree that it sounds very sane to just have 1 party that can signal a
fence.

However, that also implies that the fastpath is wrong because

a) a consumer of a fence can use that to poll on the fence and
b) because it breaks good naming pattern, where a check whether a fence
is signaled can result in the fence being signaled.

So I would kill that fast path for good.

In the past you mentioned that you had users who were wondering why
there fences are not signalling (like yeah, if you don't call
dma_fence_signal(), then your fence will not signal). What ever the
confusion was, this time we have the chance to set it straight.

P.