Message-ID: <9a90f7f14c22c01aa28d89aa91bf4dfa4049c062.camel@mailbox.org>
Date: Wed, 09 Apr 2025 16:01:53 +0200
From: Philipp Stanner <phasta@...lbox.org>
To: Christian König <christian.koenig@....com>, 
	phasta@...nel.org, Boris Brezillon <boris.brezillon@...labora.com>
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Subject: Re: [PATCH 1/2] dma-fence: Rename dma_fence_is_signaled()

On Wed, 2025-04-09 at 15:14 +0200, Christian König wrote:
> Am 09.04.25 um 14:56 schrieb Philipp Stanner:
> > On Wed, 2025-04-09 at 14:51 +0200, Philipp Stanner wrote:
> > > On Wed, 2025-04-09 at 14:39 +0200, Boris Brezillon wrote:
> > > > Hi Philipp,
> > > > 
> > > > On Wed,  9 Apr 2025 14:06:37 +0200
> > > > Philipp Stanner <phasta@...nel.org> wrote:
> > > > 
> > > > > dma_fence_is_signaled()'s name strongly reads as if this
> > > > > function
> > > > > were
> > > > > intended for checking whether a fence is already signaled.
> > > > > Also
> > > > > the
> > > > > boolean it returns hints at that.
> > > > > 
> > > > > The function's behavior, however, is more complex: it can
> > > > > check
> > > > > with a
> > > > > driver callback whether the hardware's sequence number
> > > > > indicates
> > > > > that
> > > > > the fence can already be treated as signaled, although the
> > > > > hardware's /
> > > > > driver's interrupt handler has not signaled it yet. If that's
> > > > > the
> > > > > case,
> > > > > the function also signals the fence.
> > > > > 
> > > > > (Presumably) this has caused a bug in Nouveau (unknown
> > > > > commit),
> > > > > where
> > > > > nouveau_fence_done() uses the function to check a fence,
> > > > > which
> > > > > causes a
> > > > > race.
> > > > > 
> > > > > Give the function a more obvious name.
> > > > This is just my personal view on this, but I find the new name
> > > > just
> > > > as
> > > > confusing as the old one. It sounds like something is checked,
> > > > but
> > > > it's
> > > > clear what, and then the fence is forcibly signaled like it
> > > > would
> > > > be
> > > > if
> > > > you call drm_fence_signal(). Of course, this clarified by the
> > > > doc,
> > > > but
> > > > given the goal was to make the function name clearly reflect
> > > > what
> > > > it
> > > > does, I'm not convinced it's significantly better.
> > > > 
> > > > Maybe dma_fence_check_hw_state_and_propagate(), though it might
> > > > be
> > > > too long of name. Oh well, feel free to ignore this comments if
> > > > a
> > > > majority is fine with the new name.
> > > Yoa, the name isn't perfect (the perfect name describing the
> > > whole
> > > behavior would be
> > > dma_fence_check_if_already_signaled_then_check_hardware_state_and
> > > _pro
> > > pa
> > > gate() ^^'
> > > 
> > > My intention here is to have the reader realize "watch out, the
> > > fence
> > > might get signaled here!", which is probably the most important
> > > event
> > > regarding fences, which can race, invoke the callbacks and so on.
> > > 
> > > For details readers will then check the documentation.
> > > 
> > > But I'm of course open to see if there's a majority for this or
> > > that
> > > name.
> > how about:
> > 
> > dma_fence_check_hw_and_signal() ?
> 
> I don't think that renaming the function is a good idea in the first
> place.
> 
> What the function does internally is an implementation detail of the
> framework.
> 
> For the code using this function it's completely irrelevant if the
> function might also signal the fence, what matters for the caller is
> the returned status of the fence. I think this also counts for the
> dma_fence_is_signaled() documentation.

It does obviously matter. As it's currently implemented, a lot of
important things happen implicitly.

I only see improvement by making things more obvious.

In any case, how would you call a wrapper that just does
test_bit(IS_SIGNALED, …) ?

P.

> 
> What we should improve is the documentation of the dma_fence_ops-
> >enable_signaling and dma_fence_ops->signaled callbacks.
> 
> Especially see the comment about reference counts on enable_signaling
> which is missing on the signaled callback. That is most likely the
> root cause why nouveau implemented enable_signaling correctly but not
> the other one.
> 
> But putting that aside I think we should make nails with heads and
> let the framework guarantee that the fences stay alive until they are
> signaled (one way or another). This completely removes the burden to
> keep a reference on unsignaled fences from the drivers /
> implementations and make things more over all more defensive.
> 
> Regards,
> Christian.
> 
> > 
> > P.
> > 
> > > P.
> > > 
> > > 
> > > > Regards,
> > > > 
> > > > Boris
> 

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