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

[Boris Brezillon <boris.brezillon@collabora.com>]

On Tue, 10 Feb 2026 14:11:12 +0000
Alice Ryhl <aliceryhl@google.com> wrote:

> On Tue, Feb 10, 2026 at 02:51:56PM +0100, Boris Brezillon wrote:
> > On Tue, 10 Feb 2026 13:26:48 +0000
> > Alice Ryhl <aliceryhl@google.com> wrote:
> >   
> > > On Tue, Feb 10, 2026 at 01:49:13PM +0100, Boris Brezillon wrote:  
> > > > On Tue, 10 Feb 2026 10:15:04 +0000
> > > > Alice Ryhl <aliceryhl@google.com> wrote:
> > > >     
> > > > > /// The owner of this value must ensure that this fence is signalled.
> > > > > struct MustBeSignalled<'fence> { ... }
> > > > > /// Proof value indicating that the fence has either already been
> > > > > /// signalled, or it will be. The lifetime ensures that you cannot mix
> > > > > /// up the proof value.
> > > > > struct WillBeSignalled<'fence> { ... }    
> > > > 
> > > > Sorry, I have more questions, unfortunately. Seems that
> > > > {Must,Will}BeSignalled are targeting specific fences (at least that's
> > > > what the doc and 'fence lifetime says), but in practice, the WorkItem
> > > > backing the scheduler can queue 0-N jobs (0 if no jobs have their deps
> > > > met, and N > 1 if more than one job is ready). Similarly, an IRQ
> > > > handler can signal 0-N fences (can be that the IRQ has nothing to do we
> > > > job completion, or, it can be that multiple jobs have completed). How
> > > > is this MustBeSignalled object going to be instantiated in practice if
> > > > it's done before the DmaFenceWorkItem::run() function is called?    
> > > 
> > > The {Must,Will}BeSignalled closure pair needs to wrap the piece of code
> > > that ensures a specific fence is signalled. If you have code that
> > > manages a collection of fences and invokes code for specific fences
> > > depending on outside conditions, then that's a different matter.
> > > 
> > > After all, transfer_to_wq() has two components:
> > > 1. Logic to ensure any spawned workqueue job eventually gets to run.
> > > 2. Once the individual job runs, logic specific to the one fence ensures
> > >    that this one fence gets signalled.  
> > 
> > Okay, that's a change compared to how things are modeled in C (and in
> > JobQueue) at the moment: the WorkItem is not embedded in a specific
> > job, it's something that's attached to the JobQueue. The idea being
> > that the WorkItem represents a task to be done on the queue itself
> > (check if the first element in the queue is ready for execution), not on
> > a particular job. Now, we could change that and have a per-job WorkItem,
> > but ultimately, we'll have to make sure jobs are dequeued in order
> > (deps on JobN can be met before deps on Job0, but we still want JobN to
> > be submitted after Job0), and we'd pay the WorkItem overhead once per
> > Job instead of once per JobQueue. Probably not the end of the world,
> > but it's worth considering, still.  
> 
> It sounds like the fix here is to have transfer_to_job_queue() instead
> of trying to do it at the workqueue level.

Hm, so Job would be something like that (naming/trait-def are just
suggestions to get the discussion going):

trait JobConsumer {
	type FenceType;
	type JobData;

	fn run(self: MustBeSignalled<T::FenceType>) -> Result<WillBeSignaled<Self::FenceType>>;
}

struct Job<T: JobConsumer> {
	fence: MustBeSignalled<T::FenceType>,
	data: T::JobData,
}

I guess that would do.

And then we need to flag the WorkItem that's exposed by the
JobQueue as a DmaFenceWorkItem so that
bindings::dma_fence_begin_signalling() is called before entry and
lockdep can do its job and check that nothing forbidden happens in
this WorkItem.

> 
> > > And {Must,Will}BeSignalled exists to help model part (2.). But what you
> > > described with the IRQ callback falls into (1.) instead, which is
> > > outside the scope of {Must,Will}BeSignalled (or at least requires more
> > > complex APIs).  
> > 
> > For IRQ callbacks, it's not just about making sure they run, but also
> > making sure nothing in there can lead to deadlocks, which is basically
> > #2, except it's not scoped to a particular fence. It's just a "fences
> > can be signaled from there" marker. We could restrict it to "fences of
> > this particular implementation can be signaled from there" but not
> > "this particular fence instance will be signaled next, if any", because
> > that we don't know until we've walked some HW state to figure out which
> > job is complete and thus which fence we need to signal (the interrupt
> > we get is most likely multiplexing completion on multiple GPU contexts,
> > so before we can even get to our per-context in-flight-jobs FIFO, we
> > need to demux this thing).  
> 
> All I can say is that this is a different use-case for the C api
> dma_fence_begin_signalling(). This different usage also seems useful,
> but it would be one that does not involve {Must,Will}BeSignalled
> arguments at all.
> 
> After all, dma_fence_begin_signalling() only requires those arguments if
> you want to convert a PrivateFence into a PublishedFence. (I guess a
> better name is PublishableFence.) If you're not trying to prove that a
> specific fence will be signalled, then you don't need the
> {Must,Will}BeSignalled arguments.

Okay, so that would be another function returning some sort of guard
then? What I find confusing is the fact
dma_fence::dma_fence_begin_signalling() matches the C function name
which is not per-fence, but just this lock-guard model flagging a
section from which any fence can be signalled, so maybe we should
name your dma_fence_begin_signalling() proposal differently, dunno.