On Tue, Feb 10, 2026 at 02:51:56PM +0100, Boris Brezillon wrote:
> On Tue, 10 Feb 2026 13:26:48 +0000
> Alice Ryhl <[email protected]> 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 <[email protected]> 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.
> > 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.
Alice
