On Tue, 10 Feb 2026 14:11:12 +0000 Alice Ryhl <[email protected]> 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 <[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. 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.
