Re: [RFC PATCH] rust: sync: Add dma_fence abstractions

[Philipp Stanner]

On Fri, 2025-09-26 at 09:10 -0700, Boqun Feng wrote:
> On Thu, Sep 18, 2025 at 02:30:59PM +0200, Philipp Stanner wrote:
> > dma_fence is a synchronization mechanism which is needed by virtually
> > all GPU drivers.
> > 
> > A dma_fence offers many features, among which the most important ones
> > are registering callbacks (for example to kick off a work item) which
> > get executed once a fence gets signalled.
> > 
> > dma_fence has a number of callbacks. Only the two most basic ones
> > (get_driver_name(), get_timeline_name() are abstracted since they are
> > enough to enable the basic functionality.
> > 
> > Callbacks in Rust are registered by passing driver data which implements
> > the Rust callback trait, whose function will be called by the C backend.
> > 
> > dma_fence's are always refcounted, so implement AlwaysRefcounted for
> > them. Once a reference drops to zero, the C backend calls a release
> > function, where we implement drop_in_place() to conveniently marry that
> > C-cleanup mechanism with Rust's ownership concepts.
> > 
> > This patch provides basic functionality, but is still missing:
> >   - An implementation of PinInit<T, Error> for all driver data.
> >   - A clever implementation for working dma_fence_begin_signalling()
> >     guards. See the corresponding TODO in the code.
> >   - Additional useful helper functions such as dma_fence_is_signaled().
> >     These _should_ be relatively trivial to implement, though.
> > 
> > Signed-off-by: Philipp Stanner <phasta@xxxxxxxxxx>
> > ---
> > So. ¡Hola!
> > 
> > This is a highly WIP RFC. It's obviously at many places not yet
> > conforming very well to Rust's standards.
> > 
> > Nevertheless, it has progressed enough that I want to request comments
> > from the community.
> > 
> > There are a number of TODOs in the code to which I need input.
> > 
> > Notably, it seems (half-)illegal to use a shared static reference to an
> > Atomic, which I currently use for the dma_fence unit test / docstring
> > test. I'm willing to rework that if someone suggests how.
> > (Still, shouldn't changing a global Atomic always be legal? It can race,
> > of course. But that's kind of the point of an atomic)
> > 
> > What I want comments on the most is the design of the callbacks. I think
> > it's a great opportunity to provide Rust drivers with rust-only
> > callbacks, so that they don't have to bother about the C functions.
> > 
> > dma_fence wise, only the most basic callbacks currently get implemented.
> > For Nova, AFAICS, we don't need much more than signalling fences and
> > registering callbacks.
> > 
> > 
> > Another, solvable, issue I'm having is designing the
> > dma_fence_begin_signallin() abstractions. There are TODOs about that in
> > the code. That should ideally be robust and not racy. So we might want
> > some sort of synchronized (locked?) way for using that abstraction.
> > 
> > 
> > Regarding the manually created spinlock of mine: I so far never need
> > that spinlock anywhere in Rust and wasn't sure what's then the best way
> > to pass a "raw" spinlock to C.
> > 
> > 
> > So much from my side. Hope to hear from you.
> > 
> > (I've compiled and tested this with the unit test on the current -rc3)
> > 
> > Philipp
> > ---
> >  rust/bindings/bindings_helper.h |   1 +
> >  rust/helpers/dma_fence.c        |  23 ++
> >  rust/helpers/helpers.c          |   1 +
> >  rust/helpers/spinlock.c         |   5 +
> >  rust/kernel/sync.rs             |   2 +
> >  rust/kernel/sync/dma_fence.rs   | 388 ++++++++++++++++++++++++++++++++
> 
> I missed this part, and I don't think kernel::sync is where dma_fence
> should be, as kernel::sync is mostly for the basic synchronization
> between threads/irqs. dma_fence is probably better to be grouped with
> dma-buf and other dma related primitives. Maybe in kernel::dma? Like:
> 
> rust/kernel/dma.rs
> rust/kernel/dma/dma_buf.rs
> rust/kernel/dma/dma_fence.rs
> 
> Thoughts? Miguel, Greg, Danilo and Lyude, any idea or suggestion?

@Christian König's opinion would be valuable, too.

I'm not super convinced of that because dma_fence has not really much
to do with DMA. They're not very different from completions and are a
mechanism to synchronize consumers and producers.

Actually, before f54d1867005c3 they were just called "fence" and then
renamed to "dma_fence" because someone wanted that name.


Anyways, I don't have strong objections and mostly care about having
them available somewhere.

P.

> 
> Regards,
> Boqun
> 
> >  6 files changed, 420 insertions(+)
> >  create mode 100644 rust/helpers/dma_fence.c
> >  create mode 100644 rust/kernel/sync/dma_fence.rs
> > 
> [...]