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

[Boqun Feng <boqun.feng@gmail.com>]

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@kernel.org>
> ---
> 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?

Regards,
Boqun

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