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:
[...]
> ---
> 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

The `CHECKER` static you mean? If so, it should be a `static CHECKER`
instead of `static mut CHECKER`, also for future versions please use
LKMM (Linux Kernel Memory Model) atomics [1] instead of Rust native
atomics (you probably need to define `CHECKER` as `Atomic<i32>` because
AtomicBool is not supported by LKMM and potentially sub-optimial in some
cases).

> 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.
> 

You can use `SpinLock<()>` for this purpose, no need to add new
bindings.

[1]: https://lore.kernel.org/rust-for-linux/20250905044141.77868-1-boqun.feng@gmail.com/

Regards,
Boqun

> 
> 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
> ---
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