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

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

On Fri, Sep 26, 2025 at 10:48:06AM +0200, Philipp Stanner wrote:
[..]
> > > 
> > > 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.
> 
> The dma_fence C backend needs a spinlock pointer, given to it by the
> driver (so Rust code).
> 
> How do I pass a SpinLock<()> to a C function? AFAICS SpinLock doesn't
> implement as_raw(), so I'd have to implement it, wouldn't I?
> 

Technically you can use a `&raw` to get the address of the spinlock and
just use it, since `SpinLock<()>` is `repr(C)`, so it's transparent to a 
spinlock_t. So you don't have to ;-)

However, while we are at it, it makes sense that we do it right.

> Or rather, as it looks, I'd have to implement it for SpinLockBackend?
> 

I think the below should work, we already have a special from_raw() for
Lock<()>, it makes sense to have a special as_raw() as well.

Let me know if you want me to send a proper patch, or feel free to add
one in your patchset ;-)

diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
index 27202beef90c..78f3287a1372 100644
--- a/rust/kernel/sync/lock.rs
+++ b/rust/kernel/sync/lock.rs
@@ -160,6 +160,13 @@ pub unsafe fn from_raw<'a>(ptr: *mut B::State) -> &'a Self {
         //   `B::State`.
         unsafe { &*ptr.cast() }
     }
+
+    /// Obtains the raw pointer from a [`Lock`].
+    ///
+    /// This can be useful for working with a lock user outside Rust.
+    pub fn as_raw(&self) -> *mut B::State {
+        self.state.get()
+    }
 }

 impl<T: ?Sized, B: Backend> Lock<T, B> {


Regards,
Boqun

> 
> P.
> 
> > 
> > [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
> > > ---
> > [...]
>