Re: [PATCH v2 5/6] rust: Add dma_fence abstractions

["Onur Özkan" <work@onurozkan.dev>]

On Tue, 16 Jun 2026 16:38:51 +0200
Philipp Stanner <phasta@mailbox.org> wrote:

> On Tue, 2026-06-16 at 15:47 +0300, Onur Özkan wrote:
> > On Tue, 16 Jun 2026 10:28:17 +0200
> > > +
> > > +        // DriverFenceData is repr(C) and a Fence is its first member.
> > > +        let fence_data_ptr = fence_ptr as *mut DriverFenceData<T>;
> > 
> > Either the field ordering on the type or this code is wrong because the first
> > member of DriverFenceData is `rcu_head`.
> 
> Yeah, that's a bug introduced in this revision. Thx for pointing it
> out. Already fixed it downstream.
> 
> I'm just wondering why my test did not blow up.
> 
> > > 
> > > +}
> > 
> > I am unsure whether it is safe to cast the pointer in Fence::from_raw without
> > Fence being #[repr(transparent)] as the layout compatibility is not guaranteed
> > explicitly.
> 
> Shouldn't #[repr(C)] do that trick, too? Do you need both?
> 
> Who can answer that?
> 
> 
> P.

You shouldn't use both and I think it wouldn't compile anyway. repr(C) only
makes sense when the type have multiple fields.

Thanks,
Onur