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

On Tue Jun 16, 2026 at 3:38 PM BST, Philipp Stanner 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?

You have `#[repr(C)]` for DriverFence, but not on `Fence`. `Fence::from_raw`
does require layout equivalence between `Fence` and `bindings::dma_fence` and
this is done by adding `#[repr(transparent)]` on `Fence`.

Best,
Gary