Re: [PATCH 3/4] rust: Add dma_fence abstractions

[Philipp Stanner <phasta@mailbox.org>]

On Mon, 2026-06-01 at 12:59 +0200, Boris Brezillon wrote:
> On Mon, 1 Jun 2026 10:36:06 +0000
> Alice Ryhl <aliceryhl@google.com> wrote:
> 
> > > +};
> > > +
> > > +use bindings::ECANCELED;
> > > +
> > > +use kernel::str::CString;
> > > +use kernel::sync::{
> > > +    aref::{
> > > +        ARef,
> > > +        AlwaysRefCounted, //
> > > +    },
> > > +    Arc,
> > > +    ArcBorrow, //
> > > +};
> > > +
> > > +/// VTable for dma_fence backend_ops callbacks.
> > > +//
> > > +// Mandatory dma_fence backend_ops are implemented implicitly through
> > > +// [`FenceCtx`]. Additional ones shall get implemented on this trait, which then
> > > +// shall be demanded for the fence context data.
> > > +pub trait FenceCtxOps {}  
> > 
> > This empty trait is unused.
> 
> I had initially suggested to add the F type (AKA FenceData) passed
> around in multiple places type as an associated type
> 
> pub trait FenceCtxOps {
>    type FenceData: Send + Sync;
> }
> 
> so we don't have to pass both F and C. The reasoning here is that:
> 
> 1. We expect we'll have to define more methods to the FenceCtxOps trait
> at some point, so adding it now kinda makes sense.
> 
> 2. In the current design, we've assumed that a Fence can't live/be
> created outside of a given context, so there's no world where the
> FenceData wouldn't be known by the FenceCtx implementation, and forcing
> users to pass F and C around seems needlessly verbose.

I had investigated that, but found that this causes us to write things
like

DriverFence<T> (where T is the FencCtx generic)

and then in the actual implementation use

T::FenceData

which reads very weird IMO. Because now for reasons a fence's own data
are not referred to in its own implementation, but you derive it from
the context.

I do prefer it in a way where the DriverFence generic does appear in
said fence's actual code, on equal rank with the FenceCtx.

I suppose that is actually one use case for which PhantomData does
exist.


P.