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

[Boris Brezillon <boris.brezillon@collabora.com>]

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.