Re: [RFC PATCH 2/4] rust: sync: Add dma_fence abstractions

["Danilo Krummrich" <dakr@kernel.org>]

On Fri Feb 6, 2026 at 10:32 AM CET, Philipp Stanner wrote:
> Who needs fences from another driver?

When you get VM_BIND and EXEC IOCTLs a driver takes a list of syncobjs the
submitted job should wait for before execution.

The fences of those syncobjs can be from anywhere, including other DRM drivers.

> I think we should go one step back here and question the general
> design.
>
> I only included data: T because it was among the early feedback that
> this is how you do it in Rust.
>
> I was never convinced that it's a good idea. Jobqueue doesn't need the
> 'data' field. Can anyone think of anyone who would need it?
>
> What kind of data would be in there? It seems a driver would store its
> equivalent of C's
>
> struct my_fence {
>    struct dma_fence f;
>    /* other driver data */
> }
>
> which is then accessed in C with container_of.

Your current struct is exactly this pattern:

	struct DmaFence<T> {
	    inner: Opaque<bindings::dma_fence>,
	    ...
	    data: T,
	}

So, in Rust you can just write DmaFence<MyData> rather than,

	struct my_dma_fence {
		struct dma_fence inner;
		struct my_data data;
	}

> But that data is only ever needed by that very driver.

Exactly, this is the "owned" type that is only ever used by this driver.

> They are *not* a data transfer mechanism. It seems very wrong design-
> wise to transfer generic data T from one driver to another. That's not
> a fence's purpose. Another primitive should be used for that.
>
> If another driver could touch / consume / see / use the emitter's data:
> T, that would grossly decouple us from the original dma_fence design.
> It would be akin to doing a container_of to consume foreign driver
> data.

Correct, that's why the suggestion here was to have a second type that is only

	struct ForeignDmaFence {
	    inner: Opaque<bindings::dma_fence>,
	    ...,
	    /* No data. */
	}

i.e. it does not not provide access to the rest of the allocation, since it is
private to the owning driver.

This type should also not have methods like signal(), since only the owner of
the fence should be allowed to signal the fence.