linux-kernel - Re: [RFC WIP 2/3] rust: sync: Add dma

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Message-ID: <aSQl-83Ny3seL2XK@google.com>
Date: Mon, 24 Nov 2025 09:31:39 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Lyude Paul <lyude@...hat.com>
Cc: Philipp Stanner <phasta@...nel.org>, Danilo Krummrich <dakr@...nel.org>, 
	"Christian König" <ckoenig.leichtzumerken@...il.com>, Tvrtko Ursulin <tursulin@...ulin.net>, 
	Alexandre Courbot <acourbot@...dia.com>, Daniel Almeida <daniel.almeida@...labora.com>, 
	Boris Brezillon <boris.brezillon@...labora.com>, Dave Airlie <airlied@...hat.com>, 
	Peter Colberg <pcolberg@...hat.com>, dri-devel@...ts.freedesktop.org, 
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [RFC WIP 2/3] rust: sync: Add dma_fence abstractions

On Fri, Nov 21, 2025 at 06:03:22PM -0500, Lyude Paul wrote:
> I haven't gone through this fully yet. I meant to today, but I ended up
> needing way more time to explain some of my review comments w/r/t some
> ww_mutex bindings for rust then I was expecting. But I do already have some
> comments worth reading below:
> 
> On Tue, 2025-11-18 at 14:25 +0100, Philipp Stanner wrote:
> > 
> > +
> > +/// Container for driver data which the driver gets back in its callback once the fence gets
> > +/// signalled.
> > +#[pin_data]
> > +pub struct DmaFenceCb<T: DmaFenceCbFunc> {
> > +    /// C struct needed for the backend.
> > +    #[pin]
> > +    inner: Opaque<bindings::dma_fence_cb>,
> > +    /// Driver data.
> > +    #[pin]
> > +    pub data: T,
> 
> It's entirely possible I've just never seen someone do this before but - is
> are we actually able to make pinned members of structs `pub`? I would have
> thought that wouldn't be allowed (especially if `data` was exposed as just
> `T`, since a user could then move it pretty easily and break the pinning
> guarantee).

It should be ok. If `data` is pinned, so is the entire struct meaning
that you cannot obtain a `&mut DmaFenceCb<T>`, so you cannot in turn
obtain a `&mut T`.

Alice