Message-ID: <2aa5150d913fcd4d321db52bc6bad1770f68e778.camel@mailbox.org>
Date: Sat, 27 Sep 2025 11:01:38 +0200
From: Philipp Stanner <phasta@...lbox.org>
To: Boqun Feng <boqun.feng@...il.com>, Philipp Stanner <phasta@...nel.org>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
 Gary Guo <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>,  Benno Lossin <lossin@...nel.org>, Andreas
 Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, 
 Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, Peter
 Zijlstra <peterz@...radead.org>,  Ingo Molnar <mingo@...hat.com>, Will
 Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>, Nathan
 Chancellor <nathan@...nel.org>, Nick Desaulniers
 <nick.desaulniers+lkml@...il.com>, Bill Wendling <morbo@...gle.com>, Justin
 Stitt <justinstitt@...gle.com>, Sumit Semwal <sumit.semwal@...aro.org>,
 Christian König <christian.koenig@....com>, Greg
 Kroah-Hartman <gregkh@...uxfoundation.org>,  Viresh Kumar
 <viresh.kumar@...aro.org>, Asahi Lina <lina+kernel@...hilina.net>, Daniel
 Almeida <daniel.almeida@...labora.com>, Tamir Duberstein
 <tamird@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, FUJITA
 Tomonori <fujita.tomonori@...il.com>, Krishna Ketan Rai
 <prafulrai522@...il.com>, Lyude Paul <lyude@...hat.com>, Mitchell Levy
 <levymitchell0@...il.com>, linux-kernel@...r.kernel.org, 
 rust-for-linux@...r.kernel.org, llvm@...ts.linux.dev, 
 dri-devel@...ts.freedesktop.org
Subject: Re: [RFC PATCH] rust: sync: Add dma_fence abstractions

On Fri, 2025-09-26 at 09:10 -0700, Boqun Feng wrote:
> On Thu, Sep 18, 2025 at 02:30:59PM +0200, Philipp Stanner wrote:
> > dma_fence is a synchronization mechanism which is needed by virtually
> > all GPU drivers.
> > 
> > A dma_fence offers many features, among which the most important ones
> > are registering callbacks (for example to kick off a work item) which
> > get executed once a fence gets signalled.
> > 
> > dma_fence has a number of callbacks. Only the two most basic ones
> > (get_driver_name(), get_timeline_name() are abstracted since they are
> > enough to enable the basic functionality.
> > 
> > Callbacks in Rust are registered by passing driver data which implements
> > the Rust callback trait, whose function will be called by the C backend.
> > 
> > dma_fence's are always refcounted, so implement AlwaysRefcounted for
> > them. Once a reference drops to zero, the C backend calls a release
> > function, where we implement drop_in_place() to conveniently marry that
> > C-cleanup mechanism with Rust's ownership concepts.
> > 
> > This patch provides basic functionality, but is still missing:
> >   - An implementation of PinInit<T, Error> for all driver data.
> >   - A clever implementation for working dma_fence_begin_signalling()
> >     guards. See the corresponding TODO in the code.
> >   - Additional useful helper functions such as dma_fence_is_signaled().
> >     These _should_ be relatively trivial to implement, though.
> > 
> > Signed-off-by: Philipp Stanner <phasta@...nel.org>
> > ---
> > So. ¡Hola!
> > 
> > This is a highly WIP RFC. It's obviously at many places not yet
> > conforming very well to Rust's standards.
> > 
> > Nevertheless, it has progressed enough that I want to request comments
> > from the community.
> > 
> > There are a number of TODOs in the code to which I need input.
> > 
> > Notably, it seems (half-)illegal to use a shared static reference to an
> > Atomic, which I currently use for the dma_fence unit test / docstring
> > test. I'm willing to rework that if someone suggests how.
> > (Still, shouldn't changing a global Atomic always be legal? It can race,
> > of course. But that's kind of the point of an atomic)
> > 
> > What I want comments on the most is the design of the callbacks. I think
> > it's a great opportunity to provide Rust drivers with rust-only
> > callbacks, so that they don't have to bother about the C functions.
> > 
> > dma_fence wise, only the most basic callbacks currently get implemented.
> > For Nova, AFAICS, we don't need much more than signalling fences and
> > registering callbacks.
> > 
> > 
> > Another, solvable, issue I'm having is designing the
> > dma_fence_begin_signallin() abstractions. There are TODOs about that in
> > the code. That should ideally be robust and not racy. So we might want
> > some sort of synchronized (locked?) way for using that abstraction.
> > 
> > 
> > Regarding the manually created spinlock of mine: I so far never need
> > that spinlock anywhere in Rust and wasn't sure what's then the best way
> > to pass a "raw" spinlock to C.
> > 
> > 
> > So much from my side. Hope to hear from you.
> > 
> > (I've compiled and tested this with the unit test on the current -rc3)
> > 
> > Philipp
> > ---
> >  rust/bindings/bindings_helper.h |   1 +
> >  rust/helpers/dma_fence.c        |  23 ++
> >  rust/helpers/helpers.c          |   1 +
> >  rust/helpers/spinlock.c         |   5 +
> >  rust/kernel/sync.rs             |   2 +
> >  rust/kernel/sync/dma_fence.rs   | 388 ++++++++++++++++++++++++++++++++
> 
> I missed this part, and I don't think kernel::sync is where dma_fence
> should be, as kernel::sync is mostly for the basic synchronization
> between threads/irqs. dma_fence is probably better to be grouped with
> dma-buf and other dma related primitives. Maybe in kernel::dma? Like:
> 
> rust/kernel/dma.rs
> rust/kernel/dma/dma_buf.rs
> rust/kernel/dma/dma_fence.rs
> 
> Thoughts? Miguel, Greg, Danilo and Lyude, any idea or suggestion?

@Christian König's opinion would be valuable, too.

I'm not super convinced of that because dma_fence has not really much
to do with DMA. They're not very different from completions and are a
mechanism to synchronize consumers and producers.

Actually, before f54d1867005c3 they were just called "fence" and then
renamed to "dma_fence" because someone wanted that name.


Anyways, I don't have strong objections and mostly care about having
them available somewhere.

P.

> 
> Regards,
> Boqun
> 
> >  6 files changed, 420 insertions(+)
> >  create mode 100644 rust/helpers/dma_fence.c
> >  create mode 100644 rust/kernel/sync/dma_fence.rs
> > 
> [...]

Powered by blists - more mailing lists