linux-kernel - Re: [PATCH v8 2/2] rust: add dma coherent allocator abstraction.

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Message-ID: <Z4kshDHw8NEjQONR@pollux>
Date: Thu, 16 Jan 2025 16:57:56 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Robin Murphy <robin.murphy@....com>
Cc: Christoph Hellwig <hch@....de>,
	Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
	Abdiel Janulgue <abdiel.janulgue@...il.com>,
	daniel.almeida@...labora.com, aliceryhl@...gle.com,
	rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Trevor Gross <tmgross@...ch.edu>,
	Valentin Obst <kernel@...entinobst.de>,
	open list <linux-kernel@...r.kernel.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>, airlied@...hat.com,
	"open list:DMA MAPPING HELPERS" <iommu@...ts.linux.dev>,
	Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v8 2/2] rust: add dma coherent allocator abstraction.

On Thu, Jan 16, 2025 at 01:57:50PM +0000, Robin Murphy wrote:
> On 2025-01-16 1:17 pm, Danilo Krummrich wrote:
> > On Fri, Jan 10, 2025 at 11:41:54AM +0100, Danilo Krummrich wrote:
> > > On Fri, Jan 10, 2025 at 09:39:55AM +0100, Christoph Hellwig wrote:
> > > > On Thu, Jan 09, 2025 at 09:49:47AM +0100, Danilo Krummrich wrote:
> > > > > On Thu, Jan 09, 2025 at 09:08:12AM +0100, Christoph Hellwig wrote:
> > > > > > On Wed, Jan 08, 2025 at 04:21:33PM +0100, Danilo Krummrich wrote:
> > > > > > > What does "your code" mean? Duplicated in every driver?
> > > > > > 
> > > > > > Yes, interfaces to the DMA API should stay in readable C code and not
> > > > > > in weird bindings so that it reminds greppable and maintainable.
> > > > > > 
> > > > > 
> > > > > Rust drivers shouldn't use C APIs directly, but rather use an abstraction of the
> > > > > corresponding C API.
> > > > 
> > > > Don't force me to deal with your shiny language of the day.
> > > 
> > > Again, no one asks you to deal with or maintain this piece of Rust code.
> > > 
> > > > Maintaining
> > > > multi-language projects is a pain I have no interest in dealing with.
> > > > If you want to use something that's not C, be that assembly or rust you
> > > > write to C interfaces and deal with the impedence mismatch yourself as
> > > > far as I'm concerned.
> > > > 
> > > 
> > > This is exactly what we're doing and proposing here, isn't it?
> > > 
> > > We wrote a single piece of Rust code that abstracts the C API for all Rust
> > > drivers, which we offer to maintain ourselves.
> > > 
> > > What else are you asking for?
> > 
> > Since there hasn't been a reply so far, I assume that we're good with
> > maintaining the DMA Rust abstractions separately.
> 
> Indeed, FWIW it seems like the appropriate level of abstraction to me,
> judging by the other wrappers living in rust/kernel/ already. As far as the
> interaction with C code goes, it appears to be a pretty straightforward
> midlayer consumer of the DMA API much like others we already have (e.g.
> videobuf2-dma-*), just one which happens to be a language binding rather
> than some other kind of functional abstraction.
> 
> There is a realistic chance that the C API will evolve in ways which break
> the binding, but as long as a) that won't break non-Rust builds, and b) Rust
> folks are happy to take responsibility for un-breaking the Rust build if and
> when it happens, then that seems reasonable IMO.

Surely you can expect maintainers of the Rust abstraction to help with
integrating API changes -- this isn't different compared to driver / component
maintainers helping with integrating fundamental API changes for their affected
driver / component, like you've mentioned videobuf2-dma stuff.

At last year's LPC I held a talk [1] about Rust in the kernel and there was a
question at the end where I was asked how I think about cases where Rust
abstraction break caused C API changes.

My answer was that I'm not too concerned about this. We can just rely on what
already happens every day in kernel development: people work together and
collaborate. There are a lot of very core components that are widely used and
depending on the complexity of the change may require the help of the users to
integrate changes. So, I don't think with Rust abstractions we're adding
anything that the kernel does not already has a strategy to deal with.

- Danilo

[1] https://youtu.be/3Igmx28B3BQ?si=wD0CP-zku4U6fAsN

> 
> Thanks,
> Robin.
> 
> > 
> > Hence, the next version of this patch series will have the corresponding
> > maintainer entry.
> > 
> > - Danilo
>