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

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Message-ID: <bbc60912-7ed5-4bb1-b273-8061abe49e10@arm.com>
Date: Thu, 16 Jan 2025 13:57:50 +0000
From: Robin Murphy <robin.murphy@....com>
To: Danilo Krummrich <dakr@...nel.org>, Christoph Hellwig <hch@....de>
Cc: 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 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.

Thanks,
Robin.

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