linux-kernel - Re: [PATCH v11 2/3] rust: add dma coherent allocator abstraction.

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Message-Id: <0E0FC88B-95C0-4DBE-B497-AF9101BACE70@collabora.com>
Date: Mon, 27 Jan 2025 07:59:49 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Abdiel Janulgue <abdiel.janulgue@...il.com>,
 rust-for-linux@...r.kernel.org,
 dakr@...nel.org,
 robin.murphy@....com,
 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>,
 Christoph Hellwig <hch@....de>,
 Marek Szyprowski <m.szyprowski@...sung.com>,
 airlied@...hat.com,
 "open list:DMA MAPPING HELPERS" <iommu@...ts.linux.dev>
Subject: Re: [PATCH v11 2/3] rust: add dma coherent allocator abstraction.

> 
> You were asked to rename this function because it returns a slice, but
> I wonder if it's better to take an `&mut [T]` argument and to have
> this function copy data into that argument. That way, we could make
> the function itself safe. Perhaps the actual copy needs to be
> volatile?
> 
> Well ... I understand that we did this previously and that we want to
> avoid it because it causes too much reading if T is a struct and we
> just want to read one of its fields. How about an API like this?
> 
> dma_read!(my_alloc[7].foo)
> 
> which expands to something that reads the value of the foo field of
> the 7th element, and
> 
> dma_write!(my_alloc[7].foo = 13);
> 
> That expands to something that writes 13 to field foo of the 7th element.
> 
> Thoughts? I'm proposing this to avoid going in circles between the
> same solutions.
> 
> Alice
> 

I think I missed something here. How is this any different from the *safe*
functions we currently have?

Anything that involves a copy is basically what we have already, although
your version seems to reduce the amount of data copied from sizeof(T) to
sizeof(T.field)?