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

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Message-ID: <CAH5fLgiW6n82hMhbmJ-ynd6329mEDFX6TzxXQZaaoG47HAWWcA@mail.gmail.com>
Date: Mon, 27 Jan 2025 12:34:33 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Daniel Almeida <daniel.almeida@...labora.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.

On Mon, Jan 27, 2025 at 12:00 PM Daniel Almeida
<daniel.almeida@...labora.com> wrote:
>
>
> >
> > 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)?

The difference is exactly that - you reduce the amount of data copied
to just the size of the field.

Alice