linux-kernel - Re: [PATCH v15 02/11] rust: add dma coherent allocator abstraction.

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Message-ID: <CANiq72k_2dknw9CWj=yxMeBBrvMjPp5mhu=H++BP_wTVCHtnew@mail.gmail.com>
Date: Fri, 21 Mar 2025 00:54:52 +0100
From: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: Abdiel Janulgue <abdiel.janulgue@...il.com>, rust-for-linux@...r.kernel.org, 
	daniel.almeida@...labora.com, dakr@...nel.org, robin.murphy@....com, 
	aliceryhl@...gle.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>, Trevor Gross <tmgross@...ch.edu>, 
	Valentin Obst <kernel@...entinobst.de>, linux-kernel@...r.kernel.org, 
	Christoph Hellwig <hch@....de>, Marek Szyprowski <m.szyprowski@...sung.com>, airlied@...hat.com, 
	iommu@...ts.linux.dev
Subject: Re: [PATCH v15 02/11] rust: add dma coherent allocator abstraction.

On Tue, Mar 18, 2025 at 2:13 PM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>
> The wording of in the other doc comments is different than this one and
> the two next ones. Consider aligning them:
>
>   Hints the DMA-mapping subsystem that ...
>
>   Tells the DMA-mapping subsystem ...
>
>   Indicates that the buffer ...

These come from the C side and are copy-pasted, so I suspect that was
the reason -- they may want to have them in sync, so I didn't touch
them.

> I think it would be nice to have a description of what a coherent DMA
> region is here. From Documentation/core-api/dma-api.rst:
>
>   Consistent memory is memory for which a write by either the device or
>   the processor can immediately be read by the processor or device
>   without having to worry about caching effects.  (You may however need
>   to make sure to flush the processor's write buffers before telling
>   devices to read that memory.)
>
> I think consistent and coherent are used interchangeably in the kernel.
> If so, can we settle on just one of the terms in the rust code?

Abdiel has the diff ready for this, so he may send the patch (or you),
but for the moment I decided to minimize changes -- up to the
maintainers/reviewers now :)

> > +// DMA allocations potentially carry device resources (e.g.IOMMU mappings), hence for soundness

Nit: I think this should have a space after e.g. or a comma -- I
didn't touch it, and it is a TODO anyway, but before I forget I
mention it here.

Thanks!

Cheers,
Miguel