linux-kernel - Re: [PATCH v13 2/7] rust: add dma coherent allocator abstraction.

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Message-ID: <Z92jPXvMOqhgv04k@pollux>
Date: Fri, 21 Mar 2025 18:34:53 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Abdiel Janulgue <abdiel.janulgue@...il.com>,
	rust-for-linux@...r.kernel.org, daniel.almeida@...labora.com,
	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>,
	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 v13 2/7] rust: add dma coherent allocator abstraction.

On Fri, Mar 21, 2025 at 02:23:53PM -0300, Jason Gunthorpe wrote:
> On Fri, Mar 07, 2025 at 01:06:19PM +0200, Abdiel Janulgue wrote:
> 
> > +        // SAFETY: Device pointer is guaranteed as valid by the type invariant on `Device`.
> > +        let ret = unsafe {
> > +            bindings::dma_alloc_attrs(
> > +                dev.as_raw(),
> > +                size,
> > +                &mut dma_handle,
> > +                gfp_flags.as_raw(),
> > +                dma_attrs.as_raw(),
> > +            )
> 
> This is not the correct safety statement, the device must have a driver
> bound to call this function, a struct device reference is not
> sufficient.
> 
> I belive Danilo was suggesting to ignore this unsafety for now, but if
> so it should be documented correctly.

If just landed patches [1], which are the foundation of addressing this issue.

With the next cycle, this will be ensured by the type system.

> 
> Also think the use of devres here is going to be very problematic for
> drivers to use as I said in other emails. :(

In an earlier reply today in a different thread already gave you the link [2] of
what we landed, which, besides explaining the situation, also makes clear that
there is *no* Devres wrapper around a CoherentAllocation and why.

[1] https://lore.kernel.org/lkml/20250314160932.100165-1-dakr@kernel.org/
[2] https://github.com/Rust-for-Linux/linux/blob/rust-next/rust/kernel/dma.rs#L120