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

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Message-ID: <20250321182901.GQ126678@ziepe.ca>
Date: Fri, 21 Mar 2025 15:29:01 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Danilo Krummrich <dakr@...nel.org>
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 06:34:53PM +0100, Danilo Krummrich wrote:
> 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.

Those patches say:

 The context types can be extended as required, e.g. to limit availability  of
 certain (bus) device functions to probe().

Which is not an appropriate limitation for dma_alloc_coherent, we
expect it to be called outside probe in real drivers. Is there more to
that story?

Regardless, the safety comment should not be merged with incorrect
information. :\

> > 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.

Yes, I see, I have so much email right now it is hard find all the
different versions of everything.

Jason