linux-kernel - Re: [PATCH 1/2] rust: Add dma_set_mask() and dma_set_coherent

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Message-Id: <DB6KV5SVWIYG.2FAIFGZ90ZR2I@kernel.org>
Date: Tue, 08 Jul 2025 11:48:36 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Alistair Popple" <apopple@...dia.com>
Cc: <rust-for-linux@...r.kernel.org>, "Bjorn Helgaas" <bhelgaas@...gle.com>,
 Krzysztof Wilczyński <kwilczynski@...nel.org>, "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"
 <lossin@...nel.org>, "Andreas Hindborg" <a.hindborg@...nel.org>, "Alice
 Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>, "Greg
 Kroah-Hartman" <gregkh@...uxfoundation.org>, "Rafael J. Wysocki"
 <rafael@...nel.org>, "John Hubbard" <jhubbard@...dia.com>, "Alexandre
 Courbot" <acourbot@...dia.com>, <linux-pci@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] rust: Add dma_set_mask() and
 dma_set_coherent_mask() bindings

On Tue Jul 8, 2025 at 10:40 AM CEST, Danilo Krummrich wrote:
> On Tue Jul 8, 2025 at 8:04 AM CEST, Alistair Popple wrote:
>> Add bindings to allow setting the DMA masks for both a generic device
>> and a PCI device.
>
> Nice coincidence, I was about to get back to this. I already implemented this in
> a previous patch [1], but didn't apply it yet.
>
> I think the approach below is thought a bit too simple:
>
>   (1) We want the DMA mask methods to be implemented by a trait in dma.rs.
>       Subsequently, the trait should only be implemented by bus devices where
>       the bus actually supports DMA. Allowing to set the DMA mask on any device
>       doesn't make sense.

Forgot to mention, another reason for a trait is that we can also use it as a
trait bound on dma::CoherentAllocation::new(), such that people can't pass
arbitrary devices to dma::CoherentAllocation::new(), but only those that
actually sit on a DMA capable bus.

>
>   (2) We need to consider that with this we do no prevent
>       dma_set_coherent_mask() to concurrently with dma_alloc_coherent() (not
>       even if we'd add a new `Probe` device context).
>
> (2) is the main reason why I didn't follow up yet. So far I haven't found a nice
>     solution for a sound API that doesn't need unsafe.
>
> One thing I did consider was to have some kind of per device table (similar to
> the device ID table) for drivers to specify the DMA mask already at compile
> time. However, I'm pretty sure there are cases where the DMA mask has to derived
> dynamically from probe().
>
> I think I have to think a bit more about it.
>
> [1] https://lore.kernel.org/all/20250317185345.2608976-7-abdiel.janulgue@gmail.com/