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Date: Thu, 29 Jan 2026 22:08:02 +0100
Message-Id: <DG1DP20EZCTK.2YW5G4C198BQT@kernel.org>
From: "Danilo Krummrich" <dakr@kernel.org>
Subject: Re: [PATCH v4 5/7] rust: io: add `register!` macro
Cc: "Alexandre Courbot" <acourbot@nvidia.com>, "Alice Ryhl"
 <aliceryhl@google.com>, "Daniel Almeida" <daniel.almeida@collabora.com>,
 "Miguel Ojeda" <ojeda@kernel.org>, "Boqun Feng" <boqun.feng@gmail.com>,
 =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, "Benno Lossin"
 <lossin@kernel.org>, "Andreas Hindborg" <a.hindborg@kernel.org>, "Trevor
 Gross" <tmgross@umich.edu>, "Yury Norov" <yury.norov@gmail.com>, "John
 Hubbard" <jhubbard@nvidia.com>, "Alistair Popple" <apopple@nvidia.com>,
 "Joel Fernandes" <joelagnelf@nvidia.com>, "Timur Tabi" <ttabi@nvidia.com>,
 "Edwin Peer" <epeer@nvidia.com>, "Eliot Courtney" <ecourtney@nvidia.com>,
 "Dirk Behme" <dirk.behme@de.bosch.com>, "Steven Price"
 <steven.price@arm.com>, <rust-for-linux@vger.kernel.org>,
 <linux-kernel@vger.kernel.org>
To: "Gary Guo" <gary@garyguo.net>
References: <20260128-register-v4-0-aee3a33d9649@nvidia.com>
 <20260128-register-v4-5-aee3a33d9649@nvidia.com>
 <DG0CVJN57L5F.2I6J4RXMJOGQ@garyguo.net>
 <DG0WY7S2QPQM.37S6PROC464QI@nvidia.com>
 <DG14TFQG2C13.2BFREQR1DV4I@garyguo.net>
In-Reply-To: <DG14TFQG2C13.2BFREQR1DV4I@garyguo.net>

On Thu Jan 29, 2026 at 3:10 PM CET, Gary Guo wrote:
> The issue with `Deref` bound here is that now you *require* a level of
> indirection. If something implements `Io` directly, you cannot use it wit=
h the
> method. While `&Bar` is accepted, `&Mmio` is not because `Mmio` is a dire=
ct
> implementor of `Io` and not deref to it.
>
> A `Deref` bound also does not help if, say, a type is `Arc<Bar>` which ne=
eds two
> level of dereffing before it is Io. For consistency I think it's best to =
avoid
> `Deref` call all together

I agree with that and we will probably run into this once we start implemen=
ting
the I/O traits for things like DMA memory.

We could use AsRef or a custom trait instead, but I'd rather leave this for=
 a
subsequent series, as it requires more changes. They should also be transpa=
rent
to users.