linux-kernel - Re: [PATCH] rust: driver: let probe() return impl PinInit<Self, Error>

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Message-Id: <DDK8EYINVIKJ.2ZK04J2C06GGR@nvidia.com>
Date: Fri, 17 Oct 2025 11:12:29 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Danilo Krummrich" <dakr@...nel.org>, <gregkh@...uxfoundation.org>,
 <rafael@...nel.org>
Cc: <viresh.kumar@...aro.org>, <acourbot@...dia.com>, <ira.weiny@...el.com>,
 <leon@...nel.org>, <daniel.almeida@...labora.com>, <bhelgaas@...gle.com>,
 <kwilczynski@...nel.org>, <abdiel.janulgue@...il.com>,
 <robin.murphy@....com>, <ojeda@...nel.org>, <alex.gaynor@...il.com>,
 <boqun.feng@...il.com>, <gary@...yguo.net>, <bjorn3_gh@...tonmail.com>,
 <lossin@...nel.org>, <a.hindborg@...nel.org>, <aliceryhl@...gle.com>,
 <tmgross@...ch.edu>, <rust-for-linux@...r.kernel.org>,
 <linux-pci@...r.kernel.org>, <linux-pm@...r.kernel.org>,
 <nouveau@...ts.freedesktop.org>, <dri-devel@...ts.freedesktop.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rust: driver: let probe() return impl PinInit<Self,
 Error>

On Thu Oct 16, 2025 at 9:55 PM JST, Danilo Krummrich wrote:
> The driver model defines the lifetime of the private data stored in (and
> owned by) a bus device to be valid from when the driver is bound to a
> device (i.e. from successful probe()) until the driver is unbound from
> the device.
>
> This is already taken care of by the Rust implementation of the driver
> model. However, we still ask drivers to return a Result<Pin<KBox<Self>>>
> from probe().
>
> Unlike in C, where we do not have the concept of initializers, but
> rather deal with uninitialized memory, drivers can just return an
> impl PinInit<Self, Error> instead.
>
> This contributed to more clarity to the fact that a driver returns it's

nit: s/it's/its

> device private data in probe() and the Rust driver model owns the data,
> manages the lifetime and - considering the lifetime - provides (safe)
> accessors for the driver.
>
> Hence, let probe() functions return an impl PinInit<Self, Error> instead
> of Result<Pin<KBox<Self>>>.
>
> Signed-off-by: Danilo Krummrich <dakr@...nel.org>

Short anecdote: I was curious about measuring the footprint impact of
pin initializers, so I did a `size vmlinux` before and after this patch
to compare the size of the `text` section. This patch removes exactly 60
bytes of binary code, which I guess corresponds to the duplicated `KBox`
allocations that are now gone. It's great to confirm once again how Rust
abstractions are indeed zero-overhead! :)

Also, fwiw,

Reviewed-by: Alexandre Courbot <acourbot@...dia.com>