linux-kernel - Re: [PATCH v2 1/2] rust: introduce abstractions for fwctl

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Message-Id: <DG070VH4Z7B1.26DSDJL3VQVYA@kernel.org>
Date: Wed, 28 Jan 2026 12:41:41 +0100
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Zhi Wang" <zhiw@...dia.com>
Cc: "Jason Gunthorpe" <jgg@...dia.com>, <rust-for-linux@...r.kernel.org>,
 <linux-pci@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
 <aliceryhl@...gle.com>, <bhelgaas@...gle.com>, <kwilczynski@...nel.org>,
 <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>, <tmgross@...ch.edu>, <markus.probst@...teo.de>,
 <helgaas@...nel.org>, <cjia@...dia.com>, <smitra@...dia.com>,
 <ankita@...dia.com>, <aniketa@...dia.com>, <kwankhede@...dia.com>,
 <targupta@...dia.com>, <acourbot@...dia.com>, <joelagnelf@...dia.com>,
 <jhubbard@...dia.com>, <zhiwang@...nel.org>, <daniel.almeida@...labora.com>
Subject: Re: [PATCH v2 1/2] rust: introduce abstractions for fwctl

On Wed Jan 28, 2026 at 12:36 PM CET, Zhi Wang wrote:
> On Tue, 27 Jan 2026 21:07:37 +0100
> "Danilo Krummrich" <dakr@...nel.org> wrote:
>
>> On Tue Jan 27, 2026 at 8:57 PM CET, Zhi Wang wrote:
>> > The fwctl_alloc_device() helper allocates a raw struct fwctl_device
>> > without private driver data here. The Rust driver object should be
>> > already allocated and initialized separately before reaching this
>> > point.
>> >
>> > We rely on the standard dev->parent chain to access the rust driver
>> > object from the fwctl callbacks.
>> 
>> (I will go for a thorough review soon, but for now a quick drive-by
>> comment.)
>> 
>> IIUC, you are saying that the user is supposed to use the private data
>> of the parent device in fwctl callbacks. Let's not make this a design
>> choice please. Instead, allow the user pass in separate private data for
>> the fwctl device as well.
>> 
>> This serves the purpose of clear ownership and lifetime of the data.
>> E.g. the fwctl device does not necessarily exist as long as the parent
>> device is bound.
>> 
>> It is a good thing if driver authors are forced to take a decision about
>> which object owns the data and what's the scope of the data.
>
> I wrote a version like this before. My initial concern of mixing Rust
> objects together with C objecs within C-allocated memory was about
> potential memory alignment issues when rust side doing CAST on the memory.
>
> I agree that providing a way to attach private data directly to the
> fwctl_device also has quite some benetifs.
>
> IMO, if we go this way, the private data from rust side needs to have
> #[repr(C)] to address the above issue all the time?

No that's not necessary. Please have a look at what I did in drm::Device::new()
[1], this should be the exact same case.

[1] https://rust.docs.kernel.org/src/kernel/drm/device.rs.html#98