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

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Message-ID: <20260128133651.365d876a.zhiw@nvidia.com>
Date: Wed, 28 Jan 2026 13:36:51 +0200
From: Zhi Wang <zhiw@...dia.com>
To: Danilo Krummrich <dakr@...nel.org>
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>,
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	<helgaas@...nel.org>, <cjia@...dia.com>, <smitra@...dia.com>,
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	<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 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?

Z.