linux-kernel - Re: [PATCH v6 2/5] rust: pci: provide access to PCI Vendor values

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Message-ID: <54b19bdc-5d88-4f71-ad8e-886847ccee8a@kernel.org>
Date: Tue, 26 Aug 2025 22:45:52 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: John Hubbard <jhubbard@...dia.com>
Cc: Alexandre Courbot <acourbot@...dia.com>,
 Joel Fernandes <joelagnelf@...dia.com>, Timur Tabi <ttabi@...dia.com>,
 Alistair Popple <apopple@...dia.com>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, 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>,
 nouveau@...ts.freedesktop.org, linux-pci@...r.kernel.org,
 rust-for-linux@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
 Elle Rhumsaa <elle@...thered-steel.dev>
Subject: Re: [PATCH v6 2/5] rust: pci: provide access to PCI Vendor values

On 8/26/25 10:38 PM, John Hubbard wrote:
> On 8/25/25 5:47 AM, Danilo Krummrich wrote:
>> On Mon Aug 25, 2025 at 2:33 PM CEST, Alexandre Courbot wrote:
> ...
>>> Naive question from someone with a device tree background and almost no
>>> PCI experience: one consequence of using `From` here is that if I create
>>> an non-registered Vendor value (e.g. `let vendor =
>>> Vendor::from(0xf0f0)`), then do `vendor.as_raw()`, I won't get the value
>>> passed initially but the one for `UNKNOWN`, e.g. `0xffff`. Are we ok
>>> with this?
>>
>> I think that's fine, since we shouldn't actually hit this. Drivers should only
>> ever use the pre-defined constants of Vendor; consequently the
>> Device::vendor_id() can't return UNKNOWN either.
>>
>> So, I think the From impl is not ideal, since we can't limit its visibility. In
>> order to improve this, I suggest to use Vendor::new() directly in the macro, and
>> make Vendor::new() private. The same goes for Class, I guess.
> 
> Correction: when I went to implement this, I discovered that there is a better
> way, which addresses both Alex's and your concerns.
> 
> The incremental diff below shows how. It provides:
> 
> a) .from_raw(), which in this case matches conventions slightly better
>     than new(). (I'm still learning that the Rust way is a bit different
>     that the C++ way! haha).
> 
> b) Only the parent module (in this case, that's pci:: ) can call
>     Class::from_raw(). This is exactly what we need. Fully private methods
>     wouldn't work, but leaving it open for any caller to construct a
>     Class item is also a problem.

Sorry, that's on me being not precise. When I said private I meant private to
the parent module.

The diff looks good, thanks!

Please also make sure to add #[inline] where appropriate and rebase onto
driver-core-next.