linux-kernel - Re: [PATCH v6 1/5] rust: pci: provide access to PCI Class and Class-related items

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Message-ID: <2c551130-4f2a-45ec-8191-1b14d14ddd77@nvidia.com>
Date: Mon, 25 Aug 2025 19:22:56 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Alexandre Courbot <acourbot@...dia.com>,
 Danilo Krummrich <dakr@...nel.org>
Cc: 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 1/5] rust: pci: provide access to PCI Class and
 Class-related items

On 8/25/25 5:17 AM, Alexandre Courbot wrote:
>> +        /// Convert a raw 24-bit class code value to a `Class`.
>> +        impl From<u32> for Class {
>> +            fn from(value: u32) -> Self {
>> +                match value {
>> +                    $(x if x == Self::$variant.0 => Self::$variant,)+
>> +                    _ => Self::UNKNOWN,
>> +                }
>> +            }
> 
> Should we normalize `value` to 24 bits (i.e. call `to_24bit_class`)
> before doing the match? The constants we compare against are all
> normalized, but if we pass a 16-bit class to this method the result will
> be `UNKNOWN`, unless I missed something.
> 
> Being able to store a class as either a 16-bit or 24-bit representation
> in the same type also opens the door to bugs, which we can avoid if we
> always normalize to 24-bit and make the class/subclass representation
> accessible through a convenience method only.

It's constrained by the PCI ID patterns, and should only be used in
order to get values from the C bindings, actually.

> 
>> +        }
>> +    };
>> +}
>> +
>> +/// Once constructed, a `Class` contains a valid PCI Class code.
>> +impl Class {
>> +    /// Create a new Class from a raw 24-bit class code.
>> +    pub fn new(class_code: u32) -> Self {
>> +        Self::from(class_code)
>> +    }
> 
> Do we need a `new` method when the `From` implementation does exactly
> the same thing and has the same signature?
> 

Looks like I'll remove the From, and only provide new, based on the other
thread.

thanks,
-- 
John Hubbard