linux-kernel - Re: [RFC 2/7] [!UPSTREAM] rust: pci: support configuration space access

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Message-ID: <h5pp4aqaglql5gak2ni7pw2jrcuecjaog3fmeu72p4kbcn6vgf@pzfiofsvoxe3>
Date: Wed, 10 Dec 2025 23:51:10 +0100
From: Ewan CHORYNSKI <ewan.chorynski@...me>
To: Zhi Wang <zhiw@...dia.com>
Cc: rust-for-linux@...r.kernel.org, linux-pci@...r.kernel.org, 
	nouveau@...ts.freedesktop.org, linux-kernel@...r.kernel.org, airlied@...il.com, dakr@...nel.org, 
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	markus.probst@...teo.de, helgaas@...nel.org, cjia@...dia.com, alex@...zbot.org, 
	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
Subject: Re: [RFC 2/7] [!UPSTREAM] rust: pci: support configuration space
 access

On Sat, Dec 06, 2025 at 12:42:03PM +0000, Zhi Wang wrote:
> +    /// Find the extended capability
> +    pub fn find_ext_capability(&self, cap: i32) -> Option<u16> {
> +        // SAFETY: `self.as_raw()` is a valid pointer to a `struct pci_dev`.
> +        let offset = unsafe { bindings::pci_find_ext_capability(self.as_raw(), cap) };
> +        if offset != 0 {
> +            Some(offset as u16)
> +        } else {
> +            None
> +        }
> +    }

This is a good candidate to use `Option<NonZeroU16>` for the return
type as you don't expect Some(0) to be valid. It will also enable niche
optimisation for the Option. The whole if statement can even be replaced
by `NonZeroU16::new(offset as 16)` which return an option directly.