linux-kernel - Re: [RFC 1/7] rust: pci: expose sriov_get

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Message-Id: <DETDILPA1GFY.27WND0TEC5352@nvidia.com>
Date: Tue, 09 Dec 2025 12:42:19 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Zhi Wang" <zhiw@...dia.com>, <rust-for-linux@...r.kernel.org>,
 <linux-pci@...r.kernel.org>, <nouveau@...ts.freedesktop.org>,
 <linux-kernel@...r.kernel.org>
Cc: <airlied@...il.com>, <dakr@...nel.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>, <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>, "Nouveau"
 <nouveau-bounces@...ts.freedesktop.org>
Subject: Re: [RFC 1/7] rust: pci: expose sriov_get_totalvfs() helper

On Sat Dec 6, 2025 at 9:42 PM JST, Zhi Wang wrote:
> Add a wrapper for the `pci_sriov_get_totalvfs()` helper, allowing drivers
> to query the number of total SR-IOV virtual functions a PCI device
> supports.
>
> This is useful for components that need to conditionally enable features
> based on SR-IOV capability.
>
> Signed-off-by: Zhi Wang <zhiw@...dia.com>
> ---
>  rust/kernel/pci.rs | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/rust/kernel/pci.rs b/rust/kernel/pci.rs
> index 7fcc5f6022c1..9a82e83dfd30 100644
> --- a/rust/kernel/pci.rs
> +++ b/rust/kernel/pci.rs
> @@ -514,6 +514,18 @@ pub fn pci_class(&self) -> Class {
>          // SAFETY: `self.as_raw` is a valid pointer to a `struct pci_dev`.
>          Class::from_raw(unsafe { (*self.as_raw()).class })
>      }
> +
> +    /// Returns total number of VFs, or `Err(ENODEV)` if none available.
> +    pub fn sriov_get_totalvfs(&self) -> Result<i32> {

The return type if `pci_sriov_get_totalvfs` is a bit unfortunate. It
returns `driver_max_VFs`, which is a `u16` [1], but not before
implicitly converting it to an `int` - even though it doesn't make use
of negative values to indicate errors!

Even its C callers eventually compare its returned value to a `u16` [2].

How about a cleanup patch to fix it to return `u16`, so we can make the
Rust wrapper return a `Result<NonZero<u16>>`?

[1] https://elixir.bootlin.com/linux/v6.18/source/drivers/pci/iov.c#L1281
[2] https://elixir.bootlin.com/linux/v6.18/source/drivers/pci/iov.c#L474