Re: [RFC 1/7] rust: pci: expose sriov_get_totalvfs() helper

[Dirk Behme <dirk.behme@gmail.com>]

On 06.12.25 13:42, 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@nvidia.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> {
> +        // SAFETY: `self.as_raw()` is a valid pointer to a `struct pci_dev`.
> +        let vfs = unsafe { bindings::pci_sriov_get_totalvfs(self.as_raw()) };
> +
> +        if vfs != 0 {
> +            Ok(vfs)
> +        } else {
> +            Err(ENODEV)
> +        }

In the thread [1] there was some discussion about the `if {} else {}`
"style". From that discussion I "distilled" 6 options [2] which I
liked for having an overview :) Of course not all of these applied
there (const), neither will they here. And all have pros and cons. I
think in the end option #4 was selected.

What's about to do something similar here (and in the 2/7 patch as well)?

if vfs == 0 {
    return Err(ENODEV);
}

Ok(vfs)

Dirk

[1]
https://lore.kernel.org/rust-for-linux/CANiq72kiscT5euAUjcSzvxMzM9Hdj8aQGeUN_pVF-vHf3DhBuQ@mail.gmail.com/

[2] Options distilled from the thread [1]:

1.

if let Some(sum) = addr.checked_add(PAGE_SIZE - 1) {
    return Some(sum & PAGE_MASK);
}
None


2.

addr.checked_add(PAGE_SIZE - 1).map(|sum| sum & PAGE_MASK)


3.

if let Some(sum) = addr.checked_add(PAGE_SIZE - 1) {
   Some(sum & PAGE_MASK);
} else {
   None
}


4.

let Some(sum) = addr.checked_add(PAGE_SIZE - 1) else {
    return None;
};

Some(sum & PAGE_MASK)


5.

match addr.checked_add(PAGE_SIZE - 1) {
  Some(v) => Some(v & PAGE_MASK),
  None => None,
}

6.

Some(addr.checked_add(PAGE_SIZE - 1)? & PAGE_MASK)