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

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Message-ID: <20251209162258.541a2a5b@inno-ThinkPad-X280>
Date: Tue, 9 Dec 2025 16:22:58 +0200
From: Zhi Wang <zhiw@...dia.com>
To: Dirk Behme <dirk.behme@...il.com>
CC: <rust-for-linux@...r.kernel.org>, <linux-pci@...r.kernel.org>,
	<nouveau@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>,
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Subject: Re: [RFC 1/7] rust: pci: expose sriov_get_totalvfs() helper

On Sun, 7 Dec 2025 08:12:10 +0100
Dirk Behme <dirk.behme@...il.com> wrote:

> On 06.12.25 13:42, Zhi Wang wrote:

snip

> 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
>

Hey Dirk:

Thanks for the detailed summary! As a C programmer before, I tend to do
as above, because it improves a lot of readability on success path.
While playing with rust, I tend to use 5). Personally, I try to stay
away from if let chains, as some lifecycle changes happen in rust 2024
[1]. It wouldn't bite right now, but stay away from the bumps. :)

As Miguel's comment in another thread, I would improve this as
discussed.

[1]
https://doc.rust-lang.org/edition-guide/rust-2024/temporary-if-let-scope.html
 
> [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)
>