Re: [RFC PATCH 1/2] rust: add initial scatterlist bindings

[Danilo Krummrich <dakr@kernel.org>]

On Fri, May 16, 2025 at 01:57:59PM -0300, Daniel Almeida wrote:
> Hi Danilo, 
> 
> Replying to you and Lyude here at the same time.
> 
> > On 15 May 2025, at 18:11, Danilo Krummrich <dakr@kernel.org> wrote:
> > 
> > On Mon, May 12, 2025 at 08:39:36AM -0300, Daniel Almeida wrote:
> >>> On 12 May 2025, at 06:53, Abdiel Janulgue <abdiel.janulgue@gmail.com> wrote:
> >>> +impl SGEntry {
> >>> +    /// Convert a raw `struct scatterlist *` to a `&'a SGEntry`.
> >>> +    ///
> >>> +    /// # Safety
> >>> +    ///
> >>> +    /// Callers must ensure that the `struct scatterlist` pointed to by `ptr` is valid for the lifetime
> >>> +    /// of the returned reference.
> >>> +    pub unsafe fn as_ref<'a>(ptr: *mut bindings::scatterlist) -> &'a Self {
> >>> +        // SAFETY: The pointer is valid and guaranteed by the safety requirements of the function.
> >>> +        unsafe { &*ptr.cast() }
> >>> +    }
> >> 
> >> Hmm, this name is not good. When people see as_ref() they will think of the
> >> standard library where it is used to convert from &T to &U. This is not what is
> >> happening here. Same in other places where as_ref() is used in this patch.
> > 
> > as_ref() is fine, we use this exact way commonly in the kernel, e.g. for Device,
> > GlobalLockedBy, Cpumask and for various DRM types.
> > 
> > Rust std does the same, e.g. in [1].
> > 
> > I think I also asked for this in your Resource patch for consistency, where you
> > call this from_ptr() instead.
> > 
> > [1] https://doc.rust-lang.org/std/ptr/struct.NonNull.html#method.as_ref
> > 
> 
> That is not the same thing. What you've linked to still takes &self and returns
> &T.

Fair enough. :)

> So, IMHO, the problem is not conflicting with the std AsRef, in the sense that the
> code might not compile because of it. The problem is taking a very well
> known name and then changing its semantics.

I don't see the problem, a function signature is always read as a whole, in this
case:

	fn as_ref<'a>(ptr: *mut bindings::foo) -> &'a Self

which reads as "take this pointer and give me a reference", i.e. "as ref".

Anyways, I'm not very much opinionated about the exact name, I care about
consistency.

So, if you feel like we should name it differently, please also change the
existing functions in the kernel.

> Anyways, this is just a small observation. I'll drop my case, specially
> considering that the current as_ref() is already prevalent in a lot of code upstream :)
> 
> — Daniel
>