Re: [PATCH v9 4/6] rust: enable `clippy::as_underscore` lint

[Boqun Feng <boqun.feng@gmail.com>]

On Fri, Apr 18, 2025 at 08:08:02AM -0400, Tamir Duberstein wrote:
> On Thu, Apr 17, 2025 at 4:12 PM Boqun Feng <boqun.feng@gmail.com> wrote:
> >
> > On Thu, Apr 17, 2025 at 03:26:14PM -0400, Tamir Duberstein wrote:
> > [...]
> > > >
> > > > >          Ok(())
> > > > >      }
> > > > > diff --git a/rust/kernel/device_id.rs b/rust/kernel/device_id.rs
> > > > > index e5859217a579..4063f09d76d9 100644
> > > > > --- a/rust/kernel/device_id.rs
> > > > > +++ b/rust/kernel/device_id.rs
> > > > > @@ -82,7 +82,7 @@ impl<T: RawDeviceId, U, const N: usize> IdArray<T, U, N> {
> > > > >              unsafe {
> > > > >                  raw_ids[i]
> > > > >                      .as_mut_ptr()
> > > > > -                    .byte_offset(T::DRIVER_DATA_OFFSET as _)
> > > > > +                    .byte_add(T::DRIVER_DATA_OFFSET)
> > > > >                      .cast::<usize>()
> > > > >                      .write(i);
> > > > >              }
> > > > > diff --git a/rust/kernel/devres.rs b/rust/kernel/devres.rs
> > > > > index f7e8f5f53622..70d12014e476 100644
> > > > > --- a/rust/kernel/devres.rs
> > > > > +++ b/rust/kernel/devres.rs
> > > > > @@ -45,7 +45,7 @@ struct DevresInner<T> {
> > > > >  /// # Example
> > > > >  ///
> > > > >  /// ```no_run
> > > > > -/// # use kernel::{bindings, c_str, device::Device, devres::Devres, io::{Io, IoRaw}};
> > > > > +/// # use kernel::{bindings, c_str, device::Device, devres::Devres, ffi::c_void, io::{Io, IoRaw}};
> > > > >  /// # use core::ops::Deref;
> > > > >  ///
> > > > >  /// // See also [`pci::Bar`] for a real example.
> > > > > @@ -59,19 +59,19 @@ struct DevresInner<T> {
> > > > >  ///     unsafe fn new(paddr: usize) -> Result<Self>{
> > > > >  ///         // SAFETY: By the safety requirements of this function [`paddr`, `paddr` + `SIZE`) is
> > > > >  ///         // valid for `ioremap`.
> > > > > -///         let addr = unsafe { bindings::ioremap(paddr as _, SIZE as _) };
> > > > > +///         let addr = unsafe { bindings::ioremap(paddr as bindings::phys_addr_t, SIZE) };
> > > >
> > > >
> > > > ///         let addr = unsafe { bindings::ioremap(bindings::phys_addr_t::from(paddr), SIZE) };
> > > >
> > > > better? Or even with .into()
> > > >
> > > > ///         let addr = unsafe { bindings::ioremap(paddr.into(), SIZE) };
> > >
> > > This doesn't compile because `paddr` is usize, and
> > > `bindings::phys_addr_t` is u64 (on my machine, which is aarch64).
> > >
> >
> > Ok, looks like Rust yet doesn't provide From/Into between usize and u64
> > even if the pointer size is fixed. Latest discussion can be found at:
> >
> >         https://github.com/rust-lang/rust/issues/41619#issuecomment-2056902943
> >
> > Lemme see if we can get an issue tracking this. Thanks for taking a
> > look.
> >
> > > > >  ///         if addr.is_null() {
> > > > >  ///             return Err(ENOMEM);
> > > > >  ///         }
> > > > >  ///
> > > > > -///         Ok(IoMem(IoRaw::new(addr as _, SIZE)?))
> > > > > +///         Ok(IoMem(IoRaw::new(addr as usize, SIZE)?))
> > > > >  ///     }
> > > > >  /// }
> > > > >  ///
> > > > >  /// impl<const SIZE: usize> Drop for IoMem<SIZE> {
> > > > >  ///     fn drop(&mut self) {
> > > > >  ///         // SAFETY: `self.0.addr()` is guaranteed to be properly mapped by `Self::new`.
> > > > > -///         unsafe { bindings::iounmap(self.0.addr() as _); };
> > > > > +///         unsafe { bindings::iounmap(self.0.addr() as *mut c_void); };
> > > > >  ///     }
> > > > >  /// }
> > > > >  ///
> > > > [...]
> > > > > diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
> > > > > index 43ecf3c2e860..851a6339aa90 100644
> > > > > --- a/rust/kernel/dma.rs
> > > > > +++ b/rust/kernel/dma.rs
> > > > > @@ -38,7 +38,7 @@
> > > > >  impl Attrs {
> > > > >      /// Get the raw representation of this attribute.
> > > > >      pub(crate) fn as_raw(self) -> crate::ffi::c_ulong {
> > > > > -        self.0 as _
> > > > > +        self.0 as crate::ffi::c_ulong
> > > >
> > > >         crate::ffi::c_ulong::from(self.0)
> > > >
> > > > maybe, a C unsigned long should always be able to hold the whole `Attr`
> > > > and a lossly casting is what this function does.
> > >
> > > This also doesn't compile: "the trait `core::convert::From<u32>` is
> > > not implemented for `usize`". Upstream has ambitions of running on
> > > 16-bit, I guess :)
> > >
> >
> > They do but they also have the target_pointer_width cfg, so they can
> > totally provide these functions. It's just they want to find a better
> > way (like the link I post above).
> 
> Did you want me to hold off on the respin on this point, or shall I go ahead?

No need to wait. This doesn't affect your current patch. But we do need
to start making some decisions about how we handle the conversions *32
=> *size, *size => *64 and c*long <=> *size, they should all be lossless
in the context of kernel. We have 3 options:

1.	Using `as`.
2.	Having our own to_*size(*32), to_*64(*size), to_*size(*64),
	to_c*long(*size) and to_*size(c*long) helper functions.
3.	Waiting and using what Rust upstream comes up.

I'm leaning towards to 2 and then 3, because using `as` can sometimes
surprise you when you change the type. Thoughts?

Regards,
Boqun