Re: [PATCH v2 3/3] rust: dma.rs: Migrate to zerocopy's `Frombytes/IntoBytes`

["Gary Guo" <gary@garyguo.net>]

On Sun Sep 6, 2026 at 11:29 AM BST, Sagar Taunk wrote:
> Replace explicit imports of `transmute::FromBytes`/`AsBytes` with
> zerocopy's equivalents.
>
> Link: https://github.com/Rust-for-Linux/linux/issues/1241
> Signed-off-by: Sagar Taunk <sagartaunk@proton.me>

I am surprised that just changing this is sufficient. Are all users of
`CoherentBox` already have the both zerocopy and kernel traits implemented?

Best,
Gary

> ---
>  rust/kernel/dma.rs | 22 +++++++++-------------
>  1 file changed, 9 insertions(+), 13 deletions(-)
>
> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
> index 2ce09f8e90c6..9bd47f20c784 100644
> --- a/rust/kernel/dma.rs
> +++ b/rust/kernel/dma.rs
> @@ -25,10 +25,6 @@
>      prelude::*,
>      ptr::KnownSize,
>      sync::aref::ARef,
> -    transmute::{
> -        AsBytes,
> -        FromBytes, //
> -    },
>      uaccess::UserSliceWriter, //
>  };
>  use core::{
> @@ -417,7 +413,7 @@ fn from(direction: DataDirection) -> Self {
>  /// ```
>  pub struct CoherentBox<T: KnownSize + ?Sized>(Coherent<T>);
>  
> -impl<T: AsBytes + FromBytes> CoherentBox<[T]> {
> +impl<T: IntoBytes + FromBytes> CoherentBox<[T]> {
>      /// [`CoherentBox`] variant of [`Coherent::zeroed_slice_with_attrs`].
>      #[inline]
>      pub fn zeroed_slice_with_attrs(
> @@ -454,7 +450,7 @@ pub fn init_at<E>(&mut self, i: usize, init: impl Init<T, E>) -> Result
>  
>          // SAFETY:
>          // - `ptr` is valid, properly aligned, and within this allocation.
> -        // - `T: AsBytes + FromBytes` guarantees all bit patterns are valid, so partial writes on
> +        // - `T: IntoBytes + FromBytes` guarantees all bit patterns are valid, so partial writes on
>          //   error cannot leave the element in an invalid state.
>          // - The DMA address has not been exposed yet, so there is no concurrent device access.
>          unsafe { pin_init::raw_try_init(ptr, init)? };
> @@ -523,7 +519,7 @@ pub fn from_slice(
>      }
>  }
>  
> -impl<T: AsBytes + FromBytes> CoherentBox<T> {
> +impl<T: IntoBytes + FromBytes> CoherentBox<T> {
>      /// Same as [`CoherentBox::zeroed_slice_with_attrs`], but for a single element.
>      #[inline]
>      pub fn zeroed_with_attrs(
> @@ -554,7 +550,7 @@ fn deref(&self) -> &Self::Target {
>      }
>  }
>  
> -impl<T: AsBytes + FromBytes + KnownSize + ?Sized> DerefMut for CoherentBox<T> {
> +impl<T: IntoBytes + FromBytes + KnownSize + ?Sized> DerefMut for CoherentBox<T> {
>      #[inline]
>      fn deref_mut(&mut self) -> &mut Self::Target {
>          // SAFETY:
> @@ -565,7 +561,7 @@ fn deref_mut(&mut self) -> &mut Self::Target {
>      }
>  }
>  
> -impl<T: AsBytes + FromBytes + KnownSize + ?Sized> From<CoherentBox<T>> for Coherent<T> {
> +impl<T: IntoBytes + FromBytes + KnownSize + ?Sized> From<CoherentBox<T>> for Coherent<T> {
>      #[inline]
>      fn from(value: CoherentBox<T>) -> Self {
>          value.0
> @@ -663,7 +659,7 @@ pub unsafe fn as_mut(&self) -> &mut T {
>      }
>  }
>  
> -impl<T: AsBytes + FromBytes> Coherent<T> {
> +impl<T: IntoBytes + FromBytes> Coherent<T> {
>      /// Allocates a region of `T` of coherent memory.
>      fn alloc_with_attrs(
>          dev: &device::Device<Bound>,
> @@ -753,7 +749,7 @@ pub fn init_with_attrs<E>(
>          // SAFETY:
>          // - `ptr` is valid, properly aligned, and points to exclusively owned memory.
>          // - If `raw_try_init` fails, `self` is dropped, which safely frees the underlying
> -        //   `Coherent`'s DMA memory. `T: AsBytes + FromBytes` ensures there are no complex `Drop`
> +        //   `Coherent`'s DMA memory. `T: IntoBytes + FromBytes` ensures there are no complex `Drop`
>          //   requirements we are bypassing.
>          unsafe { pin_init::raw_try_init(ptr, init)? };
>  
> @@ -948,9 +944,9 @@ unsafe impl<T: KnownSize + Send + ?Sized> Send for Coherent<T> {}
>  // methods that access the buffer contents (`field_read`, `field_write`, `as_slice`,
>  // `as_slice_mut`) are `unsafe`, and callers are responsible for ensuring no data races occur.
>  // The safe methods only return metadata or raw pointers whose use requires `unsafe`.
> -unsafe impl<T: KnownSize + ?Sized + AsBytes + FromBytes + Sync> Sync for Coherent<T> {}
> +unsafe impl<T: KnownSize + ?Sized + IntoBytes + FromBytes + Sync> Sync for Coherent<T> {}
>  
> -impl<T: KnownSize + AsBytes + ?Sized> debugfs::BinaryWriter for Coherent<T> {
> +impl<T: KnownSize + IntoBytes + ?Sized> debugfs::BinaryWriter for Coherent<T> {
>      fn write_to_slice(
>          &self,
>          writer: &mut UserSliceWriter,