Re: [PATCH 1/3] rust: uaccess: generalize write_dma() to accept any Coherent<T>

[Gary Guo]

On Wed Mar 25, 2026 at 12:39 AM GMT, Danilo Krummrich wrote:
> Generalize write_dma() from &Coherent<[u8]> to &Coherent<T> where
> T: KnownSize + AsBytes + ?Sized. The function body only uses as_ptr()
> and size(), which work for any such T, so there is no reason to
> restrict it to byte slices.

I think there is a reason, which is that it weakens the strong typing that we
have. I think it'll be better to have this be

    fn write_dma<T>(&mut self, view: View<'_, Coherent<T>, [u8]>, offset: usize) -> Result

once we have I/O view and expect people to create cast views into bytes
explicitly.

Although, given that we already have APIs like

    fn write<T: AsBytes>(&mut self, value: &T)

I'm okay with putting this in until we have a replacement solution.

The fact that our current uaccess mechanism is untyped itself is something that
I dislike (it feels a step back that C can represent `arbitary_type __user*` and
we just have a `void __user*`). It's on my list of things to fix to make a
generic `UserPtr<T>`.

Best,
Gary

>
> Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx>
> ---
>  rust/kernel/uaccess.rs | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/uaccess.rs b/rust/kernel/uaccess.rs
> index e26ef90ba8ad..6c9c1cce3c63 100644
> --- a/rust/kernel/uaccess.rs
> +++ b/rust/kernel/uaccess.rs
> @@ -12,6 +12,7 @@
>      ffi::{c_char, c_void},
>      fs::file,
>      prelude::*,
> +    ptr::KnownSize,
>      transmute::{AsBytes, FromBytes},
>  };
>  use core::mem::{size_of, MaybeUninit};
> @@ -524,7 +525,12 @@ pub fn write_slice(&mut self, data: &[u8]) -> Result {
>      ///     writer.write_dma(alloc, 0, 256)
>      /// }
>      /// ```
> -    pub fn write_dma(&mut self, alloc: &Coherent<[u8]>, offset: usize, count: usize) -> Result {
> +    pub fn write_dma<T: KnownSize + AsBytes + ?Sized>(
> +        &mut self,
> +        alloc: &Coherent<T>,
> +        offset: usize,
> +        count: usize,
> +    ) -> Result {
>          let len = alloc.size();
>          if offset.checked_add(count).ok_or(EOVERFLOW)? > len {
>              return Err(ERANGE);
>
> base-commit: dff8302ca1d0e773c90dbeeb05e759f995c95482