Re: [PATCH 5/8] rust: dma: add methods to unsafely create reference from subview

Andreas Hindborg
2026-03-26T14:37:30Z

"Gary Guo" <gary@kernel.org> writes:

> From: Gary Guo <gary@garyguo.net>
>
> Implement `Io` for `Coherent`, so now `dma::Coherent` can be used with I/O
> projections.
>
> This allows the `as_ref()` and `as_mut()` API to be used in smaller region
> than the whole DMA allocation itself. For example, if a ring buffer is shared
> between GPU and CPU, users may now use the `io_project!` API to obtain a view
> of the buffer that is unified owned by the CPU and get a reference.

What do you mean by "unified owned by the CPU"?

>
> Signed-off-by: Gary Guo <gary@garyguo.net>
> ---
> rust/kernel/dma.rs | 56 ++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 56 insertions(+)
>
> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
> index 779d4babab9a..ae2939abc166 100644
> --- a/rust/kernel/dma.rs
> +++ b/rust/kernel/dma.rs
> @@ -12,6 +12,10 @@
> Core, //
> },
> error::to_result,
> + io::{
> + Io,
> + IoCapable, //
> + },
> prelude::*,
> ptr::KnownSize,
> sync::aref::ARef,
> @@ -864,6 +868,58 @@ fn drop(&mut self) {
> // can be sent to another thread.
> unsafe impl<T: KnownSize + Send + ?Sized> Send for Coherent<T> {}
>
> +impl<T: ?Sized + KnownSize> Io for Coherent<T> {
> + type Type = T;
> +
> + #[inline]
> + fn as_ptr(&self) -> *mut Self::Type {
> + self.as_mut_ptr()
> + }
> +}
> +
> +impl<'a, B: ?Sized + KnownSize, T: ?Sized> crate::io::View<'a, Coherent, T> {

Will this break orphan rule when we split kernel crate? Or will you put
`io` and `dma` together in some core crate?

> + /// Returns a DMA handle which may be given to the device as the DMA address base of
> + /// the region.
> + #[inline]
> + pub fn dma_handle(&self) -> DmaAddress {
> + let base = self.io();
> + let offset = self.as_ptr().addr() - base.as_ptr().addr();
> + base.dma_handle() + offset as DmaAddress

Do we need a // CAST: annotation?

Best regards,
Andreas Hindborg