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

[Andreas Hindborg <a.hindborg@kernel.org>]

"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<B>, 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