linux-kernel - Re: [PATCH v2 1/5] rust: dma: implement DataDirection

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Message-Id: <DC9RED1FAC80.B7PYIPB9H5OO@nvidia.com>
Date: Sat, 23 Aug 2025 20:09:46 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Danilo Krummrich" <dakr@...nel.org>, <akpm@...ux-foundation.org>,
 <ojeda@...nel.org>, <alex.gaynor@...il.com>, <boqun.feng@...il.com>,
 <gary@...yguo.net>, <bjorn3_gh@...tonmail.com>, <lossin@...nel.org>,
 <a.hindborg@...nel.org>, <aliceryhl@...gle.com>, <tmgross@...ch.edu>,
 <abdiel.janulgue@...il.com>, <jgg@...pe.ca>, <lyude@...hat.com>,
 <robin.murphy@....com>, <daniel.almeida@...labora.com>
Cc: <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/5] rust: dma: implement DataDirection

On Thu Aug 21, 2025 at 1:52 AM JST, Danilo Krummrich wrote:
> Add the `DataDirection` struct, a newtype wrapper around the C
> `enum dma_data_direction`.
>
> This provides a type-safe Rust interface for specifying the direction of
> DMA transfers.
>
> Signed-off-by: Danilo Krummrich <dakr@...nel.org>

Reviewed-by: Alexandre Courbot <acourbot@...dia.com>

One very very minor comment below.

> ---
>  rust/bindings/bindings_helper.h |  1 +
>  rust/kernel/dma.rs              | 68 +++++++++++++++++++++++++++++++++
>  2 files changed, 69 insertions(+)
>
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index 0e140e07758b..c2cc52ee9945 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -47,6 +47,7 @@
>  #include <linux/cpumask.h>
>  #include <linux/cred.h>
>  #include <linux/device/faux.h>
> +#include <linux/dma-direction.h>
>  #include <linux/dma-mapping.h>
>  #include <linux/errname.h>
>  #include <linux/ethtool.h>
> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
> index 2bc8ab51ec28..5daba00ecc78 100644
> --- a/rust/kernel/dma.rs
> +++ b/rust/kernel/dma.rs
> @@ -244,6 +244,74 @@ pub mod attrs {
>      pub const DMA_ATTR_PRIVILEGED: Attrs = Attrs(bindings::DMA_ATTR_PRIVILEGED);
>  }
>  
> +/// DMA data direction.
> +///
> +/// Corresponds to the C [`enum dma_data_direction`].
> +///
> +/// [`enum dma_data_direction`]: srctree/include/linux/dma-direction.h
> +#[derive(Copy, Clone, PartialEq, Eq, Debug)]
> +#[repr(u32)]
> +pub enum DataDirection {
> +    /// The DMA mapping is for bidirectional data transfer.
> +    ///
> +    /// This is used when the buffer can be both read from and written to by the device.
> +    /// The cache for the corresponding memory region is both flushed and invalidated.
> +    Bidirectional = Self::const_cast(bindings::dma_data_direction_DMA_BIDIRECTIONAL),
> +
> +    /// The DMA mapping is for data transfer from memory to the device (write).
> +    ///
> +    /// The CPU has prepared data in the buffer, and the device will read it.
> +    /// The cache for the corresponding memory region is flushed.

Is flushed when? :) The comment for `FromDevice` specifies the timing
for cache invalidation, so it would be nice to have the same here imho.