linux-kernel - Re: [PATCH v4 3/3] rust: Add dma

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Message-ID: <14af50d2-f759-4d89-ab9e-0afc7f9cb280@amd.com>
Date: Fri, 12 Sep 2025 10:25:21 +0200
From: Christian König <christian.koenig@....com>
To: Lyude Paul <lyude@...hat.com>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Cc: Daniel Almeida <daniel.almeida@...labora.com>,
 Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>,
 Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
 Danilo Krummrich <dakr@...nel.org>, Sumit Semwal <sumit.semwal@...aro.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Viresh Kumar <viresh.kumar@...aro.org>,
 Wedson Almeida Filho <wedsonaf@...il.com>,
 Tamir Duberstein <tamird@...il.com>,
 Xiangfei Ding <dingxiangfei2009@...il.com>,
 "open list:DMA BUFFER SHARING FRAMEWORK:Keyword:bdma_(?:buf|fence|resv)b"
 <linux-media@...r.kernel.org>,
 "moderated list:DMA BUFFER SHARING FRAMEWORK:Keyword:bdma_(?:buf|fence|resv)b"
 <linaro-mm-sig@...ts.linaro.org>
Subject: Re: [PATCH v4 3/3] rust: Add dma_buf stub bindings

On 12.09.25 00:57, Lyude Paul wrote:
> In order to implement the gem export callback, we need a type to represent
> struct dma_buf. So - this commit introduces a set of stub bindings for
> dma_buf. These bindings provide a ref-counted DmaBuf object, but don't
> currently implement any functionality for using the DmaBuf.

Especially the last sentence is a bit problematic.

Wrapping a DMA-buf object should be pretty easy, the hard part is the operations on the DMA-buf object.

E.g. how are locking and sg_table creation handled?

Regards,
Christian.

> 
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> Reviewed-by: Daniel Almeida <daniel.almeida@...labora.com>
> 
> ---
> V3:
> * Rename as_ref() to from_raw()
> V4:
> * Add missing period to rustdoc at top of file
> 
>  rust/kernel/dma_buf.rs | 40 ++++++++++++++++++++++++++++++++++++++++
>  rust/kernel/lib.rs     |  1 +
>  2 files changed, 41 insertions(+)
>  create mode 100644 rust/kernel/dma_buf.rs
> 
> diff --git a/rust/kernel/dma_buf.rs b/rust/kernel/dma_buf.rs
> new file mode 100644
> index 0000000000000..50be3e4dd4098
> --- /dev/null
> +++ b/rust/kernel/dma_buf.rs
> @@ -0,0 +1,40 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! DMA buffer API.
> +//!
> +//! C header: [`include/linux/dma-buf.h`](srctree/include/linux/dma-buf.h)
> +
> +use bindings;
> +use kernel::types::*;
> +
> +/// A DMA buffer object.
> +///
> +/// # Invariants
> +///
> +/// The data layout of this type is equivalent to that of `struct dma_buf`.
> +#[repr(transparent)]
> +pub struct DmaBuf(Opaque<bindings::dma_buf>);
> +
> +// SAFETY: `struct dma_buf` is thread-safe
> +unsafe impl Send for DmaBuf {}
> +// SAFETY: `struct dma_buf` is thread-safe
> +unsafe impl Sync for DmaBuf {}
> +
> +#[expect(unused)]
> +impl DmaBuf {
> +    /// Convert from a `*mut bindings::dma_buf` to a [`DmaBuf`].
> +    ///
> +    /// # Safety
> +    ///
> +    /// The caller guarantees that `self_ptr` points to a valid initialized `struct dma_buf` for the
> +    /// duration of the lifetime of `'a`, and promises to not violate rust's data aliasing rules
> +    /// using the reference provided by this function.
> +    pub(crate) unsafe fn from_raw<'a>(self_ptr: *mut bindings::dma_buf) -> &'a Self {
> +        // SAFETY: Our data layout is equivalent to `dma_buf` .
> +        unsafe { &*self_ptr.cast() }
> +    }
> +
> +    pub(crate) fn as_raw(&self) -> *mut bindings::dma_buf {
> +        self.0.get()
> +    }
> +}
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index fcffc3988a903..59242d83efe21 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -81,6 +81,7 @@
>  pub mod device_id;
>  pub mod devres;
>  pub mod dma;
> +pub mod dma_buf;
>  pub mod driver;
>  #[cfg(CONFIG_DRM = "y")]
>  pub mod drm;