linux-kernel - Re: [PATCH v12 2/3] rust: add dma coherent allocator abstraction.

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Message-ID: <87h64al5c6.fsf@kernel.org>
Date: Mon, 03 Mar 2025 12:30:01 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Abdiel Janulgue" <abdiel.janulgue@...il.com>
Cc: <aliceryhl@...gle.com>,  <dakr@...nel.org>,  <robin.murphy@....com>,
  <daniel.almeida@...labora.com>,  <rust-for-linux@...r.kernel.org>,
  "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" <benno.lossin@...ton.me>,  "Trevor Gross" <tmgross@...ch.edu>,
  "Valentin Obst" <kernel@...entinobst.de>,
  <linux-kernel@...r.kernel.org>,  "Christoph Hellwig" <hch@....de>,
  "Marek Szyprowski" <m.szyprowski@...sung.com>,  <airlied@...hat.com>,
  <iommu@...ts.linux.dev>
Subject: Re: [PATCH v12 2/3] rust: add dma coherent allocator abstraction.

"Abdiel Janulgue" <abdiel.janulgue@...il.com> writes:

> Add a simple dma coherent allocator rust abstraction. Based on
> Andreas Hindborg's dma abstractions from the rnvme driver, which
> was also based on earlier work by Wedson Almeida Filho.
>
> Nacked-by: Christoph Hellwig <hch@....de>
> Signed-off-by: Abdiel Janulgue <abdiel.janulgue@...il.com>
[...]

> diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
> index 0640b7e115be..8f3808c8b7fe 100644
> --- a/rust/helpers/helpers.c
> +++ b/rust/helpers/helpers.c
> @@ -13,6 +13,7 @@
>  #include "build_bug.c"
>  #include "cred.c"
>  #include "device.c"
> +#include "dma.c"
>  #include "err.c"
>  #include "fs.c"
>  #include "io.c"
> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
> new file mode 100644
> index 000000000000..b4dd5d411711
> --- /dev/null
> +++ b/rust/kernel/dma.rs
> @@ -0,0 +1,421 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Direct memory access (DMA).
> +//!
> +//! C header: [`include/linux/dma-mapping.h`](srctree/include/linux/dma-mapping.h)
> +
> +use crate::{
> +    bindings, build_assert,
> +    device::Device,
> +    error::code::*,
> +    error::Result,
> +    transmute::{AsBytes, FromBytes},
> +    types::ARef,
> +};
> +
> +/// Inform the kernel about the device's DMA addressing capabilities. This will set the mask for
> +/// both streaming and coherent APIs together.
> +pub fn dma_set_mask_and_coherent(dev: &Device, mask: u64) -> i32 {
> +    // SAFETY: device pointer is guaranteed as valid by invariant on `Device`.
> +    unsafe { bindings::dma_set_mask_and_coherent(dev.as_raw(), mask) }
> +}
> +
> +/// Same as `dma_set_mask_and_coherent`, but set the mask only for streaming mappings.
> +pub fn dma_set_mask(dev: &Device, mask: u64) -> i32 {
> +    // SAFETY: device pointer is guaranteed as valid by invariant on `Device`.
> +    unsafe { bindings::dma_set_mask(dev.as_raw(), mask) }
> +}

I'm rebasing some of the dma pool code I'm using for NVMe on top of
these patches, and I notice that these methods in the original code from
way back (besides being on Device) has these methods return `Result`:

    pub fn dma_set_mask(&self, mask: u64) -> Result {
        let dev = self.as_raw();
        let ret = unsafe { bindings::dma_set_mask(dev as _, mask) };
        if ret != 0 {
            Err(Error::from_errno(ret))
        } else {
            Ok(())
        }
    }

Is there a reason for not returning a `Result` in this series?


Best regards,
Andreas Hindborg