linux-kernel - Re: [PATCH v13 4/7] rust: device: add dma addressing capabilities

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Message-ID: <877c5038i1.fsf@kernel.org>
Date: Fri, 07 Mar 2025 21:12:38 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Abdiel Janulgue" <abdiel.janulgue@...il.com>
Cc: <rust-for-linux@...r.kernel.org>,  <daniel.almeida@...labora.com>,
  <dakr@...nel.org>,  <robin.murphy@....com>,  <aliceryhl@...gle.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" <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 v13 4/7] rust: device: add dma addressing capabilities

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

> Add functions to set the DMA mask to inform the kernel about the
> device's DMA addressing capabilities.
>
> Signed-off-by: Abdiel Janulgue <abdiel.janulgue@...il.com>
> ---
>  rust/helpers/dma.c     |  8 ++++++++
>  rust/helpers/helpers.c |  1 +
>  rust/kernel/device.rs  | 29 +++++++++++++++++++++++++++++
>  3 files changed, 38 insertions(+)
>  create mode 100644 rust/helpers/dma.c
>
> diff --git a/rust/helpers/dma.c b/rust/helpers/dma.c
> new file mode 100644
> index 000000000000..8eb482386f93
> --- /dev/null
> +++ b/rust/helpers/dma.c
> @@ -0,0 +1,8 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/dma-mapping.h>
> +
> +int rust_helper_dma_set_mask_and_coherent(struct device *dev, u64 mask)
> +{
> +	return dma_set_mask_and_coherent(dev, mask);
> +}
> 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/device.rs b/rust/kernel/device.rs
> index db2d9658ba47..f9d3d4f60ddb 100644
> --- a/rust/kernel/device.rs
> +++ b/rust/kernel/device.rs
> @@ -6,10 +6,12 @@
>
>  use crate::{
>      bindings,
> +    error::Result,
>      str::CStr,
>      types::{ARef, Opaque},
>  };
>  use core::{fmt, ptr};
> +use kernel::prelude::*;
>
>  #[cfg(CONFIG_PRINTK)]
>  use crate::c_str;
> @@ -187,6 +189,33 @@ pub fn property_present(&self, name: &CStr) -> bool {
>          // SAFETY: By the invariant of `CStr`, `name` is null-terminated.
>          unsafe { bindings::device_property_present(self.as_raw().cast_const(), name.as_char_ptr()) }
>      }
> +
> +    /// Inform the kernel about the device's DMA addressing capabilities.
> +    ///
> +    /// Set both the DMA mask and the coherent DMA mask to the same thing.
> +    /// Note that we don't check the return value from the C `dma_set_coherent_mask`
> +    /// as the DMA API guarantees that the coherent DMA mask can be set to
> +    /// the same or smaller than the streaming DMA mask.
> +    pub fn dma_set_mask_and_coherent(&mut self, mask: u64) -> Result {
> +        // SAFETY: device pointer is guaranteed as valid by invariant on `Device`.
> +        let ret = unsafe { bindings::dma_set_mask_and_coherent(self.as_raw(), mask) };
> +        if ret != 0 {
> +            Err(Error::from_errno(ret))
> +        } else {
> +            Ok(())
> +        }
> +    }

I think we can use `Error::from_errno` here (and below). As far as I can
tell, these C functions return negative on error.

> +
> +    /// Same as [`dma_set_mask_and_coherent`], but set the mask only for streaming mappings.
> +    pub fn dma_set_mask(&mut self, mask: u64) -> Result {
> +        // SAFETY: device pointer is guaranteed as valid by invariant on `Device`.
> +        let ret = unsafe { bindings::dma_set_mask(self.as_raw(), mask) };
> +        if ret != 0 {
> +            Err(Error::from_errno(ret))
> +        } else {
> +            Ok(())
> +        }
> +    }
>  }
>
>  // SAFETY: Instances of `Device` are always reference-counted.


Best regards,
Andreas Hindborg