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

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Message-ID: <fdfb8200-1b07-4dba-9659-ef25a1391f00@arm.com>
Date: Mon, 3 Mar 2025 13:08:49 +0000
From: Robin Murphy <robin.murphy@....com>
To: Abdiel Janulgue <abdiel.janulgue@...il.com>, aliceryhl@...gle.com,
 dakr@...nel.org, daniel.almeida@...labora.com, rust-for-linux@...r.kernel.org
Cc: 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>,
 Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>,
 Valentin Obst <kernel@...entinobst.de>,
 open list <linux-kernel@...r.kernel.org>, Christoph Hellwig <hch@....de>,
 Marek Szyprowski <m.szyprowski@...sung.com>, airlied@...hat.com,
 "open list:DMA MAPPING HELPERS" <iommu@...ts.linux.dev>
Subject: Re: [PATCH v12 2/3] rust: add dma coherent allocator abstraction.

On 24/02/2025 11:49 am, Abdiel Janulgue wrote:
> 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>
> ---
>   rust/bindings/bindings_helper.h |   1 +
>   rust/helpers/dma.c              |  13 +
>   rust/helpers/helpers.c          |   1 +
>   rust/kernel/dma.rs              | 421 ++++++++++++++++++++++++++++++++
>   rust/kernel/lib.rs              |   1 +
>   5 files changed, 437 insertions(+)
>   create mode 100644 rust/helpers/dma.c
>   create mode 100644 rust/kernel/dma.rs
> 
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index 55354e4dec14..f69b05025e52 100644
> --- a/rust/bindings/bindings_helper.h
> +++ b/rust/bindings/bindings_helper.h
> @@ -11,6 +11,7 @@
>   #include <linux/blk_types.h>
>   #include <linux/blkdev.h>
>   #include <linux/cred.h>
> +#include <linux/dma-mapping.h>
>   #include <linux/errname.h>
>   #include <linux/ethtool.h>
>   #include <linux/file.h>
> diff --git a/rust/helpers/dma.c b/rust/helpers/dma.c
> new file mode 100644
> index 000000000000..30da079d366c
> --- /dev/null
> +++ b/rust/helpers/dma.c
> @@ -0,0 +1,13 @@
> +// 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);
> +}
> +
> +int rust_helper_dma_set_mask(struct device *dev, u64 mask)
> +{
> +	return dma_set_mask(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/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) }
> +}

It's a bit funny that dma_set_coherent_mask() is the only one you really 
need at this point (with only the coherent API), yet it's the one that's 
been left out.

FWIW, also +1 to the consensus that the DMA masks are, both logically 
and functionally, properties of the device, so making these Device 
methods seems sensible.

Thanks,
Robin.