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Message-ID: <1c13843f-6929-4bc6-8564-0ec2d964c41d@arm.com>
Date: Thu, 5 Dec 2024 13:07:14 +0000
From: Robin Murphy <robin.murphy@....com>
To: Abdiel Janulgue <abdiel.janulgue@...il.com>, daniel.almeida@...labora.com
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>, Alice Ryhl <aliceryhl@...gle.com>,
Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>,
Valentin Obst <kernel@...entinobst.de>,
"open list:RUST" <rust-for-linux@...r.kernel.org>,
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 v6 2/2] rust: add dma coherent allocator abstraction.
On 05/12/2024 11:32 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.
>
> Reviewed-by: Daniel Almeida <daniel.almeida@...labora.com>
> Tested-by: Daniel Almeida <daniel.almeida@...labora.com>
> Signed-off-by: Abdiel Janulgue <abdiel.janulgue@...il.com>
> ---
> rust/bindings/bindings_helper.h | 1 +
> rust/kernel/dma.rs | 136 ++++++++++++++++++++++++++++++++
> rust/kernel/lib.rs | 1 +
> 3 files changed, 138 insertions(+)
> create mode 100644 rust/kernel/dma.rs
>
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> index 5c4dfe22f41a..49bf713b9bb6 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/kernel/dma.rs b/rust/kernel/dma.rs
> new file mode 100644
> index 000000000000..dd725c571ff7
> --- /dev/null
> +++ b/rust/kernel/dma.rs
> @@ -0,0 +1,136 @@
> +// 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,
> + types::ARef,
> + transmute::{AsBytes, FromBytes},
> +};
> +
> +/// An abstraction of the `dma_alloc_coherent` API.
> +///
> +/// This is an abstraction around the `dma_alloc_coherent` API which is used to allocate and map
> +/// large consistent DMA regions.
> +///
> +/// A [`CoherentAllocation`] instance contains a pointer to the allocated region (in the
> +/// processor's virtual address space) and the device address which can be given to the device
> +/// as the DMA address base of the region. The region is released once [`CoherentAllocation`]
> +/// is dropped.
> +///
> +/// # Invariants
> +///
> +/// For the lifetime of an instance of [`CoherentAllocation`], the cpu address is a valid pointer
> +/// to an allocated region of consistent memory and we hold a reference to the device.
> +pub struct CoherentAllocation<T: AsBytes + FromBytes> {
> + dev: ARef<Device>,
> + dma_handle: bindings::dma_addr_t,
> + count: usize,
> + cpu_addr: *mut T,
> +}
> +
> +impl<T: AsBytes + FromBytes> CoherentAllocation<T> {
> + /// Allocates a region of `size_of::<T> * count` of consistent memory.
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// use kernel::device::Device;
> + /// use kernel::dma::CoherentAllocation;
> + ///
> + /// # fn test(dev: &Device) -> Result {
> + /// let c: CoherentAllocation<u64> = CoherentAllocation::alloc_coherent(dev, 4, GFP_KERNEL)?;
> + /// # Ok::<(), Error>(()) }
> + /// ```
> + pub fn alloc_coherent(
> + dev: &Device,
> + count: usize,
> + flags: kernel::alloc::Flags,
Might it be worth adding at least a placeholder type and argument for
"attrs" as well, so there's less churn involved in coming back and
adding it later? Or is the intention to have a separate
dma_alloc_attrs() constructor for such users?
(I assume adding an mmap method as and when needed won't affect any
existing users, so is fine not to worry about right now)
Thanks,
Robin.
> + ) -> Result<CoherentAllocation<T>> {
> + build_assert!(core::mem::size_of::<T>() > 0,
> + "It doesn't make sense for the allocated type to be a ZST");
> +
> + let size = count.checked_mul(core::mem::size_of::<T>()).ok_or(EOVERFLOW)?;
> + let mut dma_handle = 0;
> + // SAFETY: device pointer is guaranteed as valid by invariant on `Device`.
> + // We ensure that we catch the failure on this function and throw an ENOMEM
> + let ret = unsafe {
> + bindings::dma_alloc_attrs(
> + dev.as_raw(),
> + size,
> + &mut dma_handle, flags.as_raw(),
> + 0,
> + )
> + };
> + if ret.is_null() {
> + return Err(ENOMEM)
> + }
> + // INVARIANT: We just successfully allocated a coherent region which is accessible for
> + // `count` elements, hence the cpu address is valid. We also hold a refcounted reference
> + // to the device.
> + Ok(Self {
> + dev: dev.into(),
> + dma_handle,
> + count,
> + cpu_addr: ret as *mut T,
> + })
> + }
> +
> + /// Returns the base address to the allocated region and the dma handle. The caller takes
> + /// ownership of the returned resources.
> + pub fn into_parts(self) -> (usize, bindings::dma_addr_t) {
> + let ret = (self.cpu_addr as _, self.dma_handle);
> + core::mem::forget(self);
> + ret
> + }
> +
> + /// Returns the base address to the allocated region in the CPU's virtual address space.
> + pub fn start_ptr(&self) -> *const T {
> + self.cpu_addr as _
> + }
> +
> + /// Returns the base address to the allocated region in the CPU's virtual address space as
> + /// a mutable pointer.
> + pub fn start_ptr_mut(&mut self) -> *mut T {
> + self.cpu_addr
> + }
> +
> + /// Returns a DMA handle which may given to the device as the DMA address base of
> + /// the region.
> + pub fn dma_handle(&self) -> bindings::dma_addr_t {
> + self.dma_handle
> + }
> +
> + /// Returns the CPU-addressable region as a slice.
> + pub fn cpu_buf(&self) -> &[T]
> + {
> + // SAFETY: The pointer is valid due to type invariant on `CoherentAllocation` and
> + // is valid for reads for `self.count * size_of::<T>` bytes.
> + unsafe { core::slice::from_raw_parts(self.cpu_addr, self.count) }
> + }
> +
> + /// Performs the same functionality as `cpu_buf`, except that a mutable slice is returned.
> + pub fn cpu_buf_mut(&mut self) -> &mut [T]
> + {
> + // SAFETY: The pointer is valid due to type invariant on `CoherentAllocation` and
> + // is valid for reads for `self.count * size_of::<T>` bytes.
> + unsafe { core::slice::from_raw_parts_mut(self.cpu_addr, self.count) }
> + }
> +}
> +
> +impl<T: AsBytes + FromBytes> Drop for CoherentAllocation<T> {
> + fn drop(&mut self) {
> + let size = self.count * core::mem::size_of::<T>();
> + // SAFETY: the device, cpu address, and the dma handle is valid due to the
> + // type invariants on `CoherentAllocation`.
> + unsafe { bindings::dma_free_attrs(self.dev.as_raw(), size,
> + self.cpu_addr as _,
> + self.dma_handle, 0) }
> + }
> +}
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index e1065a7551a3..6e90ebf5a130 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -35,6 +35,7 @@
> mod build_assert;
> pub mod cred;
> pub mod device;
> +pub mod dma;
> pub mod error;
> #[cfg(CONFIG_RUST_FW_LOADER_ABSTRACTIONS)]
> pub mod firmware;
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