[PATCH v9 2/3] rust: add dma coherent allocator abstraction.

Abdiel Janulgue
2025-01-21T08:48:42Z

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.

Signed-off-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>
---
rust/bindings/bindings_helper.h | 1 +
rust/kernel/dma.rs | 272 ++++++++++++++++++++++++++++++++
rust/kernel/lib.rs | 1 +
3 files changed, 274 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..66d1bed887d3
--- /dev/null
+++ b/rust/kernel/dma.rs
@@ -0,0 +1,272 @@
+// 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,
+};
+
+/// Possible attributes associated with a DMA mapping.
+///
+/// They can be combined with the operators `|`, `&`, and `!`.
+///
+/// Values can be used from the [`attrs`] module.
+#[derive(Clone, Copy, PartialEq)]
+#[repr(transparent)]
+pub struct Attrs(u32);
+
+impl Attrs {
+ /// Get the raw representation of this attribute.
+ pub(crate) fn as_raw(self) -> crate::ffi::c_ulong {
+ self.0 as _
+ }
+
+ /// Check whether `flags` is contained in `self`.
+ pub fn contains(self, flags: Attrs) -> bool {
+ (self & flags) == flags
+ }
+}
+
+impl core::ops::BitOr for Attrs {
+ type Output = Self;
+ fn bitor(self, rhs: Self) -> Self::Output {
+ Self(self.0 | rhs.0)
+ }
+}
+
+impl core::ops::BitAnd for Attrs {
+ type Output = Self;
+ fn bitand(self, rhs: Self) -> Self::Output {
+ Self(self.0 & rhs.0)
+ }
+}
+
+impl core::ops::Not for Attrs {
+ type Output = Self;
+ fn not(self) -> Self::Output {
+ Self(!self.0)
+ }
+}
+
+/// DMA mapping attrributes.
+pub mod attrs {
+ use super::Attrs;
+
+ /// Specifies that reads and writes to the mapping may be weakly ordered, that is that reads
+ /// and writes may pass each other.
+ pub const DMA_ATTR_WEAK_ORDERING: Attrs = Attrs(bindings::DMA_ATTR_WEAK_ORDERING);
+
+ /// Specifies that writes to the mapping may be buffered to improve performance.
+ pub const DMA_ATTR_WRITE_COMBINE: Attrs = Attrs(bindings::DMA_ATTR_WRITE_COMBINE);
+
+ /// Lets the platform to avoid creating a kernel virtual mapping for the allocated buffer.
+ pub const DMA_ATTR_NO_KERNEL_MAPPING: Attrs = Attrs(bindings::DMA_ATTR_NO_KERNEL_MAPPING);
+
+ /// Allows platform code to skip synchronization of the CPU cache for the given buffer assuming
+ /// that it has been already transferred to 'device' domain.
+ pub const DMA_ATTR_SKIP_CPU_SYNC: Attrs = Attrs(bindings::DMA_ATTR_SKIP_CPU_SYNC);
+
+ /// Forces contiguous allocation of the buffer in physical memory.
+ pub const DMA_ATTR_FORCE_CONTIGUOUS: Attrs = Attrs(bindings::DMA_ATTR_FORCE_CONTIGUOUS);
+
+ /// This is a hint to the DMA-mapping subsystem that it's probably not worth the time to try
+ /// to allocate memory to in a way that gives better TLB efficiency.
+ pub const DMA_ATTR_ALLOC_SINGLE_PAGES: Attrs = Attrs(bindings::DMA_ATTR_ALLOC_SINGLE_PAGES);
+
+ /// This tells the DMA-mapping subsystem to suppress allocation failure reports (similarly to
+ /// __GFP_NOWARN).
+ pub const DMA_ATTR_NO_WARN: Attrs = Attrs(bindings::DMA_ATTR_NO_WARN);
+
+ /// Used to indicate that the buffer is fully accessible at an elevated privilege level (and
+ /// ideally inaccessible or at least read-only at lesser-privileged levels).
+ pub const DMA_ATTR_PRIVILEGED: Attrs = Attrs(bindings::DMA_ATTR_PRIVILEGED);
+}
+
+/// 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,
+ dma_attrs: Attrs,
+}
+
+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::{attrs::*, CoherentAllocation};
+ ///
+ /// # fn test(dev: &Device) -> Result {
+ /// let c: CoherentAllocation<u64> = CoherentAllocation::alloc_attrs(dev, 4, GFP_KERNEL,
+ /// DMA_ATTR_NO_WARN)?;
+ /// # Ok::<(), Error>(()) }
+ /// ```
+ pub fn alloc_attrs(
+ dev: &Device,
+ count: usize,
+ gfp_flags: kernel::alloc::Flags,
+ dma_attrs: Attrs,
+ ) -> 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,
+ gfp_flags.as_raw(),
+ dma_attrs.as_raw(),
+ )
+ };
+ 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,
+ dma_attrs,
+ })
+ }
+
+ /// Performs the same functionality as `alloc_attrs`, except the `dma_attrs` is 0 by default.
+ pub fn alloc_coherent(
+ dev: &Device,
+ count: usize,
+ gfp_flags: kernel::alloc::Flags,
+ ) -> Result<CoherentAllocation<T>> {
+ CoherentAllocation::alloc_attrs(dev, count, gfp_flags, Attrs(0))
+ }
+
+ /// Returns the base address, dma handle, attributes and the size of the allocated region.
+ /// The caller takes ownership of the returned resources, i.e., will have the responsibility
+ /// in calling `bindings::dma_free_attrs`.
+ pub fn into_parts(self) -> (*mut T, bindings::dma_addr_t, crate::ffi::c_ulong, usize) {
+ let size = self.count * core::mem::size_of::<T>();
+ let ret = (
+ self.cpu_addr,
+ self.dma_handle,
+ self.dma_attrs.as_raw(),
+ size,
+ );
+ // Drop the device's reference count associated with this object. This is needed as no
+ // destructor will be called on this object once this function returns.
+ // SAFETY: the device pointer is still valid as of this point due to the type invariants
+ // on `CoherentAllocation`.
+ unsafe { bindings::put_device(self.dev.as_raw()) }
+ 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
+ }
+
+ /// 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
+ }
+
+ /// Reads data from the region starting from `offset` as a slice.
+ /// `offset` and `count` are in units of `T`, not the number of bytes.
+ ///
+ /// Due to the safety requirements of slice, the data returned should be regarded by the
+ /// caller as a snapshot of the region when this function is called, as the region could
+ /// be modified by the device at anytime. For ringbuffer type of r/w access or use-cases
+ /// where the pointer to the live data is needed, `start_ptr()` or `start_ptr_mut()`
+ /// could be used instead.
+ ///
+ /// # Safety
+ ///
+ /// Callers must ensure that no hardware operations that involve the buffer are currently
+ /// taking place while the returned slice is live.
+ pub unsafe fn read(&self, offset: usize, count: usize) -> Result<&[T]> {
+ if offset + count >= self.count {
+ return Err(EINVAL);
+ }
+ // SAFETY: The pointer is valid due to type invariant on `CoherentAllocation`,
+ // we've just checked that the range and index is within bounds. The immutability of the
+ // of data is also guaranteed by the safety requirements of the function.
+ Ok(unsafe { core::slice::from_raw_parts(self.cpu_addr.wrapping_add(offset), count) })
+ }
+
+ /// Writes data to the region starting from `offset`. `offset` is in units of `T`, not the
+ /// number of bytes.
+ pub fn write(&self, src: &[T], offset: usize) -> Result {
+ if offset + src.len() >= self.count {
+ return Err(EINVAL);
+ }
+ // SAFETY: The pointer is valid due to type invariant on `CoherentAllocation`
+ // and we've just checked that the range and index is within bounds.
+ unsafe {
+ core::ptr::copy_nonoverlapping(
+ src.as_ptr(),
+ self.cpu_addr.wrapping_add(offset),
+ src.len(),
+ )
+ };
+ Ok(())
+ }
+}
+
+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,
+ self.dma_attrs.as_raw(),
+ )
+ }
+ }
+}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 545d1170ee63..36ac88fd91e7 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -37,6 +37,7 @@
pub mod build_assert;
pub mod cred;
pub mod device;
+pub mod dma;
pub mod error;
#[cfg(CONFIG_RUST_FW_LOADER_ABSTRACTIONS)]
pub mod firmware;
--
2.43.0