[PATCH v2 3/3] rust: dma: add as_slice/write functions for CoherentAllocation

Abdiel Janulgue
2025-04-10T08:59:27Z

Add unsafe accessors for the region for reading or writing large
blocks of data.

Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>
---
rust/kernel/dma.rs | 85 ++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 85 insertions(+)

diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
index a61da5eeb017..880f6f04ba86 100644
--- a/rust/kernel/dma.rs
+++ b/rust/kernel/dma.rs
@@ -218,6 +218,91 @@ pub fn dma_handle(&self) -> bindings::dma_addr_t {
self.dma_handle
}

+ /// Returns the data from the region starting from `offset` as a slice.
+ /// `offset` and `count` are in units of `T`, not the number of bytes.
+ ///
+ /// For ringbuffer type of r/w access or use-cases where the pointer to the live data is needed,
+ /// [`CoherentAllocation::start_ptr`] or [`CoherentAllocation::start_ptr_mut`] could be used instead.
+ ///
+ /// # Safety
+ ///
+ /// * Callers must ensure that the device does not read/write to/from memory while the returned
+ /// slice is live.
+ /// * Callers must ensure that this call does not race with a write to the same region while
+ /// while the returned slice is live.
+ pub unsafe fn as_slice(&self, offset: usize, count: usize) -> Result<&[T]> {
+ let end = offset.checked_add(count).ok_or(EOVERFLOW)?;
+ if end >= 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.
+ // - `offset` can't overflow since it is smaller than `self.count` and we've checked
+ // that `self.count` won't overflow early in the constructor.
+ Ok(unsafe { core::slice::from_raw_parts(self.cpu_addr.add(offset), count) })
+ }
+
+ /// Performs the same functionality as [`CoherentAllocation::as_slice`], except that a mutable
+ /// slice is returned.
+ ///
+ /// # Safety
+ ///
+ /// * Callers must ensure that the device does not read/write to/from memory while the returned
+ /// slice is live.
+ /// * Callers must ensure that this call does not race with a read or write to the same region
+ /// while the returned slice is live.
+ pub unsafe fn as_slice_mut(&self, offset: usize, count: usize) -> Result<&mut [T]> {
+ let end = offset.checked_add(count).ok_or(EOVERFLOW)?;
+ if end >= 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.
+ // - `offset` can't overflow since it is smaller than `self.count` and we've checked
+ // that `self.count` won't overflow early in the constructor.
+ Ok(unsafe { core::slice::from_raw_parts_mut(self.cpu_addr.add(offset), count) })
+ }
+
+ /// Writes data to the region starting from `offset`. `offset` is in units of `T`, not the
+ /// number of bytes.
+ ///
+ /// # Safety
+ ///
+ /// * Callers must ensure that the device does not read/write to/from memory while the returned
+ /// slice is live.
+ /// * Callers must ensure that this call does not race with a read or write to the same region
+ /// that overlaps with this write.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// # fn test(alloc: &mut kernel::dma::CoherentAllocation<u8>) -> Result {
+ /// let somedata: [u8; 4] = [0xf; 4];
+ /// let buf: &[u8] = &somedata;
+ /// // SAFETY: No hw operation on the device and no other r/w access to the region at this point.
+ /// unsafe { alloc.write(buf, 0)?; }
+ /// # Ok::<(), Error>(()) }
+ /// ```
+ pub unsafe fn write(&self, src: &[T], offset: usize) -> Result {
+ let end = offset.checked_add(src.len()).ok_or(EOVERFLOW)?;
+ if end >= 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.
+ // - `offset` can't overflow since it is smaller than `self.count` and we've checked
+ // that `self.count` won't overflow early in the constructor.
+ unsafe {
+ core::ptr::copy_nonoverlapping(src.as_ptr(), self.cpu_addr.add(offset), src.len())
+ };
+ Ok(())
+ }
+
/// Returns a pointer to an element from the region with bounds checking. `offset` is in
/// units of `T`, not the number of bytes.
///
--
2.43.0