rust: dma: add zeroed constructor to `Coherent`

These constructors create a coherent container of a single object
instead of slice. They are named `zeroed` and `zeroed_with_attrs` to
emphasis that they are created initialized zeroed. It is intended that
there'll be new constructors that take `PinInit` instead of zeroing.

Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://patch.msgid.link/20260320194626.36263-4-dakr@kernel.org
[ Use kernel import style. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

2 files changed, 94 insertions, 12 deletions

diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
index ff3e147f1a23..59477c865dd3 100644
--- a/rust/kernel/dma.rs
+++ b/rust/kernel/dma.rs
@@ -47,7 +47,7 @@ pub trait Device: AsRef<device::Device<Core>> {
     /// # Safety
     ///
     /// This method must not be called concurrently with any DMA allocation or mapping primitives,
-    /// such as [`CoherentAllocation::alloc_attrs`].
+    /// such as [`Coherent::zeroed`].
     unsafe fn dma_set_mask(&self, mask: DmaMask) -> Result {
         // SAFETY:
         // - By the type invariant of `device::Device`, `self.as_ref().as_raw()` is valid.
@@ -64,7 +64,7 @@ pub trait Device: AsRef<device::Device<Core>> {
     /// # Safety
     ///
     /// This method must not be called concurrently with any DMA allocation or mapping primitives,
-    /// such as [`CoherentAllocation::alloc_attrs`].
+    /// such as [`Coherent::zeroed`].
     unsafe fn dma_set_coherent_mask(&self, mask: DmaMask) -> Result {
         // SAFETY:
         // - By the type invariant of `device::Device`, `self.as_ref().as_raw()` is valid.
@@ -83,7 +83,7 @@ pub trait Device: AsRef<device::Device<Core>> {
     /// # Safety
     ///
     /// This method must not be called concurrently with any DMA allocation or mapping primitives,
-    /// such as [`CoherentAllocation::alloc_attrs`].
+    /// such as [`Coherent::zeroed`].
     unsafe fn dma_set_mask_and_coherent(&self, mask: DmaMask) -> Result {
         // SAFETY:
         // - By the type invariant of `device::Device`, `self.as_ref().as_raw()` is valid.
@@ -102,7 +102,7 @@ pub trait Device: AsRef<device::Device<Core>> {
     /// # Safety
     ///
     /// This method must not be called concurrently with any DMA allocation or mapping primitives,
-    /// such as [`CoherentAllocation::alloc_attrs`].
+    /// such as [`Coherent::zeroed`].
     unsafe fn dma_set_max_seg_size(&self, size: u32) {
         // SAFETY:
         // - By the type invariant of `device::Device`, `self.as_ref().as_raw()` is valid.
@@ -202,12 +202,12 @@ impl DmaMask {
 ///
 /// ```
 /// # use kernel::device::{Bound, Device};
-/// use kernel::dma::{attrs::*, CoherentAllocation};
+/// use kernel::dma::{attrs::*, Coherent};
 ///
 /// # fn test(dev: &Device<Bound>) -> Result {
 /// let attribs = DMA_ATTR_FORCE_CONTIGUOUS | DMA_ATTR_NO_WARN;
-/// let c: CoherentAllocation<u64> =
-///     CoherentAllocation::alloc_attrs(dev, 4, GFP_KERNEL, attribs)?;
+/// let c: Coherent<[u64]> =
+///     Coherent::zeroed_slice_with_attrs(dev, 4, GFP_KERNEL, attribs)?;
 /// # Ok::<(), Error>(()) }
 /// ```
 #[derive(Clone, Copy, PartialEq)]
@@ -492,7 +492,6 @@ impl<T: KnownSize + ?Sized> Coherent<T> {
 
 impl<T: AsBytes + FromBytes> Coherent<T> {
     /// Allocates a region of `T` of coherent memory.
-    #[expect(unused)]
     fn alloc_with_attrs(
         dev: &device::Device<Bound>,
         gfp_flags: kernel::alloc::Flags,
@@ -529,6 +528,41 @@ impl<T: AsBytes + FromBytes> Coherent<T> {
         })
     }
 
+    /// Allocates a region of type `T` of coherent memory.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use kernel::device::{
+    /// #     Bound,
+    /// #     Device,
+    /// # };
+    /// use kernel::dma::{
+    ///     attrs::*,
+    ///     Coherent,
+    /// };
+    ///
+    /// # fn test(dev: &Device<Bound>) -> Result {
+    /// let c: Coherent<[u64; 4]> =
+    ///     Coherent::zeroed_with_attrs(dev, GFP_KERNEL, DMA_ATTR_NO_WARN)?;
+    /// # Ok::<(), Error>(()) }
+    /// ```
+    #[inline]
+    pub fn zeroed_with_attrs(
+        dev: &device::Device<Bound>,
+        gfp_flags: kernel::alloc::Flags,
+        dma_attrs: Attrs,
+    ) -> Result<Self> {
+        Self::alloc_with_attrs(dev, gfp_flags | __GFP_ZERO, dma_attrs)
+    }
+
+    /// Performs the same functionality as [`Coherent::zeroed_with_attrs`], except the
+    /// `dma_attrs` is 0 by default.
+    #[inline]
+    pub fn zeroed(dev: &device::Device<Bound>, gfp_flags: kernel::alloc::Flags) -> Result<Self> {
+        Self::zeroed_with_attrs(dev, gfp_flags, Attrs(0))
+    }
+
     /// Allocates a region of `[T; len]` of coherent memory.
     fn alloc_slice_with_attrs(
         dev: &device::Device<Bound>,
@@ -572,6 +606,49 @@ impl<T: AsBytes + FromBytes> Coherent<T> {
             dma_attrs,
         })
     }
+
+    /// Allocates a zeroed region of type `T` of coherent memory.
+    ///
+    /// Unlike `Coherent::<[T; N]>::zeroed_with_attrs`, `Coherent::<T>::zeroed_slices` support
+    /// a runtime length.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use kernel::device::{
+    /// #     Bound,
+    /// #     Device,
+    /// # };
+    /// use kernel::dma::{
+    ///     attrs::*,
+    ///     Coherent,
+    /// };
+    ///
+    /// # fn test(dev: &Device<Bound>) -> Result {
+    /// let c: Coherent<[u64]> =
+    ///     Coherent::zeroed_slice_with_attrs(dev, 4, GFP_KERNEL, DMA_ATTR_NO_WARN)?;
+    /// # Ok::<(), Error>(()) }
+    /// ```
+    #[inline]
+    pub fn zeroed_slice_with_attrs(
+        dev: &device::Device<Bound>,
+        len: usize,
+        gfp_flags: kernel::alloc::Flags,
+        dma_attrs: Attrs,
+    ) -> Result<Coherent<[T]>> {
+        Coherent::alloc_slice_with_attrs(dev, len, gfp_flags | __GFP_ZERO, dma_attrs)
+    }
+
+    /// Performs the same functionality as [`Coherent::zeroed_slice_with_attrs`], except the
+    /// `dma_attrs` is 0 by default.
+    #[inline]
+    pub fn zeroed_slice(
+        dev: &device::Device<Bound>,
+        len: usize,
+        gfp_flags: kernel::alloc::Flags,
+    ) -> Result<Coherent<[T]>> {
+        Self::zeroed_slice_with_attrs(dev, len, gfp_flags, Attrs(0))
+    }
 }
 
 impl<T> Coherent<[T]> {
diff --git a/samples/rust/rust_dma.rs b/samples/rust/rust_dma.rs
index ce39b5545097..129bb4b39c04 100644
--- a/samples/rust/rust_dma.rs
+++ b/samples/rust/rust_dma.rs
@@ -6,7 +6,12 @@
 
 use kernel::{
     device::Core,
-    dma::{CoherentAllocation, DataDirection, Device, DmaMask},
+    dma::{
+        Coherent,
+        DataDirection,
+        Device,
+        DmaMask, //
+    },
     page, pci,
     prelude::*,
     scatterlist::{Owned, SGTable},
@@ -16,7 +21,7 @@ use kernel::{
 #[pin_data(PinnedDrop)]
 struct DmaSampleDriver {
     pdev: ARef<pci::Device>,
-    ca: CoherentAllocation<MyStruct>,
+    ca: Coherent<[MyStruct]>,
     #[pin]
     sgt: SGTable<Owned<VVec<u8>>>,
 }
@@ -64,8 +69,8 @@ impl pci::Driver for DmaSampleDriver {
             // SAFETY: There are no concurrent calls to DMA allocation and mapping primitives.
             unsafe { pdev.dma_set_mask_and_coherent(mask)? };
 
-            let ca: CoherentAllocation<MyStruct> =
-                CoherentAllocation::alloc_coherent(pdev.as_ref(), TEST_VALUES.len(), GFP_KERNEL)?;
+            let ca: Coherent<[MyStruct]> =
+                Coherent::zeroed_slice(pdev.as_ref(), TEST_VALUES.len(), GFP_KERNEL)?;
 
             for (i, value) in TEST_VALUES.into_iter().enumerate() {
                 kernel::dma_write!(ca, [i]?, MyStruct::new(value.0, value.1));