[PATCH v2 2/2] rust: add dma coherent allocator abstraction.

[Abdiel Janulgue <abdiel.janulgue@gmail.com>]

Add a simple dma coherent allocator rust abstraction. Based on
Andreas Hindborg's dma abstractions from the rnvme driver.

Co-developed-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Co-developed-by: Andreas Hindborg <a.hindborg@samsung.com>
Signed-off-by: Andreas Hindborg <a.hindborg@samsung.com>
Signed-off-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>
---
 rust/kernel/dma.rs | 165 +++++++++++++++++++++++++++++++++++++++++++++
 rust/kernel/lib.rs |   1 +
 2 files changed, 166 insertions(+)
 create mode 100644 rust/kernel/dma.rs

diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
new file mode 100644
index 000000000000..75a304d612f0
--- /dev/null
+++ b/rust/kernel/dma.rs
@@ -0,0 +1,165 @@
+// 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,
+    device::Device,
+    error::code::*,
+    error::Result,
+    types::ARef,
+};
+use core::ops::Add;
+
+/// Abstraction of dma_alloc_coherent
+///
+/// # 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: Add> {
+    dev: ARef<Device>,
+    dma_handle: bindings::dma_addr_t,
+    count: usize,
+    cpu_addr: *mut T,
+}
+
+impl<T: Add> CoherentAllocation<T> {
+    /// Allocates a region of `size_of::<T> * count` of consistent memory.
+    ///
+    /// Returns a CoherentAllocation object which 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.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::device::Device;
+    /// use kernel::dma::CoherentAllocation;
+    ///
+    /// # fn dox(dev: &Device) -> Result<()> {
+    /// let c: CoherentAllocation<u64> = CoherentAllocation::alloc_coherent(dev, 4, GFP_KERNEL)?;
+    /// # Ok(()) }
+    /// ```
+    pub fn alloc_coherent(
+        dev: &Device,
+        count: usize,
+        flags: kernel::alloc::Flags,
+    ) -> Result<CoherentAllocation<T>> {
+        let t_size = core::mem::size_of::<T>();
+        let size = count.checked_mul(t_size).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)
+        }
+
+        Ok(Self {
+            dev: dev.into(),
+            dma_handle,
+            count,
+            cpu_addr: ret as _,
+        })
+    }
+
+    /// Reads a value on a location specified by index.
+    pub fn read(&self, index: usize) -> Result<T>
+    where
+        T: Copy
+    {
+        if let Some(val) = self.cpu_buf().get(index) {
+            Ok(*val)
+        } else {
+            Err(EINVAL)
+        }
+    }
+
+    /// Write a value on the memory location specified by index.
+    pub fn write(&mut self, index: usize, value: &T) -> Result
+    where
+        T: Copy,
+    {
+        if let Some(elem) = self.cpu_buf_mut().get_mut(index) {
+            *elem = *value;
+            Ok(())
+        } else {
+            Err(EINVAL)
+        }
+    }
+
+    /// Performs a read and then a write of a value on a location specified by index.
+    pub fn read_write(&mut self, index: usize, value: &T) -> Result<T>
+    where
+        T: Copy,
+    {
+        if let Some(elem) = self.cpu_buf_mut().get_mut(index) {
+            let val = *elem;
+            *elem = *value;
+            Ok(val)
+        } else {
+            Err(EINVAL)
+        }
+    }
+
+    /// Returns the base address to the allocated region and the dma handle.
+    /// Caller takes ownership of 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
+    }
+
+    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) }
+    }
+
+    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: Add> 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 b62451f64f6e..b713c92eb1ef 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -32,6 +32,7 @@
 pub mod block;
 mod build_assert;
 pub mod device;
+pub mod dma;
 pub mod error;
 #[cfg(CONFIG_RUST_FW_LOADER_ABSTRACTIONS)]
 pub mod firmware;
-- 
2.43.0