rust: dma: add as_slice/write functions for CoherentAllocation

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

Signed-off-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>

Author: xyn-ack

rust/kernel/dma.rs

@@ -57,10 +57,9 @@ pub trait Device: AsRef<device::Device> {
 /// # Examples
 ///
 /// ```
-/// use kernel::device::Device;
-/// use kernel::dma::{attrs::*, CoherentAllocation};
+/// use kernel::dma::{attrs::*, Device, CoherentAllocation};
 ///
-/// # fn test(dev: &Device) -> Result {
+/// # fn test(dev: &dyn Device) -> Result {
 /// let attribs = DMA_ATTR_FORCE_CONTIGUOUS | DMA_ATTR_NO_WARN;
 /// let c: CoherentAllocation<u64> =
 ///     CoherentAllocation::alloc_attrs(dev, 4, GFP_KERNEL, attribs)?;
@@ -178,16 +177,15 @@ impl<T: AsBytes + FromBytes> CoherentAllocation<T> {
     /// # Examples
     ///
     /// ```
-    /// use kernel::device::Device;
-    /// use kernel::dma::{attrs::*, CoherentAllocation};
+    /// use kernel::dma::{attrs::*, Device, CoherentAllocation};
     ///
-    /// # fn test(dev: &Device) -> Result {
+    /// # fn test(dev: &dyn 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::Device,
+        dev: &dyn Device,
         count: usize,
         gfp_flags: kernel::alloc::Flags,
         dma_attrs: Attrs,
@@ -197,6 +195,8 @@ impl<T: AsBytes + FromBytes> CoherentAllocation<T> {
             "It doesn't make sense for the allocated type to be a ZST"
         );
 
+        let dev = dev.as_ref();
+
         let size = count
             .checked_mul(core::mem::size_of::<T>())
             .ok_or(EOVERFLOW)?;
@@ -229,7 +229,7 @@ impl<T: AsBytes + FromBytes> CoherentAllocation<T> {
     /// Performs the same functionality as [`CoherentAllocation::alloc_attrs`], except the
     /// `dma_attrs` is 0 by default.
     pub fn alloc_coherent(
-        dev: &device::Device,
+        dev: &dyn Device,
         count: usize,
         gfp_flags: kernel::alloc::Flags,
     ) -> Result<CoherentAllocation<T>> {
@@ -253,6 +253,93 @@ impl<T: AsBytes + FromBytes> CoherentAllocation<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.
+    ///
+    /// Due to the safety requirements of slice, the caller should consider that 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.
+    /// * 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 no hardware operations that involve the buffer are currently
+    ///   taking place 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 no hardware operations that involve the buffer overlaps with
+    ///   this write.
+    /// * 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.
     ///