rust: dma: implement dma::Device trait

Add a trait that defines the DMA specific methods of devices.

The `dma::Device` trait should be implemented by (bus) device
representations, where the underlying bus potentially supports DMA, such
as `pci::Device` or `platform::Device`.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Abdiel Janulgue <abdiel.janulgue@gmail.com>

Author: Danilo Krummrich

rust/kernel/dma.rs

@@ -6,13 +6,20 @@
 
 use crate::{
     bindings, build_assert,
-    device::Device,
+    device,
     error::code::*,
     error::Result,
     transmute::{AsBytes, FromBytes},
     types::ARef,
 };
 
+/// Trait to be implemented by bus specific devices.
+///
+/// The [`Device`] trait should be implemented by bus specific device representations, where the
+/// underlying bus has potential support for DMA, such as [`crate::pci::Device`] or
+/// [crate::platform::Device].
+pub trait Device: AsRef<device::Device> {}
+
 /// Possible attributes associated with a DMA mapping.
 ///
 /// They can be combined with the operators `|`, `&`, and `!`.
@@ -130,7 +137,7 @@ pub mod attrs {
 // Hence, find a way to revoke the device resources of a `CoherentAllocation`, but not the
 // entire `CoherentAllocation` including the allocated memory itself.
 pub struct CoherentAllocation<T: AsBytes + FromBytes> {
-    dev: ARef<Device>,
+    dev: ARef<device::Device>,
     dma_handle: bindings::dma_addr_t,
     count: usize,
     cpu_addr: *mut T,
@@ -152,7 +159,7 @@ impl<T: AsBytes + FromBytes> CoherentAllocation<T> {
     /// # Ok::<(), Error>(()) }
     /// ```
     pub fn alloc_attrs(
-        dev: &Device,
+        dev: &device::Device,
         count: usize,
         gfp_flags: kernel::alloc::Flags,
         dma_attrs: Attrs,
@@ -194,7 +201,7 @@ 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,
+        dev: &device::Device,
         count: usize,
         gfp_flags: kernel::alloc::Flags,
     ) -> Result<CoherentAllocation<T>> {