[PATCH 5/8] rust: dma: add methods to unsafely create reference from subview

[Gary Guo <gary@kernel.org>]

From: Gary Guo <gary@garyguo.net>

Implement `Io` for `Coherent`, so now `dma::Coherent` can be used with I/O
projections.

This allows the `as_ref()` and `as_mut()` API to be used in smaller region
than the whole DMA allocation itself. For example, if a ring buffer is shared
between GPU and CPU, users may now use the `io_project!` API to obtain a view
of the buffer that is unified owned by the CPU and get a reference.

Signed-off-by: Gary Guo <gary@garyguo.net>
---
 rust/kernel/dma.rs | 56 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)

diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
index 779d4babab9a..ae2939abc166 100644
--- a/rust/kernel/dma.rs
+++ b/rust/kernel/dma.rs
@@ -12,6 +12,10 @@
         Core, //
     },
     error::to_result,
+    io::{
+        Io,
+        IoCapable, //
+    },
     prelude::*,
     ptr::KnownSize,
     sync::aref::ARef,
@@ -864,6 +868,58 @@ fn drop(&mut self) {
 // can be sent to another thread.
 unsafe impl<T: KnownSize + Send + ?Sized> Send for Coherent<T> {}
 
+impl<T: ?Sized + KnownSize> Io for Coherent<T> {
+    type Type = T;
+
+    #[inline]
+    fn as_ptr(&self) -> *mut Self::Type {
+        self.as_mut_ptr()
+    }
+}
+
+impl<'a, B: ?Sized + KnownSize, T: ?Sized> crate::io::View<'a, Coherent<B>, T> {
+    /// Returns a DMA handle which may be given to the device as the DMA address base of
+    /// the region.
+    #[inline]
+    pub fn dma_handle(&self) -> DmaAddress {
+        let base = self.io();
+        let offset = self.as_ptr().addr() - base.as_ptr().addr();
+        base.dma_handle() + offset as DmaAddress
+    }
+
+    /// Returns a reference to the data in the region.
+    ///
+    /// # 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
+    ///   the returned slice is live.
+    #[inline]
+    pub unsafe fn as_ref(self) -> &'a T {
+        let ptr = self.as_ptr();
+        // SAFETY: pointer is aligned and valid per type invariant of `View`. Aliasing rule is
+        // satisfied per safety requirement.
+        unsafe { &*ptr }
+    }
+
+    /// Returns a mutable reference to the data in the region.
+    ///
+    /// # 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.
+    #[inline]
+    pub unsafe fn as_mut(self) -> &'a mut T {
+        let ptr = self.as_ptr();
+        // SAFETY: pointer is aligned and valid per type invariant of `View`. Aliasing rule is
+        // satisfied per safety requirement.
+        unsafe { &mut *ptr }
+    }
+}
+
 /// Reads a field of an item from an allocated region of structs.
 ///
 /// The syntax is of the form `kernel::dma_read!(dma, proj)` where `dma` is an expression evaluating
-- 
2.51.2