[PATCH v6 03/24] rust: dma: add dma_handle_with_offset method to CoherentAllocation

[Alexandre Courbot]

Sometimes one may want to obtain a DMA handle starting at a given
offset. This can be done by adding said offset to the result of
`dma_handle()`, but doing so on the client side carries the risk that
the operation will go outside the bounds of the allocation.

Thus, add a `dma_handle_with_offset` method that adds the desired offset
after checking that it is still valid.

Signed-off-by: Alexandre Courbot <acourbot at nvidia.com>
---
 rust/kernel/dma.rs | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
index 15ff639b3067d0e4a39e181bbe709a9c372a591a..04546e58252d308e7a9f17bd2eae0aebfdc3c271 100644
--- a/rust/kernel/dma.rs
+++ b/rust/kernel/dma.rs
@@ -238,6 +238,20 @@ pub fn dma_handle(&self) -> bindings::dma_addr_t {
         self.dma_handle
     }
 
+    /// Returns a DMA handle starting at `offset` (in units of `T`) which may be given to the
+    /// device as the DMA address base of the region.
+    ///
+    /// Returns `EINVAL` if `offset` is not within the bounds of the allocation.
+    pub fn dma_handle_with_offset(&self, offset: usize) -> Result<bindings::dma_addr_t> {
+        if offset >= self.count {
+            Err(EINVAL)
+        } else {
+            // INVARIANT: The type invariant of `Self` guarantees that `size_of::<T> * count` fits
+            // into a `usize`, and `offset` is inferior to `count`.
+            Ok(self.dma_handle + (offset * core::mem::size_of::<T>()) as bindings::dma_addr_t)
+        }
+    }
+
     /// Returns a pointer to an element from the region with bounds checking. `offset` is in
     /// units of `T`, not the number of bytes.
     ///

-- 
2.49.0