[PATCH v9 3/6] rust: dma: implement BinaryWriter for CoherentAllocation<u8>

[Timur Tabi <ttabi@nvidia.com>]

Implement the BinaryWriter trait for CoherentAllocation<u8>, enabling
DMA coherent allocations to be exposed as readable binary files.
The implementation handles offset tracking and bounds checking, copying
data from the coherent allocation to userspace via write_dma().

Signed-off-by: Timur Tabi <ttabi@nvidia.com>
---
 rust/kernel/dma.rs | 35 ++++++++++++++++++++++++++++++++++-
 1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
index 909d56fd5118..b38f16fc6c66 100644
--- a/rust/kernel/dma.rs
+++ b/rust/kernel/dma.rs
@@ -5,12 +5,14 @@
 //! C header: [`include/linux/dma-mapping.h`](srctree/include/linux/dma-mapping.h)
 
 use crate::{
-    bindings, build_assert, device,
+    bindings, build_assert, debugfs, device,
     device::{Bound, Core},
     error::{to_result, Result},
+    fs::file,
     prelude::*,
     sync::aref::ARef,
     transmute::{AsBytes, FromBytes},
+    uaccess::UserSliceWriter,
 };
 use core::ptr::NonNull;
 
@@ -668,6 +670,37 @@ fn drop(&mut self) {
 // can be sent to another thread.
 unsafe impl<T: AsBytes + FromBytes + Send> Send for CoherentAllocation<T> {}
 
+// SAFETY: Sharing `&CoherentAllocation` across threads is safe if `T` is `Sync`, because all
+// methods that access the buffer contents (`field_read`, `field_write`, `as_slice`,
+// `as_slice_mut`) are `unsafe`, and callers are responsible for ensuring no data races occur.
+// The safe methods only return metadata or raw pointers whose use requires `unsafe`.
+unsafe impl<T: AsBytes + FromBytes + Sync> Sync for CoherentAllocation<T> {}
+
+impl debugfs::BinaryWriter for CoherentAllocation<u8> {
+    fn write_to_slice(
+        &self,
+        writer: &mut UserSliceWriter,
+        offset: &mut file::Offset,
+    ) -> Result<usize> {
+        if offset.is_negative() {
+            return Err(EINVAL);
+        }
+
+        // If the offset is too large for a usize (e.g. on 32-bit platforms),
+        // then consider that as past EOF and just return 0 bytes.
+        let Ok(offset_val) = usize::try_from(*offset) else {
+            return Ok(0);
+        };
+
+        let count = self.count().saturating_sub(offset_val).min(writer.len());
+
+        writer.write_dma(self, offset_val, count)?;
+
+        *offset += count as i64;
+        Ok(count)
+    }
+}
+
 /// Reads a field of an item from an allocated region of structs.
 ///
 /// # Examples
-- 
2.51.0