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From: Timur Tabi <ttabi@nvidia.com>
To: Gary Guo <gary@garyguo.net>, Alice Ryhl <aliceryhl@google.com>,
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	<rust-for-linux@vger.kernel.org>
Subject: [PATCH v6 3/7] gpu: nova-core: implement BinaryWriter for CoherentAllocation<u8>
Date: Wed, 28 Jan 2026 20:28:33 -0600
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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>
---
 drivers/gpu/nova-core/gsp.rs |  1 +
 rust/kernel/dma.rs           | 36 +++++++++++++++++++++++++++++++++++-
 2 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/nova-core/gsp.rs b/drivers/gpu/nova-core/gsp.rs
index 174feaca0a6b..f7134c75b1f2 100644
--- a/drivers/gpu/nova-core/gsp.rs
+++ b/drivers/gpu/nova-core/gsp.rs
@@ -3,6 +3,7 @@
 mod boot;
 
 use kernel::{
+    debugfs,
     device,
     dma::{
         CoherentAllocation,
diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
index acc65b1e0f24..dca16fb5a2fc 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;
 
@@ -651,6 +653,38 @@ 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);
+        }
+
+        let offset_val: usize = (*offset).try_into().map_err(|_| EINVAL)?;
+        let len = self.count();
+
+        if offset_val >= len {
+            return Ok(0);
+        }
+
+        let count = (len - 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
-- 
2.52.0