linux-kernel - [PATCH v3 2/2] samples: rust: add sample code for scatterlist abstraction

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Message-ID: <20250718103359.1026240-3-abdiel.janulgue@gmail.com>
Date: Fri, 18 Jul 2025 13:33:35 +0300
From: Abdiel Janulgue <abdiel.janulgue@...il.com>
To: acourbot@...dia.com,
	dakr@...nel.org,
	jgg@...pe.ca,
	lyude@...hat.com
Cc: Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Trevor Gross <tmgross@...ch.edu>,
	Tamir Duberstein <tamird@...il.com>,
	FUJITA Tomonori <fujita.tomonori@...il.com>,
	linux-kernel@...r.kernel.org (open list),
	Andrew Morton <akpm@...ux-foundation.org>,
	Randy Dunlap <rdunlap@...radead.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Caleb Sander Mateos <csander@...estorage.com>,
	Petr Tesarik <petr@...arici.cz>,
	Sui Jingfeng <sui.jingfeng@...ux.dev>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	airlied@...hat.com,
	iommu@...ts.linux.dev (open list:DMA MAPPING HELPERS),
	rust-for-linux@...r.kernel.org,
	Abdiel Janulgue <abdiel.janulgue@...il.com>
Subject: [PATCH v3 2/2] samples: rust: add sample code for scatterlist abstraction

Add simple excercises to test the scatterlist abstraction.

Co-developed-by: Alexandre Courbot <acourbot@...dia.com>
Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
Signed-off-by: Abdiel Janulgue <abdiel.janulgue@...il.com>
---
 samples/rust/rust_dma.rs | 49 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 48 insertions(+), 1 deletion(-)

diff --git a/samples/rust/rust_dma.rs b/samples/rust/rust_dma.rs
index 9e05d5c0cdae..1fa278e8e29a 100644
--- a/samples/rust/rust_dma.rs
+++ b/samples/rust/rust_dma.rs
@@ -4,11 +4,33 @@
 //!
 //! To make this driver probe, QEMU must be run with `-device pci-testdev`.
 
-use kernel::{bindings, device::Core, dma::CoherentAllocation, pci, prelude::*, types::ARef};
+use kernel::{
+    bindings, device::Core, dma::CoherentAllocation, page::*, pci, prelude::*, scatterlist::*,
+    sync::Arc, types::ARef,
+};
 
 struct DmaSampleDriver {
     pdev: ARef<pci::Device>,
     ca: CoherentAllocation<MyStruct>,
+    _sgt: SGTable<OwnedSgt<PagesArray>, ManagedMapping>,
+}
+
+struct PagesArray(KVec<Page>);
+impl SGTablePages for PagesArray {
+    fn iter<'a>(&'a self) -> impl Iterator<Item = (&'a Page, usize, usize)> {
+        self.0.iter().map(|page| (page, kernel::page::PAGE_SIZE, 0))
+    }
+
+    fn entries(&self) -> usize {
+        self.0.len()
+    }
+}
+
+struct WrappedArc(Arc<kernel::bindings::sg_table>);
+impl core::borrow::Borrow<kernel::bindings::sg_table> for WrappedArc {
+    fn borrow(&self) -> &kernel::bindings::sg_table {
+        &self.0
+    }
 }
 
 const TEST_VALUES: [(u32, u32); 5] = [
@@ -58,10 +80,35 @@ fn probe(pdev: &pci::Device<Core>, _info: &Self::IdInfo) -> Result<Pin<KBox<Self
             kernel::dma_write!(ca[i] = MyStruct::new(value.0, value.1))?;
         }
 
+        let mut pages = KVec::new();
+        for _ in TEST_VALUES.into_iter() {
+            let _ = pages.push(Page::alloc_page(GFP_KERNEL)?, GFP_KERNEL);
+        }
+
+        // Let's pretend this is valid...
+        // SAFETY: `sg_table` is not a reference.
+        let sg_table: bindings::sg_table = unsafe { core::mem::zeroed() };
+
+        // `borrowed_sgt` cannot outlive `sg_table`.
+        // SAFETY: From above, we assume that `sg_table` is initialized and valid.
+        let _borrowed_sgt = unsafe { SGTable::new_unmapped(&sg_table) };
+
+        let sg_table = WrappedArc(Arc::new(sg_table, GFP_KERNEL)?);
+        // `refcounted_sgt` keeps a refcounted reference to the `sg_table` and is thus not
+        // tied by a compile-time lifetime.
+        // SAFETY: From above, we assume that `sg_table` is initialized and valid.
+        let _refcounted_sgt = unsafe { SGTable::new_unmapped(sg_table) };
+
+        // `owned_sgt` carries and owns the data it represents.
+        let owned_sgt = SGTable::new_owned(PagesArray(pages), GFP_KERNEL)?;
+        let sgt = owned_sgt.dma_map(pdev.as_ref(), kernel::dma::DmaDataDirection::DmaToDevice)?;
+
         let drvdata = KBox::new(
             Self {
                 pdev: pdev.into(),
                 ca,
+                // excercise the destructor
+                _sgt: sgt,
             },
             GFP_KERNEL,
         )?;
-- 
2.43.0