linux-kernel - [PATCH v2] rust: dma: allow drivers to tune max segment size

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Message-Id: <20260123131308.3218421-1-beata.michalska@arm.com>
Date: Fri, 23 Jan 2026 14:13:08 +0100
From: Beata Michalska <beata.michalska@....com>
To: dakr@...nel.org,
	ojeda@...nel.org,
	rust-for-linux@...r.kernel.org
Cc: abdiel.janulgue@...il.com,
	daniel.almeida@...labora.com,
	aliceryhl@...gle.com,
	robin.murphy@....com,
	a.hindborg@...nel.org,
	boqun.feng@...il.com,
	gary@...yguo.net,
	bjorn3_gh@...tonmail.com,
	lossin@...nel.org,
	tmgross@...ch.edu,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2] rust: dma: allow drivers to tune max segment size

Make dma_set_max_seg_size() available to Rust so drivers can perform
standard DMA setup steps.

Signed-off-by: Beata Michalska <beata.michalska@....com>
---

v2:
- Aligned safety requirements

 rust/helpers/dma.c |  5 +++++
 rust/kernel/dma.rs | 17 +++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/rust/helpers/dma.c b/rust/helpers/dma.c
index 2afa32c21c94..220b9308830f 100644
--- a/rust/helpers/dma.c
+++ b/rust/helpers/dma.c
@@ -40,3 +40,8 @@ size_t rust_helper_dma_max_mapping_size(struct device *dev)
 {
 	return dma_max_mapping_size(dev);
 }
+
+void rust_helper_dma_set_max_seg_size(struct device *dev, unsigned int size)
+{
+	dma_set_max_seg_size(dev, size);
+}
diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
index acc65b1e0f24..909d56fd5118 100644
--- a/rust/kernel/dma.rs
+++ b/rust/kernel/dma.rs
@@ -85,6 +85,23 @@ unsafe fn dma_set_mask_and_coherent(&self, mask: DmaMask) -> Result {
             bindings::dma_set_mask_and_coherent(self.as_ref().as_raw(), mask.value())
         })
     }
+
+    /// Set the maximum size of a single DMA segment the device may request.
+    ///
+    /// This method is usually called once from `probe()` as soon as the device capabilities are
+    /// known.
+    ///
+    /// # Safety
+    ///
+    /// This method must not be called concurrently with any DMA allocation or mapping primitives,
+    /// such as [`CoherentAllocation::alloc_attrs`].
+    unsafe fn dma_set_max_seg_size(&self, size: u32) {
+        // SAFETY:
+        // - By the type invariant of `device::Device`, `self.as_ref().as_raw()` is valid.
+        // - The safety requirement of this function guarantees that there are no concurrent calls
+        //   to DMA allocation and mapping primitives using this parameter.
+        unsafe { bindings::dma_set_max_seg_size(self.as_ref().as_raw(), size) }
+    }
 }
 
 /// A DMA mask that holds a bitmask with the lowest `n` bits set.
-- 
2.25.1