[PATCH 2/2] rust: scatterlist: honor the device's maximum segment size

[Matteo Kloiber]

SGTable::new() caps segment length at dma_max_mapping_size() only, which
limits the DMA mapping path (e.g. swiotlb), not the device itself. The
per-device limit from dma_set_max_seg_size() is ignored, so contiguous
page segments can be longer than the declared max segment size,
potentially causing problems for future drivers that use this
abstraction.

nova-core declares an unlimited segment size, so this does not change
its behavior.

Fixes: 05aa6fb1c21d ("rust: scatterlist: Add abstraction for sg_table")
Signed-off-by: Matteo Kloiber <kernel@xxxxxxxxxxx>
---
 rust/helpers/dma.c         |  5 +++++
 rust/kernel/scatterlist.rs | 12 ++++++++++--
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/rust/helpers/dma.c b/rust/helpers/dma.c
index 9fbeb507b08c..ff8f24dae9df 100644
--- a/rust/helpers/dma.c
+++ b/rust/helpers/dma.c
@@ -49,3 +49,8 @@ __rust_helper void rust_helper_dma_set_max_seg_size(struct device *dev,
 {
 	dma_set_max_seg_size(dev, size);
 }
+
+__rust_helper unsigned int rust_helper_dma_get_max_seg_size(struct device *dev)
+{
+	return dma_get_max_seg_size(dev);
+}
diff --git a/rust/kernel/scatterlist.rs b/rust/kernel/scatterlist.rs
index b83c468b5c63..d677dcbe7aac 100644
--- a/rust/kernel/scatterlist.rs
+++ b/rust/kernel/scatterlist.rs
@@ -350,15 +350,23 @@ fn new(
             page_vec.push(page.as_ptr(), flags)?;
         }
 
+        // Cap segments at both the DMA mapping-path limit and the device's declared
+        // max segment size.
+        //
         // `dma_max_mapping_size` returns `size_t`, but `sg_alloc_table_from_pages_segment()` takes
         // an `unsigned int`.
         //
         // SAFETY: `dev.as_raw()` is a valid pointer to a `struct device`.
-        let max_segment = match unsafe { bindings::dma_max_mapping_size(dev.as_raw()) } {
+        let max_mapping = match unsafe { bindings::dma_max_mapping_size(dev.as_raw()) } {
             0 => u32::MAX,
-            max_segment => u32::try_from(max_segment).unwrap_or(u32::MAX),
+            max_mapping => u32::try_from(max_mapping).unwrap_or(u32::MAX),
         };
 
+        // SAFETY: `dev.as_raw()` is a valid pointer to a `struct device`.
+        let max_seg_size = unsafe { bindings::dma_get_max_seg_size(dev.as_raw()) };
+
+        let max_segment = max_mapping.min(max_seg_size);
+
         Ok(try_pin_init!(&this in Self {
             // SAFETY:
             // - `page_vec` is a `KVec` of valid `struct page *` obtained from `pages`.
-- 
2.51.2