[PATCH v2 2/2] rust: scatterlist: honor the device's maximum segment size
From: Matteo Kloiber <hidden>
Date: 2026-09-13 21:13:39
Also in:
dri-devel, driver-core, lkml, nova-gpu
Subsystem:
dma mapping & scatterlist api [rust], rust, the rest · Maintainers:
Danilo Krummrich, Miguel Ojeda, Linus Torvalds
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.
Fixes: 05aa6fb1c21d ("rust: scatterlist: Add abstraction for sg_table")
Signed-off-by: Matteo Kloiber <redacted>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
---
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..44eeafed05e3 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_size = 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_size => u32::try_from(max_mapping_size).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_size.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