[PATCH v6 01/16] dma-mapping: introduce new DMA attribute to indicate MMIO memory

Leon Romanovsky
2025-09-09T13:28:20Z

This patch introduces the DMA_ATTR_MMIO attribute to mark DMA buffers
that reside in memory-mapped I/O (MMIO) regions, such as device BARs
exposed through the host bridge, which are accessible for peer-to-peer
(P2P) DMA.

This attribute is especially useful for exporting device memory to other
devices for DMA without CPU involvement, and avoids unnecessary or
potentially detrimental CPU cache maintenance calls.

DMA_ATTR_MMIO is supposed to provide dma_map_resource() functionality
without need to call to special function and perform branching when
processing generic containers like bio_vec by the callers.

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
---
Documentation/core-api/dma-attributes.rst | 18 ++++++++++++++++++
include/linux/dma-mapping.h | 20 ++++++++++++++++++++
include/trace/events/dma.h | 3 ++-
rust/kernel/dma.rs | 3 +++
4 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/Documentation/core-api/dma-attributes.rst b/Documentation/core-api/dma-attributes.rst
index 1887d92e8e926..0bdc2be65e575 100644
--- a/Documentation/core-api/dma-attributes.rst
+++ b/Documentation/core-api/dma-attributes.rst
@@ -130,3 +130,21 @@ accesses to DMA buffers in both privileged "supervisor" and unprivileged
subsystem that the buffer is fully accessible at the elevated privilege
level (and ideally inaccessible or at least read-only at the
lesser-privileged levels).
+
+DMA_ATTR_MMIO
+-------------
+
+This attribute indicates the physical address is not normal system
+memory. It may not be used with kmap*()/phys_to_virt()/phys_to_page()
+functions, it may not be cacheable, and access using CPU load/store
+instructions may not be allowed.
+
+Usually this will be used to describe MMIO addresses, or other non-cacheable
+register addresses. When DMA mapping this sort of address we call
+the operation Peer to Peer as a one device is DMA'ing to another device.
+For PCI devices the p2pdma APIs must be used to determine if
+DMA_ATTR_MMIO is appropriate.
+
+For architectures that require cache flushing for DMA coherence
+DMA_ATTR_MMIO will not perform any cache flushing. The address
+provided must never be mapped cacheable into the CPU.
diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index 55c03e5fe8cb3..4254fd9bdf5dd 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -58,6 +58,26 @@
*/
#define DMA_ATTR_PRIVILEGED (1UL << 9)

+/*
+ * DMA_ATTR_MMIO - Indicates memory-mapped I/O (MMIO) region for DMA mapping
+ *
+ * This attribute indicates the physical address is not normal system
+ * memory. It may not be used with kmap*()/phys_to_virt()/phys_to_page()
+ * functions, it may not be cacheable, and access using CPU load/store
+ * instructions may not be allowed.
+ *
+ * Usually this will be used to describe MMIO addresses, or other non-cacheable
+ * register addresses. When DMA mapping this sort of address we call
+ * the operation Peer to Peer as a one device is DMA'ing to another device.
+ * For PCI devices the p2pdma APIs must be used to determine if DMA_ATTR_MMIO
+ * is appropriate.
+ *
+ * For architectures that require cache flushing for DMA coherence
+ * DMA_ATTR_MMIO will not perform any cache flushing. The address
+ * provided must never be mapped cacheable into the CPU.
+ */
+#define DMA_ATTR_MMIO (1UL << 10)
+
/*
* A dma_addr_t can hold any valid DMA or bus address for the platform. It can
* be given to a device to use as a DMA source or target. It is specific to a
diff --git a/include/trace/events/dma.h b/include/trace/events/dma.h
index d8ddc27b6a7c8..ee90d6f1dcf35 100644
--- a/include/trace/events/dma.h
+++ b/include/trace/events/dma.h
@@ -31,7 +31,8 @@ TRACE_DEFINE_ENUM(DMA_NONE);
{ DMA_ATTR_FORCE_CONTIGUOUS, "FORCE_CONTIGUOUS" }, \
{ DMA_ATTR_ALLOC_SINGLE_PAGES, "ALLOC_SINGLE_PAGES" }, \
{ DMA_ATTR_NO_WARN, "NO_WARN" }, \
- { DMA_ATTR_PRIVILEGED, "PRIVILEGED" })
+ { DMA_ATTR_PRIVILEGED, "PRIVILEGED" }, \
+ { DMA_ATTR_MMIO, "MMIO" })

DECLARE_EVENT_CLASS(dma_map,
TP_PROTO(struct device *dev, phys_addr_t phys_addr, dma_addr_t dma_addr,
diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
index 2bc8ab51ec280..61d9eed7a786e 100644
--- a/rust/kernel/dma.rs
+++ b/rust/kernel/dma.rs
@@ -242,6 +242,9 @@ pub mod attrs {
/// Indicates that the buffer is fully accessible at an elevated privilege level (and
/// ideally inaccessible or at least read-only at lesser-privileged levels).
pub const DMA_ATTR_PRIVILEGED: Attrs = Attrs(bindings::DMA_ATTR_PRIVILEGED);
+
+ /// Indicates that the buffer is MMIO memory.
+ pub const DMA_ATTR_MMIO: Attrs = Attrs(bindings::DMA_ATTR_MMIO);
}

/// An abstraction of the `dma_alloc_coherent` API.
--
2.51.0