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[PATCH v3 4/8] dma-mapping: Introduce DMA require coherency attribute

Leon Romanovsky posted 8 patches 3 weeks ago
[PATCH v3 4/8] dma-mapping: Introduce DMA require coherency attribute
Posted by Leon Romanovsky 3 weeks ago 
From: Leon Romanovsky <leonro@nvidia.com>

The mapping buffers which carry this attribute require DMA coherent system.
This means that they can't take SWIOTLB path, can perform CPU cache overlap
and doesn't perform cache flushing.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
---
 Documentation/core-api/dma-attributes.rst | 16 ++++++++++++++++
 include/linux/dma-mapping.h               |  7 +++++++
 include/trace/events/dma.h                |  3 ++-
 kernel/dma/debug.c                        |  3 ++-
 kernel/dma/mapping.c                      |  6 ++++++
 5 files changed, 33 insertions(+), 2 deletions(-)

diff --git a/Documentation/core-api/dma-attributes.rst b/Documentation/core-api/dma-attributes.rst
index 48cfe86cc06d7..441bdc9d08318 100644
--- a/Documentation/core-api/dma-attributes.rst
+++ b/Documentation/core-api/dma-attributes.rst
@@ -163,3 +163,19 @@ data corruption.
 
 All mappings that share a cache line must set this attribute to suppress DMA
 debug warnings about overlapping mappings.
+
+DMA_ATTR_REQUIRE_COHERENT
+-------------------------
+
+DMA mapping requests with the DMA_ATTR_REQUIRE_COHERENT fail on any
+system where SWIOTLB or cache management is required. This should only
+be used to support uAPI designs that require continuous HW DMA
+coherence with userspace processes, for example RDMA and DRM. At a
+minimum the memory being mapped must be userspace memory from
+pin_user_pages() or similar.
+
+Drivers should consider using dma_mmap_pages() instead of this
+interface when building their uAPIs, when possible.
+
+It must never be used in an in-kernel driver that only works with
+kernal memory.
diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index da44394b3a1a7..482b919f040f7 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -86,6 +86,13 @@
  */
 #define DMA_ATTR_DEBUGGING_IGNORE_CACHELINES	(1UL << 11)
 
+/*
+ * DMA_ATTR_REQUIRE_COHERENT: Indicates that DMA coherency is required.
+ * All mappings that carry this attribute can't work with SWIOTLB and cache
+ * flushing.
+ */
+#define DMA_ATTR_REQUIRE_COHERENT	(1UL << 12)
+
 /*
  * 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 8c64bc0721fe4..63597b0044247 100644
--- a/include/trace/events/dma.h
+++ b/include/trace/events/dma.h
@@ -33,7 +33,8 @@ TRACE_DEFINE_ENUM(DMA_NONE);
 		{ DMA_ATTR_NO_WARN, "NO_WARN" }, \
 		{ DMA_ATTR_PRIVILEGED, "PRIVILEGED" }, \
 		{ DMA_ATTR_MMIO, "MMIO" }, \
-		{ DMA_ATTR_DEBUGGING_IGNORE_CACHELINES, "CACHELINES_OVERLAP" })
+		{ DMA_ATTR_DEBUGGING_IGNORE_CACHELINES, "CACHELINES_OVERLAP" }, \
+		{ DMA_ATTR_REQUIRE_COHERENT, "REQUIRE_COHERENT" })
 
 DECLARE_EVENT_CLASS(dma_map,
 	TP_PROTO(struct device *dev, phys_addr_t phys_addr, dma_addr_t dma_addr,
diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c
index 83e1cfe05f08d..0677918f06a80 100644
--- a/kernel/dma/debug.c
+++ b/kernel/dma/debug.c
@@ -601,7 +601,8 @@ static void add_dma_entry(struct dma_debug_entry *entry, unsigned long attrs)
 	unsigned long flags;
 	int rc;
 
-	entry->is_cache_clean = attrs & DMA_ATTR_DEBUGGING_IGNORE_CACHELINES;
+	entry->is_cache_clean = attrs & (DMA_ATTR_DEBUGGING_IGNORE_CACHELINES |
+					 DMA_ATTR_REQUIRE_COHERENT);
 
 	bucket = get_hash_bucket(entry, &flags);
 	hash_bucket_add(bucket, entry);
diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
index 3928a509c44c2..6d3dd0bd3a886 100644
--- a/kernel/dma/mapping.c
+++ b/kernel/dma/mapping.c
@@ -164,6 +164,9 @@ dma_addr_t dma_map_phys(struct device *dev, phys_addr_t phys, size_t size,
 	if (WARN_ON_ONCE(!dev->dma_mask))
 		return DMA_MAPPING_ERROR;
 
+	if (!dev_is_dma_coherent(dev) && (attrs & DMA_ATTR_REQUIRE_COHERENT))
+		return DMA_MAPPING_ERROR;
+
 	if (dma_map_direct(dev, ops) ||
 	    (!is_mmio && arch_dma_map_phys_direct(dev, phys + size)))
 		addr = dma_direct_map_phys(dev, phys, size, dir, attrs);
@@ -235,6 +238,9 @@ static int __dma_map_sg_attrs(struct device *dev, struct scatterlist *sg,
 
 	BUG_ON(!valid_dma_direction(dir));
 
+	if (!dev_is_dma_coherent(dev) && (attrs & DMA_ATTR_REQUIRE_COHERENT))
+		return -EOPNOTSUPP;
+
 	if (WARN_ON_ONCE(!dev->dma_mask))
 		return 0;
 

-- 
2.53.0
Re: [PATCH v3 4/8] dma-mapping: Introduce DMA require coherency attribute
Posted by Randy Dunlap 3 weeks ago 

On 3/16/26 12:06 PM, Leon Romanovsky wrote:
> diff --git a/Documentation/core-api/dma-attributes.rst b/Documentation/core-api/dma-attributes.rst
> index 48cfe86cc06d7..441bdc9d08318 100644
> --- a/Documentation/core-api/dma-attributes.rst
> +++ b/Documentation/core-api/dma-attributes.rst
> @@ -163,3 +163,19 @@ data corruption.
>  
>  All mappings that share a cache line must set this attribute to suppress DMA
>  debug warnings about overlapping mappings.
> +
> +DMA_ATTR_REQUIRE_COHERENT
> +-------------------------
> +
> +DMA mapping requests with the DMA_ATTR_REQUIRE_COHERENT fail on any
> +system where SWIOTLB or cache management is required. This should only
> +be used to support uAPI designs that require continuous HW DMA
> +coherence with userspace processes, for example RDMA and DRM. At a
> +minimum the memory being mapped must be userspace memory from
> +pin_user_pages() or similar.
> +
> +Drivers should consider using dma_mmap_pages() instead of this
> +interface when building their uAPIs, when possible.
> +
> +It must never be used in an in-kernel driver that only works with
> +kernal memory.

   kernel

-- 
~Randy
Re: [PATCH v3 4/8] dma-mapping: Introduce DMA require coherency attribute
Posted by Leon Romanovsky 3 weeks ago 
On Mon, Mar 16, 2026 at 12:17:39PM -0700, Randy Dunlap wrote:
> 
> 
> On 3/16/26 12:06 PM, Leon Romanovsky wrote:
> > diff --git a/Documentation/core-api/dma-attributes.rst b/Documentation/core-api/dma-attributes.rst
> > index 48cfe86cc06d7..441bdc9d08318 100644
> > --- a/Documentation/core-api/dma-attributes.rst
> > +++ b/Documentation/core-api/dma-attributes.rst
> > @@ -163,3 +163,19 @@ data corruption.
> >  
> >  All mappings that share a cache line must set this attribute to suppress DMA
> >  debug warnings about overlapping mappings.
> > +
> > +DMA_ATTR_REQUIRE_COHERENT
> > +-------------------------
> > +
> > +DMA mapping requests with the DMA_ATTR_REQUIRE_COHERENT fail on any
> > +system where SWIOTLB or cache management is required. This should only
> > +be used to support uAPI designs that require continuous HW DMA
> > +coherence with userspace processes, for example RDMA and DRM. At a
> > +minimum the memory being mapped must be userspace memory from
> > +pin_user_pages() or similar.
> > +
> > +Drivers should consider using dma_mmap_pages() instead of this
> > +interface when building their uAPIs, when possible.
> > +
> > +It must never be used in an in-kernel driver that only works with
> > +kernal memory.
> 
>    kernel

Thanks, let's hope that it is the only one comment :).

> 
> -- 
> ~Randy
>

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