This is a note to let you know that I've just added the patch titled
dma-mapping: add dma_opt_mapping_size()
to the 5.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
dma-mapping-add-dma_opt_mapping_size.patch
and it can be found in the queue-5.15 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit f3accc328fee29391f06da3097bc0cc49b7f84b3
Author: John Garry <john.garry@xxxxxxxxxx>
Date: Thu Jul 14 19:15:24 2022 +0800
dma-mapping: add dma_opt_mapping_size()
[ Upstream commit a229cc14f3395311b899e5e582b71efa8dd01df0 ]
Streaming DMA mapping involving an IOMMU may be much slower for larger
total mapping size. This is because every IOMMU DMA mapping requires an
IOVA to be allocated and freed. IOVA sizes above a certain limit are not
cached, which can have a big impact on DMA mapping performance.
Provide an API for device drivers to know this "optimal" limit, such that
they may try to produce mapping which don't exceed it.
Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
Reviewed-by: Damien Le Moal <damien.lemoal@xxxxxxxxxxxxxxxxxx>
Acked-by: Martin K. Petersen <martin.petersen@xxxxxxxxxx>
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
Stable-dep-of: afc5aa46ed56 ("iommu/dma: Force swiotlb_max_mapping_size on an untrusted device")
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/Documentation/core-api/dma-api.rst b/Documentation/core-api/dma-api.rst
index 6d6d0edd2d278..829f20a193cab 100644
--- a/Documentation/core-api/dma-api.rst
+++ b/Documentation/core-api/dma-api.rst
@@ -204,6 +204,20 @@ Returns the maximum size of a mapping for the device. The size parameter
of the mapping functions like dma_map_single(), dma_map_page() and
others should not be larger than the returned value.
+::
+
+ size_t
+ dma_opt_mapping_size(struct device *dev);
+
+Returns the maximum optimal size of a mapping for the device.
+
+Mapping larger buffers may take much longer in certain scenarios. In
+addition, for high-rate short-lived streaming mappings, the upfront time
+spent on the mapping may account for an appreciable part of the total
+request lifetime. As such, if splitting larger requests incurs no
+significant performance penalty, then device drivers are advised to
+limit total DMA streaming mappings length to the returned value.
+
::
bool
diff --git a/include/linux/dma-map-ops.h b/include/linux/dma-map-ops.h
index bfffe494356ad..2ff55ec902f48 100644
--- a/include/linux/dma-map-ops.h
+++ b/include/linux/dma-map-ops.h
@@ -69,6 +69,7 @@ struct dma_map_ops {
int (*dma_supported)(struct device *dev, u64 mask);
u64 (*get_required_mask)(struct device *dev);
size_t (*max_mapping_size)(struct device *dev);
+ size_t (*opt_mapping_size)(void);
unsigned long (*get_merge_boundary)(struct device *dev);
};
diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index dca2b1355bb13..fe3849434b2a2 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -144,6 +144,7 @@ int dma_set_mask(struct device *dev, u64 mask);
int dma_set_coherent_mask(struct device *dev, u64 mask);
u64 dma_get_required_mask(struct device *dev);
size_t dma_max_mapping_size(struct device *dev);
+size_t dma_opt_mapping_size(struct device *dev);
bool dma_need_sync(struct device *dev, dma_addr_t dma_addr);
unsigned long dma_get_merge_boundary(struct device *dev);
struct sg_table *dma_alloc_noncontiguous(struct device *dev, size_t size,
@@ -266,6 +267,10 @@ static inline size_t dma_max_mapping_size(struct device *dev)
{
return 0;
}
+static inline size_t dma_opt_mapping_size(struct device *dev)
+{
+ return 0;
+}
static inline bool dma_need_sync(struct device *dev, dma_addr_t dma_addr)
{
return false;
diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
index 9478eccd1c8e6..c9dbc8f5812b8 100644
--- a/kernel/dma/mapping.c
+++ b/kernel/dma/mapping.c
@@ -777,6 +777,18 @@ size_t dma_max_mapping_size(struct device *dev)
}
EXPORT_SYMBOL_GPL(dma_max_mapping_size);
+size_t dma_opt_mapping_size(struct device *dev)
+{
+ const struct dma_map_ops *ops = get_dma_ops(dev);
+ size_t size = SIZE_MAX;
+
+ if (ops && ops->opt_mapping_size)
+ size = ops->opt_mapping_size();
+
+ return min(dma_max_mapping_size(dev), size);
+}
+EXPORT_SYMBOL_GPL(dma_opt_mapping_size);
+
bool dma_need_sync(struct device *dev, dma_addr_t dma_addr)
{
const struct dma_map_ops *ops = get_dma_ops(dev);
