[PATCH] swiotlb: Introduce DMA_ATTR_SKIP_DEVICE_SYNC

[Florian Fainelli]

From: Justin Chen <justin.chen@xxxxxxxxxxxx>

Network device driver's receive path typically do the following:

- dma_map_single(.., DMA_FROM_DEVICE)
- dma_sync_single_for_cpu() to allow the CPU to inspect packet
  descriptors
- dma_unmap_single(.., DMA_FROM_DEVICE) when releasing the buffer

Each of those operations incurs a copy from the original buffer to the
TLB buffer, even if the device is known to be writing full buffers.

Add a DMA_ATTR_SKIP_DEVICE_SYNC flag which can be set by device drivers
to skip the copy at dma_map_single() to speed up the RX path when the
device is known to be doing full buffer writes.

This has been seen to provide a 20% speedup for Wi-Fi RX throughput
testing.

Signed-off-by: Justin Chen <justin.chen@xxxxxxxxxxxx>
[florian: commit message, add DMA-API attribute flag]
Signed-off-by: Florian Fainelli <florian.fainelli@xxxxxxxxxxxx>
---
 Documentation/core-api/dma-attributes.rst | 9 +++++++++
 Documentation/core-api/swiotlb.rst        | 4 +++-
 include/linux/dma-mapping.h               | 6 ++++++
 include/trace/events/dma.h                | 3 ++-
 kernel/dma/swiotlb.c                      | 8 ++++++++
 5 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/Documentation/core-api/dma-attributes.rst b/Documentation/core-api/dma-attributes.rst
index 1887d92e8e92..ccd9c1891200 100644
--- a/Documentation/core-api/dma-attributes.rst
+++ b/Documentation/core-api/dma-attributes.rst
@@ -130,3 +130,12 @@ 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_SKIP_DEVICE_SYNC
+-------------------------
+
+Device drivers can set DMA_ATTR_SKIP_DEVICE_SYNC in order to avoid doing a copy
+from the original buffer to the TLB buffer for dma_map_single() with a
+DMA_FROM_DEVICE direction. This can be used to save an extra copy in a device
+driver's data path when using swiotlb bounce buffering.
+
diff --git a/Documentation/core-api/swiotlb.rst b/Documentation/core-api/swiotlb.rst
index 9e0fe027dd3b..3bc1f9ba67b2 100644
--- a/Documentation/core-api/swiotlb.rst
+++ b/Documentation/core-api/swiotlb.rst
@@ -67,7 +67,9 @@ to the driver for programming into the device. If a DMA operation specifies
 multiple memory buffer segments, a separate bounce buffer must be allocated for
 each segment. swiotlb_tbl_map_single() always does a "sync" operation (i.e., a
 CPU copy) to initialize the bounce buffer to match the contents of the original
-buffer.
+buffer, except if DMA_ATTR_SKIP_DEVICE_SYNC is specified and the direction is
+DMA_FROM_DEVICE. This is a performance optimization that may not be suitable for
+all platforms.
 
 swiotlb_tbl_unmap_single() does the reverse. If the DMA operation might have
 updated the bounce buffer memory and DMA_ATTR_SKIP_CPU_SYNC is not set, the
diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index b79925b1c433..bfdaa65f8e9d 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -58,6 +58,12 @@
  */
 #define DMA_ATTR_PRIVILEGED		(1UL << 9)
 
+/*
+ * DMA_ATTR_SKIP_DEVICE_SYNC: used to indicate that the buffer does not need to
+ * be synchronized to the device.
+ */
+#define DMA_ATTR_SKIP_DEVICE_SYNC	(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 d8ddc27b6a7c..6eb8fd7e3515 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_SKIP_DEVICE_SYNC, "SKIP_DEVICE_SYNC" })
 
 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/swiotlb.c b/kernel/dma/swiotlb.c
index abcf3fa63a56..8dab89bf5e33 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -1435,8 +1435,16 @@ phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
 	 * the original data, even if it's garbage, is necessary to match
 	 * hardware behavior.  Use of swiotlb is supposed to be transparent,
 	 * i.e. swiotlb must not corrupt memory by clobbering unwritten bytes.
+	 *
+	 * Setting DMA_ATTR_SKIP_DEVICE_SYNC will negate the behavior described
+	 * before and avoid the copy from the original buffer to the TLB
+	 * buffer.
 	 */
+	if (dir == DMA_FROM_DEVICE && (attrs & DMA_ATTR_SKIP_DEVICE_SYNC))
+		goto out;
+
 	swiotlb_bounce(dev, tlb_addr, mapping_size, DMA_TO_DEVICE, pool);
+out:
 	return tlb_addr;
 }
 
-- 
2.43.0