[RFC PATCH 1/3] add full_buffer_write flag to struct device_dma_parameters

[Jianxiong Gao]

When devices write to a buffer, some devices are guaranteed to
overwrite the entire buffer. These devices may benefit from such
behaviors by reducing the need to pre-condition the original
buffer. For example when bouncing data through swiotlb, the buffer
is forced to be synced before the device writes to it. For devices
that we know for sure overwrites the entire buffer, this flag can
be utilized to eliminate the extra copy on every IO bounced
through the swiotlb.

Signed-off-by: Jianxiong Gao <jxgao at google.com>
---
 include/linux/device.h      |  1 +
 include/linux/dma-mapping.h | 15 +++++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/include/linux/device.h b/include/linux/device.h
index 80a5b3268986..003007ad6ad3 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -460,6 +460,7 @@ struct device_dma_parameters {
 	 * a low level driver may set these to teach IOMMU code about
 	 * sg limitations.
 	 */
+	bool full_buffer_write;
 	unsigned int max_segment_size;
 	unsigned int min_align_mask;
 	unsigned long segment_boundary_mask;
diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index b79925b1c433..e93b909865d3 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -513,6 +513,21 @@ static inline int dma_coerce_mask_and_coherent(struct device *dev, u64 mask)
 	return dma_set_mask_and_coherent(dev, mask);
 }
 
+static inline bool dma_is_full_buffer_write(struct device *dev)
+{
+	if (dev->dma_parms)
+		return dev->dma_parms->full_buffer_write;
+	return false;
+}
+
+static inline int dma_set_full_buffer_write(struct device *dev, bool full_buffer_write)
+{
+	if (WARN_ON_ONCE(!dev->dma_parms))
+		return -EIO;
+	dev->dma_parms->full_buffer_write = full_buffer_write;
+	return 0;
+}
+
 static inline unsigned int dma_get_max_seg_size(struct device *dev)
 {
 	if (dev->dma_parms && dev->dma_parms->max_segment_size)
-- 
2.48.1.362.g079036d154-goog