[RFCv3 1/2] device: add dma_params->max_segment_count

[sumit.semwal@linaro.org (Sumit Semwal)]

From: Rob Clark <robdclark@gmail.com>

For devices which have constraints about maximum number of segments in
an sglist.  For example, a device which could only deal with contiguous
buffers would set max_segment_count to 1.

The initial motivation is for devices sharing buffers via dma-buf,
to allow the buffer exporter to know the constraints of other
devices which have attached to the buffer.  The dma_mask and fields
in 'struct device_dma_parameters' tell the exporter everything else
that is needed, except whether the importer has constraints about
maximum number of segments.

Signed-off-by: Rob Clark <robdclark@gmail.com>
 [sumits: Minor updates wrt comments]
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
---

v3: include Robin Murphy's fix[1] for handling '0' as a value for
     max_segment_count
v2: minor updates wrt comments on the first version

[1]: http://article.gmane.org/gmane.linux.kernel.iommu/8175/

 include/linux/device.h      |  1 +
 include/linux/dma-mapping.h | 19 +++++++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/include/linux/device.h b/include/linux/device.h
index fb506738f7b7..a32f9b67315c 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -647,6 +647,7 @@ struct device_dma_parameters {
 	 * sg limitations.
 	 */
 	unsigned int max_segment_size;
+	unsigned int max_segment_count;    /* INT_MAX for unlimited */
 	unsigned long segment_boundary_mask;
 };
 
diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index c3007cb4bfa6..d3351a36d5ec 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -154,6 +154,25 @@ static inline unsigned int dma_set_max_seg_size(struct device *dev,
 		return -EIO;
 }
 
+#define DMA_SEGMENTS_MAX_SEG_COUNT ((unsigned int) INT_MAX)
+
+static inline unsigned int dma_get_max_seg_count(struct device *dev)
+{
+	if (dev->dma_parms && dev->dma_parms->max_segment_count)
+		return dev->dma_parms->max_segment_count;
+	return DMA_SEGMENTS_MAX_SEG_COUNT;
+}
+
+static inline int dma_set_max_seg_count(struct device *dev,
+						unsigned int count)
+{
+	if (dev->dma_parms) {
+		dev->dma_parms->max_segment_count = count;
+		return 0;
+	}
+	return -EIO;
+}
+
 static inline unsigned long dma_get_seg_boundary(struct device *dev)
 {
 	return dev->dma_parms ?
-- 
1.9.1