Re: [RFCv2 1/2] device: add dma_params->max_segment_count

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Hi Sumit,

On 21/01/15 04:16, Sumit Semwal wrote:
From: Rob Clark <robdclark@xxxxxxxxx>

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@xxxxxxxxx>
  [sumits: Minor updates wrt comments on the first version]
Signed-off-by: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
---
  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 fb50673..a32f9b6 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 c3007cb..38e2835 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)
+{
+	return dev->dma_parms ?
+			dev->dma_parms->max_segment_count :
+			DMA_SEGMENTS_MAX_SEG_COUNT;
+}

I know this copies the style of the existing code, but unfortunately it also copies the subtle brokenness. Plenty of drivers seem to set up a dma_parms struct just for max_segment_size, thus chances are you'll come across a max_segment_count of 0 sooner or later. How badly is that going to break things? I posted a fix recently[1] having hit this problem with segment_boundary_mask in IOMMU code.

+
+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;
+	} else

This "else" is just as unnecessary as the other two I've taken out ;)


Robin.

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

+		return -EIO;
+}
+
  static inline unsigned long dma_get_seg_boundary(struct device *dev)
  {
  	return dev->dma_parms ?



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