Hi Robin!

On 22 January 2015 at 00:26, Robin Murphy robin.murphy@arm.com wrote:

Hi Sumit,

On 21/01/15 04:16, Sumit Semwal wrote:

...

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 on the first version] Signed-off-by: Sumit Semwal sumit.semwal@linaro.org

---

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.

Thanks very much for reviewing this code; and apologies for missing your patch that you mentioned here; sure, I will update my patch accordingly as well.

...

+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.

...

•           return -EIO;
  

+}

• static inline unsigned long dma_get_seg_boundary(struct device *dev) { return dev->dma_parms ?

BR, Sumit.