On Thu, Mar 1, 2012 at 12:04 AM, Marek Szyprowski
[off-list ref] wrote:

+/**
+ * arm_iommu_map_sg - map a set of SG buffers for streaming mode DMA
+ * @dev: valid struct device pointer
+ * @sg: list of buffers
+ * @nents: number of buffers to map
+ * @dir: DMA transfer direction
+ *
+ * Map a set of buffers described by scatterlist in streaming mode for DMA.
+ * The scatter gather list elements are merged together (if possible) and
+ * tagged with the appropriate dma address and length. They are obtained via
+ * sg_dma_{address,length}.
+ */
+int arm_iommu_map_sg(struct device *dev, struct scatterlist *sg, int nents,
+ ? ? ? ? ? ? ? ? ? ?enum dma_data_direction dir, struct dma_attrs *attrs)
+{
+ ? ? ? struct scatterlist *s = sg, *dma = sg, *start = sg;
+ ? ? ? int i, count = 0;
+ ? ? ? unsigned int offset = s->offset;
+ ? ? ? unsigned int size = s->offset + s->length;
+ ? ? ? unsigned int max = dma_get_max_seg_size(dev);
+
+ ? ? ? for (i = 1; i < nents; i++) {
+ ? ? ? ? ? ? ? s->dma_address = ARM_DMA_ERROR;
+ ? ? ? ? ? ? ? s->dma_length = 0;
+
+ ? ? ? ? ? ? ? s = sg_next(s);
+
+ ? ? ? ? ? ? ? if (s->offset || (size & ~PAGE_MASK) || size + s->length > max) {
+ ? ? ? ? ? ? ? ? ? ? ? if (__map_sg_chunk(dev, start, size, &dma->dma_address,
+ ? ? ? ? ? ? ? ? ? ? ? ? ? dir) < 0)
+ ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? goto bad_mapping;
+
+ ? ? ? ? ? ? ? ? ? ? ? dma->dma_address += offset;
+ ? ? ? ? ? ? ? ? ? ? ? dma->dma_length = size - offset;
+
+ ? ? ? ? ? ? ? ? ? ? ? size = offset = s->offset;
+ ? ? ? ? ? ? ? ? ? ? ? start = s;
+ ? ? ? ? ? ? ? ? ? ? ? dma = sg_next(dma);
+ ? ? ? ? ? ? ? ? ? ? ? count += 1;
+ ? ? ? ? ? ? ? }
+ ? ? ? ? ? ? ? size += s->length;
+ ? ? ? }
+ ? ? ? if (__map_sg_chunk(dev, start, size, &dma->dma_address, dir) < 0)
+ ? ? ? ? ? ? ? goto bad_mapping;
+
+ ? ? ? dma->dma_address += offset;
+ ? ? ? dma->dma_length = size - offset;
+
+ ? ? ? return count+1;
+
+bad_mapping:
+ ? ? ? for_each_sg(sg, s, count, i)
+ ? ? ? ? ? ? ? __iommu_remove_mapping(dev, sg_dma_address(s), sg_dma_len(s));
+ ? ? ? return 0;
+}
+

This looks that the given sg list specifies the list of physical
memory chunks and
the list of IO virtual memory chunks at the same time after calling
arm_dma_map_sg().
It can happen that dma_address and dma_length of a sg entry does not
correspond to
physical memory information of the sg entry.

I think it is beneficial for handling IO virtual memory.

However, I worry about any other problems caused by a single sg entry contains
information from 2 different context.

Regards,

Cho KyongHo.