drm: prime: add common helper to check scatterlist contiguity

It is a common operation done by DRM drivers to check the contiguity
of the DMA-mapped buffer described by a scatterlist in the
sg_table object. Let's add a common helper for this operation.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>

drivers/gpu/drm/drm_prime.c

@@ -825,6 +825,37 @@ out:
 }
 EXPORT_SYMBOL(drm_prime_pages_to_sg);
 
+/**
+ * drm_prime_get_contiguous_size - returns the contiguous size of the buffer
+ * @sgt: sg_table describing the buffer to check
+ *
+ * This helper calculates the contiguous size in the DMA address space
+ * of the the buffer described by the provided sg_table.
+ *
+ * This is useful for implementing
+ * &drm_gem_object_funcs.gem_prime_import_sg_table.
+ */
+unsigned long drm_prime_get_contiguous_size(struct sg_table *sgt)
+{
+	dma_addr_t expected = sg_dma_address(sgt->sgl);
+	struct scatterlist *sg;
+	unsigned long size = 0;
+	int i;
+
+	for_each_sgtable_dma_sg(sgt, sg, i) {
+		unsigned int len = sg_dma_len(sg);
+
+		if (!len)
+			break;
+		if (sg_dma_address(sg) != expected)
+			break;
+		expected += len;
+		size += len;
+	}
+	return size;
+}
+EXPORT_SYMBOL(drm_prime_get_contiguous_size);
+
 /**
  * drm_gem_prime_export - helper library implementation of the export callback
  * @obj: GEM object to export