Hi Boris,

Thank you for the patch.

On Tuesday 08 March 2016 12:15:12 Boris Brezillon wrote:

This will lead to page-sized scatter-gather entries even for pages of the 
vmalloc memory that happen to be physically contiguous. That works, but I 
wonder whether we'd want to be more efficient.

+	if (!IS_ALIGNED((unsigned long)buf, cons->preferred_alignment)) {
+		const void *aligned_buf = PTR_ALIGN(buf,
+						    cons->preferred_alignment);
+		size_t unaligned_len = (unsigned long)(aligned_buf - buf);
+
+		chunk_len = min_t(size_t, chunk_len, unaligned_len);
+	} else if (chunk_len > cons->preferred_alignment) {
+		chunk_len &= ~(cons->preferred_alignment - 1);
+	}
+
+	return chunk_len;
+}
+
+#define sg_for_each_chunk_in_buf(buf, len, chunk_len, constraints)	\
+	for (chunk_len = sg_buf_chunk_len(buf, len, constraints);	\
+	     len;							\
+	     len -= chunk_len, buf += chunk_len,			\
+	     chunk_len = sg_buf_chunk_len(buf, len, constraints))
+
+static int sg_check_constraints(struct sg_constraints *cons,
+				const void *buf, size_t len)
+{
+	if (!cons->required_alignment)
+		cons->required_alignment = 1;
+
+	if (!cons->preferred_alignment)
+		cons->preferred_alignment = cons->required_alignment;
+
+	/* Test if buf and len are properly aligned. */
+	if (!IS_ALIGNED((unsigned long)buf, cons->required_alignment) ||
+	    !IS_ALIGNED(len, cons->required_alignment))
+		return -EINVAL;
+
+	/*
+	 * if the buffer has been vmallocated and required_alignment is
+	 * more than PAGE_SIZE we cannot guarantee it.
+	 */
+	if (is_vmalloc_addr(buf) && cons->required_alignment > PAGE_SIZE)
+		return -EINVAL;
+
+	/*
+	 * max_chunk_len has to be aligned to required_alignment to
+	 * guarantee that all buffer chunks are aligned correctly.
+	 */
+	if (!IS_ALIGNED(cons->max_chunk_len, cons->required_alignment))
+		return -EINVAL;
+
+	/*
+	 * preferred_alignment has to be aligned to required_alignment
+	 * to avoid misalignment of buffer chunks.
+	 */
+	if (!IS_ALIGNED(cons->preferred_alignment, cons->required_alignment))
+		return -EINVAL;
+
+	return 0;
+}
+
+/**
+ * sg_alloc_table_from_buf - create an SG table from a buffer
+ *
+ * @sgt: SG table
+ * @buf: buffer you want to create this SG table from
+ * @len: length of buf
+ * @constraints: optional constraints to take into account when creating
+ *		 the SG table. Can be NULL if no specific constraints are
+ *		 required.
+ * @gfp_mask: type of allocation to use when creating the table
+ *
+ * This function creates an SG table from a buffer, its length and some
+ * SG constraints.
+ *
+ * Note: This function supports vmallocated buffers.

What other types of memory does it support ? kmalloc() quite obviously, are 
there others ? I think you should explicitly list the memory types that the 
function intends to support.

+ */
+int sg_alloc_table_from_buf(struct sg_table *sgt, const void *buf, size_t
len,
+			    const struct sg_constraints *constraints,
+			    gfp_t gfp_mask)
+{
+	struct sg_constraints cons = { };
+	size_t remaining, chunk_len;
+	const void *sg_buf;
+	int i, ret;
+
+	if (constraints)
+		cons = *constraints;
+
+	ret = sg_check_constraints(&cons, buf, len);
+	if (ret)
+		return ret;
+
+	sg_buf = buf;
+	remaining = len;
+	i = 0;
+	sg_for_each_chunk_in_buf(sg_buf, remaining, chunk_len, &cons)
+		i++;
+
+	ret = sg_alloc_table(sgt, i, gfp_mask);
+	if (ret)
+		return ret;
+
+	sg_buf = buf;
+	remaining = len;
+	i = 0;
+	sg_for_each_chunk_in_buf(sg_buf, remaining, chunk_len, &cons) {
+		if (is_vmalloc_addr(sg_buf)) {
+			struct page *vm_page;
+
+			vm_page = vmalloc_to_page(sg_buf);
+			if (!vm_page) {
+				ret = -ENOMEM;
+				goto err_free_table;
+			}
+
+			sg_set_page(&sgt->sgl[i], vm_page, chunk_len,
+				    offset_in_page(sg_buf));
+		} else {
+			sg_set_buf(&sgt->sgl[i], sg_buf, chunk_len);
+		}
+
+		i++;
+	}
+
+	return 0;
+
+err_free_table:
+	sg_free_table(sgt);
+
+	return ret;
+}
+EXPORT_SYMBOL(sg_alloc_table_from_buf);
+
 void __sg_page_iter_start(struct sg_page_iter *piter,
 			  struct scatterlist *sglist, unsigned int nents,
 			  unsigned long pgoffset)

-- 
Regards,

Laurent Pinchart

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