From: Rusty Russell <rusty@rustcorp.com.au>
Date: Fri, 9 Nov 2007 20:29:16 +1100
Subject: Introduce sg_ring: a ring of scatterlist arrays.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 include/linux/sg_ring.h |   74 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 74 insertions(+), 0 deletions(-)
 create mode 100644 include/linux/sgring.h

diff --git a/include/linux/sgring.h b/include/linux/sg_ring.h
new file mode 100644
index 0000000..286376f
--- /dev/null
+++ b/include/linux/sg_ring.h
@@ -0,0 +1,74 @@
+#ifndef _LINUX_SG_RING_H
+#define _LINUX_SG_RING_H
+#include <linux/scatterlist.h>
+
+/**
+ * struct sg_ring - a ring of scatterlists
+ * @list: the list_head chaining them together
+ * @num: the number of valid sg entries
+ * @max: the maximum number of sg entries (size of the sg array).
+ * @sg: the array of scatterlist entries.
+ *
+ * This provides a convenient encapsulation of one or more scatter gather
+ * arrays.
+ */
+struct sg_ring
+{
+	struct list_head list;
+	unsigned int num, max;
+	struct scatterlist sg[0];
+};
+
+/* This helper declares an sg ring on the stack or in a struct. */
+#define DECLARE_SG_RING(name, max)		\
+	struct {				\
+		struct sg_ring ring;		\
+		struct scatterlist sg[max];	\
+	} name
+
+/**
+ * sg_ring_init - initialize a scatterlist ring.
+ * @sg: the sg_ring.
+ * @max: the size of the trailing sg array.
+ *
+ * After initialization sg is alone in the ring.
+ */
+static inline void sg_ring_init(struct sg_ring *sg, unsigned int max)
+{
+#ifdef CONFIG_DEBUG_SG
+	unsigned int i;
+	for (i = 0; i < max; i++)
+		sg->sg[i].sg_magic = SG_MAGIC;
+#endif
+	INIT_LIST_HEAD(&sg->list);
+	sg->max = max;
+}
+
+/**
+ * sg_ring_next - next array in a scatterlist ring.
+ * @sg: the sg_ring.
+ *
+ * After initialization sg is alone in the ring.
+ */
+static inline struct sg_ring *sg_ring_next(struct sg_ring *sg)
+{
+	return list_first_entry(&sg->list, struct sg_ring, list);
+}
+
+/**
+ * sg_ring_single - initialize a one-element scatterlist ring.
+ * @sg: the sg_ring.
+ * @buf: the pointer to the buffer.
+ * @buflen: the length of the buffer.
+ *
+ * Does sg_ring_init and also sets up first (and only) sg element.
+ */
+static inline void sg_ring_single(struct sg_ring *sg,
+				  const void *buf,
+				  unsigned int buflen)
+{
+	sg_ring_init(sg, 1);
+	sg->num = 1;
+	sg_init_one(&sg->sg[0], buf, buflen);
+}
+#endif /* _LINUX_SG_RING_H */