[RFC PATCH 1/2] sg_ring instead of scatterlist chaining

[Rusty Russell <rusty@rustcorp.com.au>]

Hi all,

	This patch implements a header for a linked list of scatterlist
 arrays, rather than using an extra entry and low pointer bits to chain them
together.  I've tested that it's sane for virtio (which uses struct
scatterlist).

Features:
1) Neatens code by including length in structure.
2) Avoids end ambiguity by including maximum length too.
3) Works fine with old "sg is an array" interfaces.
4) Kinda icky for stack declaration, so hence a helper is created.
5) Lacks magic.

I reverted (most of?) the scatterlist chaining changes to create these
patches, so it won't apply to your kernels.  The reversion patch isn't
interesting, so I haven't posted it.

Thanks,
Rusty.

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 4efbd9c..ce7e581 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -5,6 +5,51 @@
 #include <linux/mm.h>
 #include <linux/string.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(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)
+{
+	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);
+}
+
 static inline void sg_set_buf(struct scatterlist *sg, const void *buf,
 			      unsigned int buflen)
 {
@@ -20,4 +65,20 @@ static inline void sg_init_one(struct scatterlist *sg, const void *buf,
 	sg_set_buf(sg, buf, buflen);
 }
 
+/**
+ * sg_init_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_init_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_SCATTERLIST_H */