Change table chaining layout

Change the page member of the scatterlist structure to be an unsigned
long, and encode more stuff in the lower bits:

- Bits 0 and 1 zero: this is a normal sg entry. Next sg entry is located
  at sg + 1.
- Bit 0 set: this is a chain entry, the next real entry is at ->page_link
  with the two low bits masked off.
- Bit 1 set: this is the final entry in the sg entry. sg_next() will return
  NULL when passed such an entry.

It's thus important that sg table users use the proper accessors to get
and set the page member.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 54 insertions, 24 deletions

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 1645795..c6136e8 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -2,9 +2,26 @@
 #define _LINUX_SCATTERLIST_H
 
 #include <asm/scatterlist.h>
-#include <asm/io.h>
 #include <linux/mm.h>
 #include <linux/string.h>
+#include <asm/io.h>
+
+/*
+ * Notes on SG table design.
+ *
+ * Architectures must provide an unsigned long page_link field in the
+ * scatterlist struct. We use that to place the page pointer AND encode
+ * information about the sg table as well. The two lower bits are reserved
+ * for this information.
+ *
+ * If bit 0 is set, then the page_link contains a pointer to the next sg
+ * table list. Otherwise the next entry is at sg + 1.
+ *
+ * If bit 1 is set, then this sg entry is the last element in a list.
+ *
+ * See sg_next().
+ *
+ */
 
 /**
  * sg_set_page - Set sg entry to point at given page
@@ -20,11 +37,20 @@
  **/
 static inline void sg_set_page(struct scatterlist *sg, struct page *page)
 {
-	sg->page = page;
+	unsigned long page_link = sg->page_link & 0x3;
+
+	sg->page_link = page_link | (unsigned long) page;
 }
 
-#define sg_page(sg)	((sg)->page)
+#define sg_page(sg)	((struct page *) ((sg)->page_link & ~0x3))
 
+/**
+ * sg_set_buf - Set sg entry to point at given data
+ * @sg:		 SG entry
+ * @buf:	 Data
+ * @buflen:	 Data length
+ *
+ **/
 static inline void sg_set_buf(struct scatterlist *sg, const void *buf,
 			      unsigned int buflen)
 {
@@ -38,26 +64,27 @@ static inline void sg_set_buf(struct scatterlist *sg, const void *buf,
  * a valid sg entry, or whether it points to the start of a new scatterlist.
  * Those low bits are there for everyone! (thanks mason :-)
  */
-#define sg_is_chain(sg)		((unsigned long) (sg)->page & 0x01)
+#define sg_is_chain(sg)		((sg)->page_link & 0x01)
+#define sg_is_last(sg)		((sg)->page_link & 0x02)
 #define sg_chain_ptr(sg)	\
-	((struct scatterlist *) ((unsigned long) (sg)->page & ~0x01))
+	((struct scatterlist *) ((sg)->page_link & ~0x03))
 
 /**
  * sg_next - return the next scatterlist entry in a list
  * @sg:		The current sg entry
  *
- * Usually the next entry will be @sg@ + 1, but if this sg element is part
- * of a chained scatterlist, it could jump to the start of a new
- * scatterlist array.
- *
- * Note that the caller must ensure that there are further entries after
- * the current entry, this function will NOT return NULL for an end-of-list.
+ * Description:
+ *   Usually the next entry will be @sg@ + 1, but if this sg element is part
+ *   of a chained scatterlist, it could jump to the start of a new
+ *   scatterlist array.
  *
- */
+ **/
 static inline struct scatterlist *sg_next(struct scatterlist *sg)
 {
-	sg++;
+	if (sg_is_last(sg))
+		return NULL;
 
+	sg++;
 	if (unlikely(sg_is_chain(sg)))
 		sg = sg_chain_ptr(sg);
 
@@ -75,14 +102,15 @@ static inline struct scatterlist *sg_next(struct scatterlist *sg)
  * @sgl:	First entry in the scatterlist
  * @nents:	Number of entries in the scatterlist
  *
- * Should only be used casually, it (currently) scan the entire list
- * to get the last entry.
+ * Description:
+ *   Should only be used casually, it (currently) scan the entire list
+ *   to get the last entry.
  *
- * Note that the @sgl@ pointer passed in need not be the first one,
- * the important bit is that @nents@ denotes the number of entries that
- * exist from @sgl@.
+ *   Note that the @sgl@ pointer passed in need not be the first one,
+ *   the important bit is that @nents@ denotes the number of entries that
+ *   exist from @sgl@.
  *
- */
+ **/
 static inline struct scatterlist *sg_last(struct scatterlist *sgl,
 					  unsigned int nents)
 {
@@ -105,16 +133,17 @@ static inline struct scatterlist *sg_last(struct scatterlist *sgl,
  * @prv_nents:	Number of entries in prv
  * @sgl:	Second scatterlist
  *
- * Links @prv@ and @sgl@ together, to form a longer scatterlist.
+ * Description:
+ *   Links @prv@ and @sgl@ together, to form a longer scatterlist.
  *
- */
+ **/
 static inline void sg_chain(struct scatterlist *prv, unsigned int prv_nents,
 			    struct scatterlist *sgl)
 {
 #ifndef ARCH_HAS_SG_CHAIN
 	BUG();
 #endif
-	prv[prv_nents - 1].page = (struct page *) ((unsigned long) sgl | 0x01);
+	prv[prv_nents - 1].page_link = (unsigned long) sgl | 0x01;
 }
 
 /**
@@ -128,13 +157,14 @@ static inline void sg_chain(struct scatterlist *prv, unsigned int prv_nents,
  **/
 static inline void sg_mark_end(struct scatterlist *sgl, unsigned int nents)
 {
+	sgl[nents - 1].page_link = 0x02;
 }
 
 static inline void __sg_mark_end(struct scatterlist *sg)
 {
+	sg->page_link |= 0x02;
 }
 
-
 /**
  * sg_init_one - Initialize a single entry sg list
  * @sg:		 SG entry
@@ -187,7 +217,7 @@ static inline unsigned long sg_phys(struct scatterlist *sg)
 
 /**
  * sg_virt - Return virtual address of an sg entry
- * @sg:	     SG entry
+ * @sg:      SG entry
  *
  * Description:
  *   This calls page_address() on the page in this sg entry, and adds the