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>
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 @@
* 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.
+ * 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.
*
- * 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.
- *
- */
+ **/
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 @@
* @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 @@
* @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_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 @@
/**
* 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
