[PATCH] Clean up scatterlist.h and introduce macros for readability.

["Robert P. J. Day" <rpjday@crashcourse.ca>]

Add some macros to <linux/scatterlist.h> to make the bit manipulation
more readable, and expand on some of the documentation.

This patch incorporates content from Cyrill Gorcunov
<gorcunov@gmail.com> as well.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>

---

  compile-tested with "make defconfig" under i386.  (what kernel
subsection would this fall under?)


diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 4571231..ae296c0 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -1,10 +1,11 @@
 #ifndef _LINUX_SCATTERLIST_H
 #define _LINUX_SCATTERLIST_H

-#include <asm/types.h>
-#include <asm/scatterlist.h>
 #include <linux/mm.h>
 #include <linux/string.h>
+
+#include <asm/types.h>
+#include <asm/scatterlist.h>
 #include <asm/io.h>

 /*
@@ -12,19 +13,36 @@
  *
  * 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.
+ * information about the sg table as well. The two lower bits (which we
+ * will call the "mask bits" for future reference) 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 0 is *not* set, then the next sg entry is simply the next one
+ * consecutively in memory.  If, however, bit 0 *is* set, we must still
+ * go to the next consecutive sg entry in memory, but then we must
+ * additionally (and immediately) follow that pointer to the next
+ * sg entry in the list.
  *
  * If bit 1 is set, then this sg entry is the last element in a list.
  *
  * See sg_next().
- *
  */
+
+/*
+ *  Define macros for the mask bits and associated operations.
+ */
+
+#define SG_CHAIN	0x1
+#define SG_LAST		0x2
+#define SG_MASK		0x3
+
+#define sg_is_chain(sg)		((sg)->page_link & SG_CHAIN)
+#define sg_is_last(sg)		((sg)->page_link & SG_LAST)
+#define sg_chain_ptr(sg)	\
+	((struct scatterlist *) ((sg)->page_link & ~SG_MASK))
+#define sg_page(sg)	((struct page *) ((sg)->page_link & ~SG_MASK))

-#define SG_MAGIC	0x87654321
+#define SG_MAGIC	0x87654321	/* only for CONFIG_DEBUG_SG */

 /**
  * sg_assign_page - Assign a given page to an SG entry
@@ -38,17 +56,17 @@
  **/
 static inline void sg_assign_page(struct scatterlist *sg, struct page *page)
 {
-	unsigned long page_link = sg->page_link & 0x3;
+	unsigned long saved_mask_bits = sg->page_link & SG_MASK;

 	/*
 	 * In order for the low bit stealing approach to work, pages
 	 * must be aligned at a 32-bit boundary as a minimum.
 	 */
-	BUG_ON((unsigned long) page & 0x03);
+	BUG_ON((unsigned long) page & SG_MASK);
 #ifdef CONFIG_DEBUG_SG
 	BUG_ON(sg->sg_magic != SG_MAGIC);
 #endif
-	sg->page_link = page_link | (unsigned long) page;
+	sg->page_link = saved_mask_bits | (unsigned long) page;
 }

 /**
@@ -73,7 +91,6 @@ static inline void sg_set_page(struct scatterlist *sg, struct page *page,
 	sg->length = len;
 }

-#define sg_page(sg)	((struct page *) ((sg)->page_link & ~0x3))

 /**
  * sg_set_buf - Set sg entry to point at given data
@@ -88,15 +105,6 @@ static inline void sg_set_buf(struct scatterlist *sg, const void *buf,
 	sg_set_page(sg, virt_to_page(buf), buflen, offset_in_page(buf));
 }

-/*
- * We overload the LSB of the page pointer to indicate whether it's
- * 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)		((sg)->page_link & 0x01)
-#define sg_is_last(sg)		((sg)->page_link & 0x02)
-#define sg_chain_ptr(sg)	\
-	((struct scatterlist *) ((sg)->page_link & ~0x03))

 /**
  * sg_next - return the next scatterlist entry in a list
@@ -179,7 +187,7 @@ static inline void sg_chain(struct scatterlist *prv, unsigned int prv_nents,
 #ifndef ARCH_HAS_SG_CHAIN
 	BUG();
 #endif
-	prv[prv_nents - 1].page_link = (unsigned long) sgl | 0x01;
+	prv[prv_nents - 1].page_link = (unsigned long) sgl | SG_CHAIN;
 }

 /**
@@ -193,12 +201,12 @@ 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;
+	sgl[nents - 1].page_link = SG_LAST;
 }

 static inline void __sg_mark_end(struct scatterlist *sg)
 {
-	sg->page_link |= 0x02;
+	sg->page_link |= SG_LAST;
 }

 /**
-- 
========================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA

http://crashcourse.ca
========================================================================
-
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