linux-kernel - [PATCH V3] lib/scatterlist: Add SG_CHAIN and SG

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Date:   Mon, 19 Feb 2018 19:37:39 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
        bart.vanassche@....com, jthumshirn@...e.de, axboe@...nel.dk,
        chris@...is-wilson.co.uk, tvrtko.ursulin@...el.com
Cc:     khandual@...ux.vnet.ibm.com
Subject: [PATCH V3] lib/scatterlist: Add SG_CHAIN and SG_LAST macros for LSB encodings

This replaces scatterlist->page_link LSB encodings with SG_CHAIN and
SG_LAST definitions without any functional change. While here also
add macro definitions SG_PAGE_BITS and SG_PAGE_MASK.

Signed-off-by: Anshuman Khandual <khandual@...ux.vnet.ibm.com>
---
Changes in V3:
- Changed SG_END as SG_LAST due to a build failure
- Added SG_PAGE_BITS and SG_PAGE_MASK as per Tvrtko
- Used BIT() macro as per Chris

Changes in V2:
- Changed SG_EMARK as SG_END as per Johannes and Tvrtko
- Added 'UL' to the constants as per Bart

 include/linux/scatterlist.h | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 22b2131bcdcd..494b84c7f3c8 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -66,15 +66,20 @@ struct sg_table {
 
 #define SG_MAGIC	0x87654321
 
+#define SG_CHAIN	BIT(0)
+#define SG_LAST		BIT(1)
+#define SG_PAGE_BITS	(SG_CHAIN | SG_LAST)
+#define SG_PAGE_MASK	(~SG_PAGE_BITS)
+
 /*
  * 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_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 & ~0x03))
+	((struct scatterlist *) ((sg)->page_link & SG_PAGE_MASK))
 
 /**
  * sg_assign_page - Assign a given page to an SG entry
@@ -88,13 +93,13 @@ struct sg_table {
  **/
 static inline void sg_assign_page(struct scatterlist *sg, struct page *page)
 {
-	unsigned long page_link = sg->page_link & 0x3;
+	unsigned long page_link = sg->page_link & SG_PAGE_BITS;
 
 	/*
 	 * 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_PAGE_BITS);
 #ifdef CONFIG_DEBUG_SG
 	BUG_ON(sg->sg_magic != SG_MAGIC);
 	BUG_ON(sg_is_chain(sg));
@@ -130,7 +135,7 @@ static inline struct page *sg_page(struct scatterlist *sg)
 	BUG_ON(sg->sg_magic != SG_MAGIC);
 	BUG_ON(sg_is_chain(sg));
 #endif
-	return (struct page *)((sg)->page_link & ~0x3);
+	return (struct page *)((sg)->page_link & SG_PAGE_MASK);
 }
 
 /**
@@ -178,7 +183,8 @@ static inline void sg_chain(struct scatterlist *prv, unsigned int prv_nents,
 	 * Set lowest bit to indicate a link pointer, and make sure to clear
 	 * the termination bit if it happens to be set.
 	 */
-	prv[prv_nents - 1].page_link = ((unsigned long) sgl | 0x01) & ~0x02;
+	prv[prv_nents - 1].page_link = ((unsigned long) sgl | SG_CHAIN)
+					& ~SG_LAST;
 }
 
 /**
@@ -198,8 +204,8 @@ static inline void sg_mark_end(struct scatterlist *sg)
 	/*
 	 * Set termination bit, clear potential chain bit
 	 */
-	sg->page_link |= 0x02;
-	sg->page_link &= ~0x01;
+	sg->page_link |= SG_LAST;
+	sg->page_link &= ~SG_CHAIN;
 }
 
 /**
@@ -215,7 +221,7 @@ static inline void sg_unmark_end(struct scatterlist *sg)
 #ifdef CONFIG_DEBUG_SG
 	BUG_ON(sg->sg_magic != SG_MAGIC);
 #endif
-	sg->page_link &= ~0x02;
+	sg->page_link &= ~SG_LAST;
 }
 
 /**
-- 
2.11.0