Add chained sg support to linux/scatterlist.h

The core of the patch - allow the last sg element in a scatterlist
table to point to the start of a new table. We overload the LSB of
the page pointer to indicate whether this is a valid sg entry, or
merely a link to the next list.

Includes a fix from Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
correcting the ifdef ARCH_HAS_SG_CHAIN guarding sg_last().

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

1 files changed, 77 insertions, 2 deletions

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index bed5ab4d756a..2dc7464cce52 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -20,8 +20,36 @@ static inline void sg_init_one(struct scatterlist *sg, const void *buf,
         sg_set_buf(sg, buf, buflen);
 }
 
-#define sg_next(sg)             ((sg) + 1)
-#define sg_last(sg, nents)      (&(sg[(nents) - 1]))
+/*
+ * 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)         ((unsigned long) (sg)->page & 0x01)
+#define sg_chain_ptr(sg)        \
+        ((struct scatterlist *) ((unsigned long) (sg)->page & ~0x01))
+
+/**
+ * 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.
+ *
+ */
+static inline struct scatterlist *sg_next(struct scatterlist *sg)
+{
+        sg++;
+
+        if (unlikely(sg_is_chain(sg)))
+                sg = sg_chain_ptr(sg);
+
+        return sg;
+}
 
 /*
  * Loop over each sg element, following the pointer to a new list if necessary
@@ -29,4 +57,51 @@ static inline void sg_init_one(struct scatterlist *sg, const void *buf,
 #define for_each_sg(sglist, sg, nr, __i)        \
         for (__i = 0, sg = (sglist); __i < (nr); __i++, sg = sg_next(sg))
 
+/**
+ * sg_last - return the last scatterlist entry in a list
+ * @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.
+ *
+ * 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)
+{
+#ifndef ARCH_HAS_SG_CHAIN
+        struct scatterlist *ret = &sgl[nents - 1];
+#else
+        struct scatterlist *sg, *ret = NULL;
+        int i;
+
+        for_each_sg(sgl, sg, nents, i)
+                ret = sg;
+
+#endif
+        return ret;
+}
+
+/**
+ * sg_chain - Chain two sglists together
+ * @prv:        First scatterlist
+ * @prv_nents:  Number of entries in prv
+ * @sgl:        Second scatterlist
+ *
+ * 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);
+}
+
 #endif /* _LINUX_SCATTERLIST_H */