[PATCH 7/13] i386 sg: add support for chaining scatterlists

Jens Axboe
2007-05-10T10:24:41Z

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.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
---
include/asm-i386/scatterlist.h | 2 +
include/linux/scatterlist.h | 52 +++++++++++++++++++++++++++++++++++++--
2 files changed, 51 insertions(+), 3 deletions(-)

diff --git a/include/asm-i386/scatterlist.h b/include/asm-i386/scatterlist.h
index d7e45a8..bd5164a 100644
--- a/include/asm-i386/scatterlist.h
+++ b/include/asm-i386/scatterlist.h
@@ -10,6 +10,8 @@ struct scatterlist {
unsigned int length;
};

+#define ARCH_HAS_SG_CHAIN
+
/* These macros should be used after a pci_map_sg call has been done
* to get bus addresses of each of the SG entries and their lengths.
* You should only work with the number of sg entries pci_map_sg
diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index c5bffde..bad6b9e 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -20,13 +20,59 @@ 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]))
+#define sg_is_chain(sg) ((unsigned long) (sg)->page & 0x01)
+#define sg_chain_ptr(sg) \
+ ((struct scatterlist *) ((unsigned long) (sg)->page & ~0x01))
+
+/*
+ * We overload the meaning of ->page for sg chaining. If the LSB is
+ * set, the page member contains a pointer to the next sgtable.
+ */
+static inline struct scatterlist *sg_next(struct scatterlist *sg)
+{
+ if (sg_is_chain(sg))
+ return sg_chain_ptr(sg);
+
+ return sg + 1;
+}

/*
* Loop over each sg element, following the pointer to a new list if necessary
*/
#define for_each_sg(sglist, sg, nr, __i) \
- for (__i = 0, sg = (sglist); __i < nr; __i++, sg = sg_next(sg))
+ for (__i = 0, sg = (sglist); __i < (nr); __i++, sg = sg_next(sg))
+
+/*
+ * We could improve this by passing in the maximum size of an sglist, so
+ * we could jump directly to the last table. That would eliminate this
+ * (potentially) lengthy scan.
+ */
+static inline struct scatterlist *sg_last(struct scatterlist *sgl,
+ unsigned int nents)
+{
+#ifdef 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;
+}
+
+/*
+ * Chain previous sglist to this one
+ */
+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 */
--
1.5.2.rc1