Both the SCSI and NVMe subsystems receive user data from the block
layer in scatterlist_s (aka scatter gather lists (sgl) which are
often arrays). If drivers in those subsystems represent storage
(e.g. a ramdisk) or cache "hot" user data then they may also
choose to use scatterlist_s. Currently there are no sgl to sgl
operations in the kernel. Start with a copy.
Signed-off-by: Douglas Gilbert <dgilbert@xxxxxxxxxxxx>
---
include/linux/scatterlist.h | 4 ++
lib/scatterlist.c | 74 +++++++++++++++++++++++++++++++++++++
2 files changed, 78 insertions(+)
diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 80178afc2a4a..6649414c0749 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -321,6 +321,10 @@ size_t sg_pcopy_to_buffer(struct scatterlist *sgl, unsigned int nents,
size_t sg_zero_buffer(struct scatterlist *sgl, unsigned int nents,
size_t buflen, off_t skip);
+size_t sgl_copy_sgl(struct scatterlist *d_sgl, unsigned int d_nents, off_t d_skip,
+ struct scatterlist *s_sgl, unsigned int s_nents, off_t s_skip,
+ size_t n_bytes);
+
/*
* Maximum number of entries that will be allocated in one piece, if
* a list larger than this is required then chaining will be utilized.
diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index d5770e7f1030..a0a86059c10e 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -974,3 +974,77 @@ size_t sg_zero_buffer(struct scatterlist *sgl, unsigned int nents,
return offset;
}
EXPORT_SYMBOL(sg_zero_buffer);
+
+/**
+ * sgl_copy_sgl - Copy over a destination sgl from a source sgl
+ * @d_sgl: Destination sgl
+ * @d_nents: Number of SG entries in destination sgl
+ * @d_skip: Number of bytes to skip in destination before starting
+ * @s_sgl: Source sgl
+ * @s_nents: Number of SG entries in source sgl
+ * @s_skip: Number of bytes to skip in source before starting
+ * @n_bytes: The (maximum) number of bytes to copy
+ *
+ * Returns the number of copied bytes.
+ *
+ * Notes:
+ * Destination arguments appear before the source arguments, as with memcpy().
+ *
+ * Stops copying if either d_sgl, s_sgl or n_bytes is exhausted.
+ *
+ * Since memcpy() is used, overlapping copies (where d_sgl and s_sgl belong
+ * to the same sgl and the copy regions overlap) are not supported.
+ *
+ * Large copies are broken into copy segments whose sizes may vary. Those
+ * copy segment sizes are chosen by the min3() statement in the code below.
+ * Since SG_MITER_ATOMIC is used for both sides, each copy segment is started
+ * with kmap_atomic() [in sg_miter_next()] and completed with kunmap_atomic()
+ * [in sg_miter_stop()]. This means pre-emption is inhibited for relatively
+ * short periods even in very large copies.
+ *
+ * If d_skip is large, potentially spanning multiple d_nents then some
+ * integer arithmetic to adjust d_sgl may improve performance. For example
+ * if d_sgl is built using sgl_alloc_order(chainable=false) then the sgl
+ * will be an array with equally sized segments facilitating that
+ * arithmetic. The suggestion applies to s_skip, s_sgl and s_nents as well.
+ *
+ **/
+size_t sgl_copy_sgl(struct scatterlist *d_sgl, unsigned int d_nents, off_t d_skip,
+ struct scatterlist *s_sgl, unsigned int s_nents, off_t s_skip,
+ size_t n_bytes)
+{
+ size_t len;
+ size_t offset = 0;
+ struct sg_mapping_iter d_iter, s_iter;
+
+ if (n_bytes == 0)
+ return 0;
+ sg_miter_start(&s_iter, s_sgl, s_nents, SG_MITER_ATOMIC | SG_MITER_FROM_SG);
+ sg_miter_start(&d_iter, d_sgl, d_nents, SG_MITER_ATOMIC | SG_MITER_TO_SG);
+ if (!sg_miter_skip(&s_iter, s_skip))
+ goto fini;
+ if (!sg_miter_skip(&d_iter, d_skip))
+ goto fini;
+
+ while (offset < n_bytes) {
+ if (!sg_miter_next(&s_iter))
+ break;
+ if (!sg_miter_next(&d_iter))
+ break;
+ len = min3(d_iter.length, s_iter.length, n_bytes - offset);
+
+ memcpy(d_iter.addr, s_iter.addr, len);
+ offset += len;
+ /* LIFO order (stop d_iter before s_iter) needed with SG_MITER_ATOMIC */
+ d_iter.consumed = len;
+ sg_miter_stop(&d_iter);
+ s_iter.consumed = len;
+ sg_miter_stop(&s_iter);
+ }
+fini:
+ sg_miter_stop(&d_iter);
+ sg_miter_stop(&s_iter);
+ return offset;
+}
+EXPORT_SYMBOL(sgl_copy_sgl);
+
--
2.25.1
