[SCSI] block: add sg buffer copy helper functions

This patch adds new three helper functions to copy data between an SG
list and a linear buffer.

- sg_copy_from_buffer copies data from linear buffer to an SG list

- sg_copy_to_buffer copies data from an SG list to a linear buffer

When the APIs copy data from a linear buffer to an SG list,
flush_kernel_dcache_page is called. It's not necessary for everyone
but it's a no-op on most architectures and in general the API is not
used in performance critical path.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

2 files changed, 107 insertions, 0 deletions

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index a3d567a974e8..71fc81360048 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -213,6 +213,11 @@ int __sg_alloc_table(struct sg_table *, unsigned int, unsigned int, gfp_t,
                      sg_alloc_fn *);
 int sg_alloc_table(struct sg_table *, unsigned int, gfp_t);
 
+size_t sg_copy_from_buffer(struct scatterlist *sgl, unsigned int nents,
+                           void *buf, size_t buflen);
+size_t sg_copy_to_buffer(struct scatterlist *sgl, unsigned int nents,
+                         void *buf, size_t buflen);
+
 /*
  * 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 acca4901046c..b80c21100d78 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -8,6 +8,7 @@
  */
 #include <linux/module.h>
 #include <linux/scatterlist.h>
+#include <linux/highmem.h>
 
 /**
  * sg_next - return the next scatterlist entry in a list
@@ -292,3 +293,104 @@ int sg_alloc_table(struct sg_table *table, unsigned int nents, gfp_t gfp_mask)
         return ret;
 }
 EXPORT_SYMBOL(sg_alloc_table);
+
+/**
+ * sg_copy_buffer - Copy data between a linear buffer and an SG list
+ * @sgl:                 The SG list
+ * @nents:               Number of SG entries
+ * @buf:                 Where to copy from
+ * @buflen:              The number of bytes to copy
+ * @to_buffer:           transfer direction (non zero == from an sg list to a
+ *                       buffer, 0 == from a buffer to an sg list
+ *
+ * Returns the number of copied bytes.
+ *
+ **/
+static size_t sg_copy_buffer(struct scatterlist *sgl, unsigned int nents,
+                             void *buf, size_t buflen, int to_buffer)
+{
+        struct scatterlist *sg;
+        size_t buf_off = 0;
+        int i;
+
+        WARN_ON(!irqs_disabled());
+
+        for_each_sg(sgl, sg, nents, i) {
+                struct page *page;
+                int n = 0;
+                unsigned int sg_off = sg->offset;
+                unsigned int sg_copy = sg->length;
+
+                if (sg_copy > buflen)
+                        sg_copy = buflen;
+                buflen -= sg_copy;
+
+                while (sg_copy > 0) {
+                        unsigned int page_copy;
+                        void *p;
+
+                        page_copy = PAGE_SIZE - sg_off;
+                        if (page_copy > sg_copy)
+                                page_copy = sg_copy;
+
+                        page = nth_page(sg_page(sg), n);
+                        p = kmap_atomic(page, KM_BIO_SRC_IRQ);
+
+                        if (to_buffer)
+                                memcpy(buf + buf_off, p + sg_off, page_copy);
+                        else {
+                                memcpy(p + sg_off, buf + buf_off, page_copy);
+                                flush_kernel_dcache_page(page);
+                        }
+
+                        kunmap_atomic(p, KM_BIO_SRC_IRQ);
+
+                        buf_off += page_copy;
+                        sg_off += page_copy;
+                        if (sg_off == PAGE_SIZE) {
+                                sg_off = 0;
+                                n++;
+                        }
+                        sg_copy -= page_copy;
+                }
+
+                if (!buflen)
+                        break;
+        }
+
+        return buf_off;
+}
+
+/**
+ * sg_copy_from_buffer - Copy from a linear buffer to an SG list
+ * @sgl:                 The SG list
+ * @nents:               Number of SG entries
+ * @buf:                 Where to copy from
+ * @buflen:              The number of bytes to copy
+ *
+ * Returns the number of copied bytes.
+ *
+ **/
+size_t sg_copy_from_buffer(struct scatterlist *sgl, unsigned int nents,
+                           void *buf, size_t buflen)
+{
+        return sg_copy_buffer(sgl, nents, buf, buflen, 0);
+}
+EXPORT_SYMBOL(sg_copy_from_buffer);
+
+/**
+ * sg_copy_to_buffer - Copy from an SG list to a linear buffer
+ * @sgl:                 The SG list
+ * @nents:               Number of SG entries
+ * @buf:                 Where to copy to
+ * @buflen:              The number of bytes to copy
+ *
+ * Returns the number of copied bytes.
+ *
+ **/
+size_t sg_copy_to_buffer(struct scatterlist *sgl, unsigned int nents,
+                         void *buf, size_t buflen)
+{
+        return sg_copy_buffer(sgl, nents, buf, buflen, 1);
+}
+EXPORT_SYMBOL(sg_copy_to_buffer);