2 files changed, 168 insertions, 46 deletions

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 71fc81360048..e5996984ddd0 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -224,4 +224,42 @@ size_t sg_copy_to_buffer(struct scatterlist *sgl, unsigned int nents,
  */
 #define SG_MAX_SINGLE_ALLOC             (PAGE_SIZE / sizeof(struct scatterlist))
 
+
+/*
+ * Mapping sg iterator
+ *
+ * Iterates over sg entries mapping page-by-page.  On each successful
+ * iteration, @miter->page points to the mapped page and
+ * @miter->length bytes of data can be accessed at @miter->addr.  As
+ * long as an interation is enclosed between start and stop, the user
+ * is free to choose control structure and when to stop.
+ *
+ * @miter->consumed is set to @miter->length on each iteration.  It
+ * can be adjusted if the user can't consume all the bytes in one go.
+ * Also, a stopped iteration can be resumed by calling next on it.
+ * This is useful when iteration needs to release all resources and
+ * continue later (e.g. at the next interrupt).
+ */
+
+#define SG_MITER_ATOMIC         (1 << 0)         /* use kmap_atomic */
+
+struct sg_mapping_iter {
+        /* the following three fields can be accessed directly */
+        struct page             *page;          /* currently mapped page */
+        void                    *addr;          /* pointer to the mapped area */
+        size_t                  length;         /* length of the mapped area */
+        size_t                  consumed;       /* number of consumed bytes */
+
+        /* these are internal states, keep away */
+        struct scatterlist      *__sg;          /* current entry */
+        unsigned int            __nents;        /* nr of remaining entries */
+        unsigned int            __offset;       /* offset within sg */
+        unsigned int            __flags;
+};
+
+void sg_miter_start(struct sg_mapping_iter *miter, struct scatterlist *sgl,
+                    unsigned int nents, unsigned int flags);
+bool sg_miter_next(struct sg_mapping_iter *miter);
+void sg_miter_stop(struct sg_mapping_iter *miter);
+
 #endif /* _LINUX_SCATTERLIST_H */
diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index b80c21100d78..876ba6d5b670 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -295,6 +295,117 @@ int sg_alloc_table(struct sg_table *table, unsigned int nents, gfp_t gfp_mask)
 EXPORT_SYMBOL(sg_alloc_table);
 
 /**
+ * sg_miter_start - start mapping iteration over a sg list
+ * @miter: sg mapping iter to be started
+ * @sgl: sg list to iterate over
+ * @nents: number of sg entries
+ *
+ * Description:
+ *   Starts mapping iterator @miter.
+ *
+ * Context:
+ *   Don't care.
+ */
+void sg_miter_start(struct sg_mapping_iter *miter, struct scatterlist *sgl,
+                    unsigned int nents, unsigned int flags)
+{
+        memset(miter, 0, sizeof(struct sg_mapping_iter));
+
+        miter->__sg = sgl;
+        miter->__nents = nents;
+        miter->__offset = 0;
+        miter->__flags = flags;
+}
+EXPORT_SYMBOL(sg_miter_start);
+
+/**
+ * sg_miter_next - proceed mapping iterator to the next mapping
+ * @miter: sg mapping iter to proceed
+ *
+ * Description:
+ *   Proceeds @miter@ to the next mapping.  @miter@ should have been
+ *   started using sg_miter_start().  On successful return,
+ *   @miter@->page, @miter@->addr and @miter@->length point to the
+ *   current mapping.
+ *
+ * Context:
+ *   IRQ disabled if SG_MITER_ATOMIC.  IRQ must stay disabled till
+ *   @miter@ is stopped.  May sleep if !SG_MITER_ATOMIC.
+ *
+ * Returns:
+ *   true if @miter contains the next mapping.  false if end of sg
+ *   list is reached.
+ */
+bool sg_miter_next(struct sg_mapping_iter *miter)
+{
+        unsigned int off, len;
+
+        /* check for end and drop resources from the last iteration */
+        if (!miter->__nents)
+                return false;
+
+        sg_miter_stop(miter);
+
+        /* get to the next sg if necessary.  __offset is adjusted by stop */
+        if (miter->__offset == miter->__sg->length && --miter->__nents) {
+                miter->__sg = sg_next(miter->__sg);
+                miter->__offset = 0;
+        }
+
+        /* map the next page */
+        off = miter->__sg->offset + miter->__offset;
+        len = miter->__sg->length - miter->__offset;
+
+        miter->page = nth_page(sg_page(miter->__sg), off >> PAGE_SHIFT);
+        off &= ~PAGE_MASK;
+        miter->length = min_t(unsigned int, len, PAGE_SIZE - off);
+        miter->consumed = miter->length;
+
+        if (miter->__flags & SG_MITER_ATOMIC)
+                miter->addr = kmap_atomic(miter->page, KM_BIO_SRC_IRQ) + off;
+        else
+                miter->addr = kmap(miter->page) + off;
+
+        return true;
+}
+EXPORT_SYMBOL(sg_miter_next);
+
+/**
+ * sg_miter_stop - stop mapping iteration
+ * @miter: sg mapping iter to be stopped
+ *
+ * Description:
+ *   Stops mapping iterator @miter.  @miter should have been started
+ *   started using sg_miter_start().  A stopped iteration can be
+ *   resumed by calling sg_miter_next() on it.  This is useful when
+ *   resources (kmap) need to be released during iteration.
+ *
+ * Context:
+ *   IRQ disabled if the SG_MITER_ATOMIC is set.  Don't care otherwise.
+ */
+void sg_miter_stop(struct sg_mapping_iter *miter)
+{
+        WARN_ON(miter->consumed > miter->length);
+
+        /* drop resources from the last iteration */
+        if (miter->addr) {
+                miter->__offset += miter->consumed;
+
+                if (miter->__flags & SG_MITER_ATOMIC) {
+                        WARN_ON(!irqs_disabled());
+                        kunmap_atomic(miter->addr, KM_BIO_SRC_IRQ);
+                } else
+                        kunmap(miter->addr);
+
+                miter->page = NULL;
+                miter->addr = NULL;
+                miter->length = 0;
+                miter->consumed = 0;
+        }
+}
+EXPORT_SYMBOL(sg_miter_stop);
+
+/**
  * sg_copy_buffer - Copy data between a linear buffer and an SG list
  * @sgl:                 The SG list
  * @nents:               Number of SG entries
@@ -309,56 +420,29 @@ EXPORT_SYMBOL(sg_alloc_table);
 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;
+        unsigned int offset = 0;
+        struct sg_mapping_iter miter;
+
+        sg_miter_start(&miter, sgl, nents, SG_MITER_ATOMIC);
+
+        while (sg_miter_next(&miter) && offset < buflen) {
+                unsigned int len;
+
+                len = min(miter.length, buflen - offset);
+
+                if (to_buffer)
+                        memcpy(buf + offset, miter.addr, len);
+                else {
+                        memcpy(miter.addr, buf + offset, len);
+                        flush_kernel_dcache_page(miter.page);
                 }
 
-                if (!buflen)
-                        break;
+                offset += len;
         }
 
-        return buf_off;
+        sg_miter_stop(&miter);
+
+        return offset;
 }
 
 /**