+/* allocate an SG list to store bytes_to_transfer bytes and copy the
+ * subset of the in_sg that matches the buffer subset
+ * we are about to transfer. */
+static struct scatterlist *wa_xfer_create_subset_sg(struct scatterlist *in_sg,
+ const unsigned int bytes_transferred,
+ const unsigned int bytes_to_transfer, unsigned int *out_num_sgs)
+{
+ struct scatterlist *out_sg;
+ unsigned int bytes_processed = 0, offset_into_current_page_data = 0,
+ nents;
+ struct scatterlist *current_xfer_sg = in_sg;
+ struct scatterlist *current_seg_sg, *last_seg_sg;
+
+ /* skip previously transferred pages. */
+ while ((current_xfer_sg) &&
+ (bytes_processed < bytes_transferred)) {
+ bytes_processed += current_xfer_sg->length;
+
+ /* advance the sg if current segment starts on or past the
+ next page. */
+ if (bytes_processed <= bytes_transferred)
+ current_xfer_sg = sg_next(current_xfer_sg);
+ }
+
+ /* the data for the current segment starts in current_xfer_sg.
+ calculate the offset. */
+ if (bytes_processed > bytes_transferred) {
+ offset_into_current_page_data = current_xfer_sg->length -
+ (bytes_processed - bytes_transferred);
+ }
+
+ /* calculate the number of pages needed by this segment. */
+ nents = DIV_ROUND_UP((bytes_to_transfer +
+ offset_into_current_page_data +
+ current_xfer_sg->offset),
+ PAGE_SIZE);
+
+ out_sg = kmalloc((sizeof(struct scatterlist) * nents), GFP_ATOMIC);
+ if (out_sg) {
+ sg_init_table(out_sg, nents);
+
+ /* copy the portion of the incoming SG that correlates to the
+ * data to be transferred by this segment to the segment SG. */
+ last_seg_sg = current_seg_sg = out_sg;
+ bytes_processed = 0;
+
+ /* reset nents and calculate the actual number of sg entries
+ needed. */
+ nents = 0;
+ while ((bytes_processed < bytes_to_transfer) &&
+ current_seg_sg && current_xfer_sg) {
+ unsigned int page_len = min((current_xfer_sg->length -
+ offset_into_current_page_data),
+ (bytes_to_transfer - bytes_processed));
+
+ sg_set_page(current_seg_sg, sg_page(current_xfer_sg),
+ page_len,
+ current_xfer_sg->offset +
+ offset_into_current_page_data);
+
+ bytes_processed += page_len;
+
+ last_seg_sg = current_seg_sg;
+ current_seg_sg = sg_next(current_seg_sg);
+ current_xfer_sg = sg_next(current_xfer_sg);
+
+ /* only the first page may require additional offset. */
+ offset_into_current_page_data = 0;
+ nents++;
+ }
+
+ /* update num_sgs and terminate the list since we may have
+ * concatenated pages. */
+ sg_mark_end(last_seg_sg);
+ *out_num_sgs = nents;
+ }
+
+ return out_sg;
+}
+