block: update bio according to DMA alignment padding
Tejun Heo [Tue, 19 Feb 2008 10:35:38 +0000 (11:35 +0100)]
DMA start address and transfer size alignment for PC requests are
achieved using bio_copy_user() instead of bio_map_user().  This works
because bio_copy_user() always uses full pages and block DMA alignment
isn't allowed to go over PAGE_SIZE.

However, the implementation didn't update the last bio of the request
to make this padding visible to lower layers.  This patch makes
blk_rq_map_user() extend the last bio such that it includes the
padding area and the size of area pointed to by the request is
properly aligned.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/blk-map.c

index bc5ce60..a7cf63c 100644 (file)
@@ -139,6 +139,23 @@ int blk_rq_map_user(struct request_queue *q, struct request *rq,
                ubuf += ret;
        }
 
+       /*
+        * __blk_rq_map_user() copies the buffers if starting address
+        * or length isn't aligned.  As the copied buffer is always
+        * page aligned, we know that there's enough room for padding.
+        * Extend the last bio and update rq->data_len accordingly.
+        *
+        * On unmap, bio_uncopy_user() will use unmodified
+        * bio_map_data pointed to by bio->bi_private.
+        */
+       if (len & queue_dma_alignment(q)) {
+               unsigned int pad_len = (queue_dma_alignment(q) & ~len) + 1;
+               struct bio *bio = rq->biotail;
+
+               bio->bi_io_vec[bio->bi_vcnt - 1].bv_len += pad_len;
+               bio->bi_size += pad_len;
+       }
+
        rq->buffer = rq->data = NULL;
        return 0;
 unmap_rq: