firewire: Add a comment to describe why we split the sg list.

Signed-off-by: Kristian Hoegsberg <krh@redhat.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

1 files changed, 4 insertions, 1 deletions

diff --git a/drivers/firewire/fw-sbp2.c b/drivers/firewire/fw-sbp2.c
index 07be24e2d201..cb808d525314 100644
--- a/drivers/firewire/fw-sbp2.c
+++ b/drivers/firewire/fw-sbp2.c
@@ -926,7 +926,10 @@ static int sbp2_command_orb_map_scatterlist(struct sbp2_command_orb *orb)
 
         /*
          * Convert the scatterlist to an sbp2 page table.  If any
-         * scatterlist entries are too big for sbp2 we split the as we go.
+         * scatterlist entries are too big for sbp2, we split them as we
+         * go.  Even if we ask the block I/O layer to not give us sg
+         * elements larger than 65535 bytes, some IOMMUs may merge sg elements
+         * during DMA mapping, and Linux currently doesn't prevent this.
          */
         for (i = 0, j = 0; i < count; i++) {
                 sg_len = sg_dma_len(sg + i);