- /* Scatter-gather buffers (all but the last) must have a length
- * divisible by the bulk maxpacket size. Otherwise a data packet
- * would end up being short, causing a premature end to the data
- * transfer. We'll use the maxpacket value of the bulk-IN pipe
- * to set the SCSI device queue's DMA alignment mask.
+ /* USB has unusual DMA-alignment requirements: Although the
+ * starting address of each scatter-gather element doesn't matter,
+ * the length of each element except the last must be divisible
+ * by the Bulk maxpacket value. There's currently no way to
+ * express this by block-layer constraints, so we'll cop out
+ * and simply require addresses to be aligned at 512-byte
+ * boundaries. This is okay since most block I/O involves
+ * hardware sectors that are multiples of 512 bytes in length,
+ * and since host controllers up through USB 2.0 have maxpacket
+ * values no larger than 512.
+ *
+ * But it doesn't suffice for Wireless USB, where Bulk maxpacket
+ * values can be as large as 2048. To make that work properly
+ * will require changes to the block layer.