UAS: fix alignment of scatter/gather segments

{

	struct uas_dev_info *devinfo =

		(struct uas_dev_info *)sdev->host->hostdata;

	int maxp;

	sdev->hostdata = devinfo;

	/*

	 * 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.

	 * We have two requirements here. We must satisfy the requirements

	 * of the physical HC and the demands of the protocol, as we

	 * definitely want no additional memory allocation in this path

	 * ruling out using bounce buffers.

	 *

	 * 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.

	 * For a transmission on USB to continue we must never send

	 * a package that is smaller than maxpacket. Hence the length of each

         * scatterlist element except the last must be divisible by the

         * Bulk maxpacket value.

	 * If the HC does not ensure that through SG,

	 * the upper layer must do that. We must assume nothing

	 * about the capabilities off the HC, so we use the most

	 * pessimistic requirement.

	 */

	maxp = usb_maxpacket(devinfo->udev, devinfo->data_in_pipe, 0);

	blk_queue_virt_boundary(sdev->request_queue, maxp - 1);

	/*

	 * The protocol has no requirements on alignment in the strict sense.

	 * Controllers may or may not have alignment restrictions.

	 * As this is not exported, we use an extremely conservative guess.

	 */

	blk_queue_update_dma_alignment(sdev->request_queue, (512 - 1));