scsi: usb: image: microtek: use sg helper to iterate over scatterlist

Unlike the legacy I/O path, scsi-mq preallocates a large array to hold
the scatterlist for each request. This static allocation can consume
substantial amounts of memory on modern controllers which support a
large number of concurrently outstanding requests.

To facilitate a switch to a smaller static allocation combined with a
dynamic allocation for requests that need it, we need to make sure all
SCSI drivers handle chained scatterlists correctly.

Convert remaining drivers that directly dereference the scatterlist
array to using the iterator functions.

[mkp: clarified commit message]

Cc: Oliver Neukum <oliver@neukum.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/usb/image/microtek.c

@@ -488,7 +488,6 @@ static void mts_command_done( struct urb *transfer )
 
 static void mts_do_sg (struct urb* transfer)
 {
-	struct scatterlist * sg;
 	int status = transfer->status;
 	MTS_INT_INIT();
 
@@ -500,13 +499,12 @@ static void mts_do_sg (struct urb* transfer)
 		mts_transfer_cleanup(transfer);
         }
 
-	sg = scsi_sglist(context->srb);
-	context->fragment++;
+	context->curr_sg = sg_next(context->curr_sg);
 	mts_int_submit_urb(transfer,
 			   context->data_pipe,
-			   sg_virt(&sg[context->fragment]),
-			   sg[context->fragment].length,
-			   context->fragment + 1 == scsi_sg_count(context->srb) ?
+			   sg_virt(context->curr_sg),
+			   context->curr_sg->length,
+			   sg_is_last(context->curr_sg) ?
 			   mts_data_done : mts_do_sg);
 }
 
@@ -526,22 +524,20 @@ static void
 mts_build_transfer_context(struct scsi_cmnd *srb, struct mts_desc* desc)
 {
 	int pipe;
-	struct scatterlist * sg;
-	
+
 	MTS_DEBUG_GOT_HERE();
 
 	desc->context.instance = desc;
 	desc->context.srb = srb;
-	desc->context.fragment = 0;
 
 	if (!scsi_bufflen(srb)) {
 		desc->context.data = NULL;
 		desc->context.data_length = 0;
 		return;
 	} else {
-		sg = scsi_sglist(srb);
-		desc->context.data = sg_virt(&sg[0]);
-		desc->context.data_length = sg[0].length;
+		desc->context.curr_sg = scsi_sglist(srb);
+		desc->context.data = sg_virt(desc->context.curr_sg);
+		desc->context.data_length = desc->context.curr_sg->length;
 	}
 
 

drivers/usb/image/microtek.h

@@ -21,7 +21,7 @@ struct mts_transfer_context
 	void *data;
 	unsigned data_length;
 	int data_pipe;
-	int fragment;
+	struct scatterlist *curr_sg;
 
 	u8 *scsi_status; /* status returned from ep_response after command completion */
 };