usb: gadget: introduce support for sg lists

Some controllers support scatter/gather transfers
and that might be very useful for some gadget drivers.

This means that we can make use of larger buffer
allocations which means we will have less completion
IRQs overtime, thus improving the perceived performance.
Signed-off-by: Felipe Balbi <balbi@ti.com>

include/linux/usb/gadget.h

@@ -20,6 +20,7 @@
 #include <linux/init.h>
 #include <linux/list.h>
 #include <linux/slab.h>
+#include <linux/scatterlist.h>
 #include <linux/types.h>
 #include <linux/usb/ch9.h>
 
@@ -32,6 +33,9 @@ struct usb_ep;
  * @dma: DMA address corresponding to 'buf'.  If you don't set this
  *	field, and the usb controller needs one, it is responsible
  *	for mapping and unmapping the buffer.
+ * @sg: a scatterlist for SG-capable controllers.
+ * @num_sgs: number of SG entries
+ * @num_mapped_sgs: number of SG entries mapped to DMA (internal)
  * @length: Length of that data
  * @stream_id: The stream id, when USB3.0 bulk streams are being used
  * @no_interrupt: If true, hints that no completion irq is needed.
@@ -88,6 +92,10 @@ struct usb_request {
 	unsigned		length;
 	dma_addr_t		dma;
 
+	struct scatterlist	*sg;
+	unsigned		num_sgs;
+	unsigned		num_mapped_sgs;
+
 	unsigned		stream_id:16;
 	unsigned		no_interrupt:1;
 	unsigned		zero:1;
@@ -479,6 +487,7 @@ struct usb_gadget_ops {
  * @speed: Speed of current connection to USB host.
  * @max_speed: Maximal speed the UDC can handle.  UDC must support this
  *      and all slower speeds.
+ * @sg_supported: true if we can handle scatter-gather
  * @is_otg: True if the USB device port uses a Mini-AB jack, so that the
  *	gadget driver must provide a USB OTG descriptor.
  * @is_a_peripheral: False unless is_otg, the "A" end of a USB cable
@@ -519,6 +528,7 @@ struct usb_gadget {
 	struct list_head		ep_list;	/* of usb_ep */
 	enum usb_device_speed		speed;
 	enum usb_device_speed		max_speed;
+	unsigned			sg_supported:1;
 	unsigned			is_otg:1;
 	unsigned			is_a_peripheral:1;
 	unsigned			b_hnp_enable:1;