docs: driver-api: usb: update dma info

We should not hide the recommend APIs in a obscure place.
Signed-off-by: Randy Li <ayaka@soulik.info>
Link: https://lore.kernel.org/r/20230914172336.18761-3-ayaka@soulik.infoSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/usb/dma.rst

@@ -93,44 +93,18 @@ DMA address space of the device.  However, most buffers passed to your
 driver can safely be used with such DMA mapping.  (See the first section
 of Documentation/core-api/dma-api-howto.rst, titled "What memory is DMA-able?")
 
-- When you're using scatterlists, you can map everything at once.  On some
-  systems, this kicks in an IOMMU and turns the scatterlists into single
-  DMA transactions::
+- When you have the scatterlists which have been mapped for the USB controller,
+  you could use the new ``usb_sg_*()`` calls, which would turn scatterlist
+  into URBs::
 
-	int usb_buffer_map_sg (struct usb_device *dev, unsigned pipe,
-		struct scatterlist *sg, int nents);
+	int usb_sg_init(struct usb_sg_request *io, struct usb_device *dev,
+		unsigned pipe, unsigned	period, struct scatterlist *sg,
+		int nents, size_t length, gfp_t mem_flags);
 
-	void usb_buffer_dmasync_sg (struct usb_device *dev, unsigned pipe,
-		struct scatterlist *sg, int n_hw_ents);
+	void usb_sg_wait(struct usb_sg_request *io);
 
-	void usb_buffer_unmap_sg (struct usb_device *dev, unsigned pipe,
-		struct scatterlist *sg, int n_hw_ents);
+	void usb_sg_cancel(struct usb_sg_request *io);
 
-  It's probably easier to use the new ``usb_sg_*()`` calls, which do the DMA
-  mapping and apply other tweaks to make scatterlist i/o be fast.
-
-- Some drivers may prefer to work with the model that they're mapping large
-  buffers, synchronizing their safe re-use.  (If there's no re-use, then let
-  usbcore do the map/unmap.)  Large periodic transfers make good examples
-  here, since it's cheaper to just synchronize the buffer than to unmap it
-  each time an urb completes and then re-map it on during resubmission.
-
-  These calls all work with initialized urbs:  ``urb->dev``, ``urb->pipe``,
-  ``urb->transfer_buffer``, and ``urb->transfer_buffer_length`` must all be
-  valid when these calls are used (``urb->setup_packet`` must be valid too
-  if urb is a control request)::
-
-	struct urb *usb_buffer_map (struct urb *urb);
-
-	void usb_buffer_dmasync (struct urb *urb);
-
-	void usb_buffer_unmap (struct urb *urb);
-
-  The calls manage ``urb->transfer_dma`` for you, and set
-  ``URB_NO_TRANSFER_DMA_MAP`` so that usbcore won't map or unmap the buffer.
-  They cannot be used for setup_packet buffers in control requests.
-
-Note that several of those interfaces are currently commented out, since
-they don't have current users.  See the source code.  Other than the dmasync
-calls (where the underlying DMA primitives have changed), most of them can
-easily be commented back in if you want to use them.
+  When the USB controller doesn't support DMA, the ``usb_sg_init()`` would try
+  to submit URBs in PIO way as long as the page in scatterlists is not in the
+  Highmem, which could be very rare in modern architectures.