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+In Linux 2.5 kernels (and later), USB device drivers have additional control
+over how DMA may be used to perform I/O operations.  The APIs are detailed
+in the kernel usb programming guide (kerneldoc, from the source code).
+
+
+API OVERVIEW
+
+The big picture is that USB drivers can continue to ignore most DMA issues,
+though they still must provide DMA-ready buffers (see DMA-mapping.txt).
+That's how they've worked through the 2.4 (and earlier) kernels.
+
+OR:  they can now be DMA-aware.
+
+- New calls enable DMA-aware drivers, letting them allocate dma buffers and
+  manage dma mappings for existing dma-ready buffers (see below).
+
+- URBs have an additional "transfer_dma" field, as well as a transfer_flags
+  bit saying if it's valid.  (Control requests also have "setup_dma" and a
+  corresponding transfer_flags bit.)
+
+- "usbcore" will map those DMA addresses, if a DMA-aware driver didn't do
+  it first and set URB_NO_TRANSFER_DMA_MAP or URB_NO_SETUP_DMA_MAP.  HCDs
+  don't manage dma mappings for URBs.
+
+- There's a new "generic DMA API", parts of which are usable by USB device
+  drivers.  Never use dma_set_mask() on any USB interface or device; that
+  would potentially break all devices sharing that bus.
+
+
+ELIMINATING COPIES
+
+It's good to avoid making CPUs copy data needlessly.  The costs can add up,
+and effects like cache-trashing can impose subtle penalties.
+
+- When you're allocating a buffer for DMA purposes anyway, use the buffer
+  primitives.  Think of them as kmalloc and kfree that give you the right
+  kind of addresses to store in urb->transfer_buffer and urb->transfer_dma,
+  while guaranteeing that no hidden copies through DMA "bounce" buffers will
+  slow things down.  You'd also set URB_NO_TRANSFER_DMA_MAP in
+  urb->transfer_flags:
+
+        void *usb_buffer_alloc (struct usb_device *dev, size_t size,
+                int mem_flags, dma_addr_t *dma);
+
+        void usb_buffer_free (struct usb_device *dev, size_t size,
+                void *addr, dma_addr_t dma);
+
+  For control transfers you can use the buffer primitives or not for each
+  of the transfer buffer and setup buffer independently.  Set the flag bits
+  URB_NO_TRANSFER_DMA_MAP and URB_NO_SETUP_DMA_MAP to indicate which
+  buffers you have prepared.  For non-control transfers URB_NO_SETUP_DMA_MAP
+  is ignored.
+
+  The memory buffer returned is "dma-coherent"; sometimes you might need to
+  force a consistent memory access ordering by using memory barriers.  It's
+  not using a streaming DMA mapping, so it's good for small transfers on
+  systems where the I/O would otherwise tie up an IOMMU mapping.  (See
+  Documentation/DMA-mapping.txt for definitions of "coherent" and "streaming"
+  DMA mappings.)
+
+  Asking for 1/Nth of a page (as well as asking for N pages) is reasonably
+  space-efficient.
+
+- Devices on some EHCI controllers could handle DMA to/from high memory.
+  Driver probe() routines can notice this using a generic DMA call, then
+  tell higher level code (network, scsi, etc) about it like this:
+
+        if (dma_supported (&intf->dev, 0xffffffffffffffffULL))
+                net->features |= NETIF_F_HIGHDMA;
+
+  That can eliminate dma bounce buffering of requests that originate (or
+  terminate) in high memory, in cases where the buffers aren't allocated
+  with usb_buffer_alloc() but instead are dma-mapped.
+
+
+WORKING WITH EXISTING BUFFERS
+
+Existing buffers aren't usable for DMA without first being mapped into the
+DMA address space of the device.
+
+- 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:
+
+        int usb_buffer_map_sg (struct usb_device *dev, unsigned pipe,
+                struct scatterlist *sg, int nents);
+
+        void usb_buffer_dmasync_sg (struct usb_device *dev, unsigned pipe,
+                struct scatterlist *sg, int n_hw_ents);
+
+        void usb_buffer_unmap_sg (struct usb_device *dev, unsigned pipe,
+                struct scatterlist *sg, int n_hw_ents);
+
+  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.  The same goes for
+  urb->setup_dma and URB_NO_SETUP_DMA_MAP for control requests.