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+USB DMA
+~~~~~~~
+
+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
+Documentation/core-api/dma-api-howto.rst).  That's how they've worked through
+the 2.4 (and earlier) kernels, or they can now be DMA-aware.
+
+DMA-aware usb drivers:
+
+- 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", but
+  drivers must not use it.)
+
+- "usbcore" will map this DMA address, if a DMA-aware driver didn't do
+  it first and set ``URB_NO_TRANSFER_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.
+
+- If you're doing lots of small data transfers from the same buffer all
+  the time, that can really burn up resources on systems which use an
+  IOMMU to manage the DMA mappings.  It can cost MUCH more to set up and
+  tear down the IOMMU mappings with each request than perform the I/O!
+
+  For those specific cases, USB has primitives to allocate less expensive
+  memory.  They work like kmalloc and kfree versions that give you the right
+  kind of addresses to store in urb->transfer_buffer and urb->transfer_dma.
+  You'd also set ``URB_NO_TRANSFER_DMA_MAP`` in urb->transfer_flags::
+
+	void *usb_alloc_coherent (struct usb_device *dev, size_t size,
+		int mem_flags, dma_addr_t *dma);
+
+	void usb_free_coherent (struct usb_device *dev, size_t size,
+		void *addr, dma_addr_t dma);
+
+  Most drivers should **NOT** be using these primitives; they don't need
+  to use this type of memory ("dma-coherent"), and memory returned from
+  :c:func:`kmalloc` will work just fine.
+
+  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 thrash an IOMMU mapping.  (See
+  Documentation/core-api/dma-api-howto.rst 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.
+
+  On most systems the memory returned will be uncached, because the
+  semantics of dma-coherent memory require either bypassing CPU caches
+  or using cache hardware with bus-snooping support.  While x86 hardware
+  has such bus-snooping, many other systems use software to flush cache
+  lines to prevent DMA conflicts.
+
+- Devices on some EHCI controllers could handle DMA to/from high memory.
+
+  Unfortunately, the current Linux DMA infrastructure doesn't have a sane
+  way to expose these capabilities ... and in any case, HIGHMEM is mostly a
+  design wart specific to x86_32.  So your best bet is to ensure you never
+  pass a highmem buffer into a USB driver.  That's easy; it's the default
+  behavior.  Just don't override it; e.g. with ``NETIF_F_HIGHDMA``.
+
+  This may force your callers to do some bounce buffering, copying from
+  high memory to "normal" DMA memory.  If you can come up with a good way
+  to fix this issue (for x86_32 machines with over 1 GByte of memory),
+  feel free to submit patches.
+
+Working with existing buffers
+=============================
+
+Existing buffers aren't usable for DMA without first being mapped into the
+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::
+
+	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.
+  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.