usb/dma.txt: convert to ReST and add to driver-api book

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

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/DMA-API-HOWTO.txt).  That's how they've worked through

the 2.4 (and earlier) kernels.

``Documentation/DMA-API-HOWTO.txt``).  That's how they've worked through

the 2.4 (and earlier) kernels, or they can now be DMA-aware.

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).

  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

  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

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.

  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:

  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

  Most drivers should **NOT** be using these primitives; they don't need

  to use this type of memory ("dma-coherent"), and memory returned from

  kmalloc() will work just fine.

  :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/DMA-API-HOWTO.txt for definitions of "coherent" and

  ``Documentation/DMA-API-HOWTO.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

  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.

  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

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

- 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:

  DMA transactions::

	int usb_buffer_map_sg (struct usb_device *dev, unsigned pipe,

		struct scatterlist *sg, int nents);

	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

  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

  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):

  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_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.

  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

   anchors

   bulk-streams

   callbacks

   dma

   power-management

   writing_usb_driver

   writing_musb_glue_layer