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

This document describe some USB core features. Add it to the
driver-api book.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 0 insertions, 133 deletions

diff --git a/Documentation/usb/dma.txt b/Documentation/usb/dma.txt
deleted file mode 100644
index 444651e70d95..000000000000
--- a/Documentation/usb/dma.txt
+++ /dev/null
@@ -1,133 +0,0 @@
-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/DMA-API-HOWTO.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", 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
-  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
-  "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/DMA-API-HOWTO.txt, 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.