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Based on kernel version 3.16. Page generated on 2014-08-06 21:41 EST.

1	In Linux 2.5 kernels (and later), USB device drivers have additional control
2	over how DMA may be used to perform I/O operations.  The APIs are detailed
3	in the kernel usb programming guide (kerneldoc, from the source code).
4	
5	
6	API OVERVIEW
7	
8	The big picture is that USB drivers can continue to ignore most DMA issues,
9	though they still must provide DMA-ready buffers (see
10	Documentation/DMA-API-HOWTO.txt).  That's how they've worked through
11	the 2.4 (and earlier) kernels.
12	
13	OR:  they can now be DMA-aware.
14	
15	- New calls enable DMA-aware drivers, letting them allocate dma buffers and
16	  manage dma mappings for existing dma-ready buffers (see below).
17	
18	- URBs have an additional "transfer_dma" field, as well as a transfer_flags
19	  bit saying if it's valid.  (Control requests also have "setup_dma", but
20	  drivers must not use it.)
21	
22	- "usbcore" will map this DMA address, if a DMA-aware driver didn't do
23	  it first and set URB_NO_TRANSFER_DMA_MAP.  HCDs
24	  don't manage dma mappings for URBs.
25	
26	- There's a new "generic DMA API", parts of which are usable by USB device
27	  drivers.  Never use dma_set_mask() on any USB interface or device; that
28	  would potentially break all devices sharing that bus.
29	
30	
31	ELIMINATING COPIES
32	
33	It's good to avoid making CPUs copy data needlessly.  The costs can add up,
34	and effects like cache-trashing can impose subtle penalties.
35	
36	- If you're doing lots of small data transfers from the same buffer all
37	  the time, that can really burn up resources on systems which use an
38	  IOMMU to manage the DMA mappings.  It can cost MUCH more to set up and
39	  tear down the IOMMU mappings with each request than perform the I/O!
40	
41	  For those specific cases, USB has primitives to allocate less expensive
42	  memory.  They work like kmalloc and kfree versions that give you the right
43	  kind of addresses to store in urb->transfer_buffer and urb->transfer_dma.
44	  You'd also set URB_NO_TRANSFER_DMA_MAP in urb->transfer_flags:
45	
46		void *usb_alloc_coherent (struct usb_device *dev, size_t size,
47			int mem_flags, dma_addr_t *dma);
48	
49		void usb_free_coherent (struct usb_device *dev, size_t size,
50			void *addr, dma_addr_t dma);
51	
52	  Most drivers should *NOT* be using these primitives; they don't need
53	  to use this type of memory ("dma-coherent"), and memory returned from
54	  kmalloc() will work just fine.
55	
56	  The memory buffer returned is "dma-coherent"; sometimes you might need to
57	  force a consistent memory access ordering by using memory barriers.  It's
58	  not using a streaming DMA mapping, so it's good for small transfers on
59	  systems where the I/O would otherwise thrash an IOMMU mapping.  (See
60	  Documentation/DMA-API-HOWTO.txt for definitions of "coherent" and
61	  "streaming" DMA mappings.)
62	
63	  Asking for 1/Nth of a page (as well as asking for N pages) is reasonably
64	  space-efficient.
65	
66	  On most systems the memory returned will be uncached, because the
67	  semantics of dma-coherent memory require either bypassing CPU caches
68	  or using cache hardware with bus-snooping support.  While x86 hardware
69	  has such bus-snooping, many other systems use software to flush cache
70	  lines to prevent DMA conflicts.
71	
72	- Devices on some EHCI controllers could handle DMA to/from high memory.
73	
74	  Unfortunately, the current Linux DMA infrastructure doesn't have a sane
75	  way to expose these capabilities ... and in any case, HIGHMEM is mostly a
76	  design wart specific to x86_32.  So your best bet is to ensure you never
77	  pass a highmem buffer into a USB driver.  That's easy; it's the default
78	  behavior.  Just don't override it; e.g. with NETIF_F_HIGHDMA.
79	
80	  This may force your callers to do some bounce buffering, copying from
81	  high memory to "normal" DMA memory.  If you can come up with a good way
82	  to fix this issue (for x86_32 machines with over 1 GByte of memory),
83	  feel free to submit patches.
84	
85	
86	WORKING WITH EXISTING BUFFERS
87	
88	Existing buffers aren't usable for DMA without first being mapped into the
89	DMA address space of the device.  However, most buffers passed to your
90	driver can safely be used with such DMA mapping.  (See the first section
91	of Documentation/DMA-API-HOWTO.txt, titled "What memory is DMA-able?")
92	
93	- When you're using scatterlists, you can map everything at once.  On some
94	  systems, this kicks in an IOMMU and turns the scatterlists into single
95	  DMA transactions:
96	
97		int usb_buffer_map_sg (struct usb_device *dev, unsigned pipe,
98			struct scatterlist *sg, int nents);
99	
100		void usb_buffer_dmasync_sg (struct usb_device *dev, unsigned pipe,
101			struct scatterlist *sg, int n_hw_ents);
102	
103		void usb_buffer_unmap_sg (struct usb_device *dev, unsigned pipe,
104			struct scatterlist *sg, int n_hw_ents);
105	
106	  It's probably easier to use the new usb_sg_*() calls, which do the DMA
107	  mapping and apply other tweaks to make scatterlist i/o be fast.
108	
109	- Some drivers may prefer to work with the model that they're mapping large
110	  buffers, synchronizing their safe re-use.  (If there's no re-use, then let
111	  usbcore do the map/unmap.)  Large periodic transfers make good examples
112	  here, since it's cheaper to just synchronize the buffer than to unmap it
113	  each time an urb completes and then re-map it on during resubmission.
114	
115	  These calls all work with initialized urbs:  urb->dev, urb->pipe,
116	  urb->transfer_buffer, and urb->transfer_buffer_length must all be
117	  valid when these calls are used (urb->setup_packet must be valid too
118	  if urb is a control request):
119	
120		struct urb *usb_buffer_map (struct urb *urb);
121	
122		void usb_buffer_dmasync (struct urb *urb);
123	
124		void usb_buffer_unmap (struct urb *urb);
125	
126	  The calls manage urb->transfer_dma for you, and set URB_NO_TRANSFER_DMA_MAP
127	  so that usbcore won't map or unmap the buffer.  They cannot be used for
128	  setup_packet buffers in control requests.
129	
130	Note that several of those interfaces are currently commented out, since
131	they don't have current users.  See the source code.  Other than the dmasync
132	calls (where the underlying DMA primitives have changed), most of them can
133	easily be commented back in if you want to use them.
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