1USB DMA
2~~~~~~~
3
4In Linux 2.5 kernels (and later), USB device drivers have additional control
5over how DMA may be used to perform I/O operations.  The APIs are detailed
6in the kernel usb programming guide (kerneldoc, from the source code).
7
8API overview
9============
10
11The big picture is that USB drivers can continue to ignore most DMA issues,
12though they still must provide DMA-ready buffers (see
13Documentation/core-api/dma-api-howto.rst).  That's how they've worked through
14the 2.4 (and earlier) kernels, or they can now be DMA-aware.
15
16DMA-aware usb drivers:
17
18- New calls enable DMA-aware drivers, letting them allocate dma buffers and
19  manage dma mappings for existing dma-ready buffers (see below).
20
21- URBs have an additional "transfer_dma" field, as well as a transfer_flags
22  bit saying if it's valid.  (Control requests also have "setup_dma", but
23  drivers must not use it.)
24
25- "usbcore" will map this DMA address, if a DMA-aware driver didn't do
26  it first and set ``URB_NO_TRANSFER_DMA_MAP``.  HCDs
27  don't manage dma mappings for URBs.
28
29- There's a new "generic DMA API", parts of which are usable by USB device
30  drivers.  Never use dma_set_mask() on any USB interface or device; that
31  would potentially break all devices sharing that bus.
32
33Eliminating copies
34==================
35
36It's good to avoid making CPUs copy data needlessly.  The costs can add up,
37and effects like cache-trashing can impose subtle penalties.
38
39- If you're doing lots of small data transfers from the same buffer all
40  the time, that can really burn up resources on systems which use an
41  IOMMU to manage the DMA mappings.  It can cost MUCH more to set up and
42  tear down the IOMMU mappings with each request than perform the I/O!
43
44  For those specific cases, USB has primitives to allocate less expensive
45  memory.  They work like kmalloc and kfree versions that give you the right
46  kind of addresses to store in urb->transfer_buffer and urb->transfer_dma.
47  You'd also set ``URB_NO_TRANSFER_DMA_MAP`` in urb->transfer_flags::
48
49	void *usb_alloc_coherent (struct usb_device *dev, size_t size,
50		int mem_flags, dma_addr_t *dma);
51
52	void usb_free_coherent (struct usb_device *dev, size_t size,
53		void *addr, dma_addr_t dma);
54
55  Most drivers should **NOT** be using these primitives; they don't need
56  to use this type of memory ("dma-coherent"), and memory returned from
57  :c:func:`kmalloc` will work just fine.
58
59  The memory buffer returned is "dma-coherent"; sometimes you might need to
60  force a consistent memory access ordering by using memory barriers.  It's
61  not using a streaming DMA mapping, so it's good for small transfers on
62  systems where the I/O would otherwise thrash an IOMMU mapping.  (See
63  Documentation/core-api/dma-api-howto.rst for definitions of "coherent" and
64  "streaming" DMA mappings.)
65
66  Asking for 1/Nth of a page (as well as asking for N pages) is reasonably
67  space-efficient.
68
69  On most systems the memory returned will be uncached, because the
70  semantics of dma-coherent memory require either bypassing CPU caches
71  or using cache hardware with bus-snooping support.  While x86 hardware
72  has such bus-snooping, many other systems use software to flush cache
73  lines to prevent DMA conflicts.
74
75- Devices on some EHCI controllers could handle DMA to/from high memory.
76
77  Unfortunately, the current Linux DMA infrastructure doesn't have a sane
78  way to expose these capabilities ... and in any case, HIGHMEM is mostly a
79  design wart specific to x86_32.  So your best bet is to ensure you never
80  pass a highmem buffer into a USB driver.  That's easy; it's the default
81  behavior.  Just don't override it; e.g. with ``NETIF_F_HIGHDMA``.
82
83  This may force your callers to do some bounce buffering, copying from
84  high memory to "normal" DMA memory.  If you can come up with a good way
85  to fix this issue (for x86_32 machines with over 1 GByte of memory),
86  feel free to submit patches.
87
88Working with existing buffers
89=============================
90
91Existing buffers aren't usable for DMA without first being mapped into the
92DMA address space of the device.  However, most buffers passed to your
93driver can safely be used with such DMA mapping.  (See the first section
94of Documentation/core-api/dma-api-howto.rst, titled "What memory is DMA-able?")
95
96- When you're using scatterlists, you can map everything at once.  On some
97  systems, this kicks in an IOMMU and turns the scatterlists into single
98  DMA transactions::
99
100	int usb_buffer_map_sg (struct usb_device *dev, unsigned pipe,
101		struct scatterlist *sg, int nents);
102
103	void usb_buffer_dmasync_sg (struct usb_device *dev, unsigned pipe,
104		struct scatterlist *sg, int n_hw_ents);
105
106	void usb_buffer_unmap_sg (struct usb_device *dev, unsigned pipe,
107		struct scatterlist *sg, int n_hw_ents);
108
109  It's probably easier to use the new ``usb_sg_*()`` calls, which do the DMA
110  mapping and apply other tweaks to make scatterlist i/o be fast.
111
112- Some drivers may prefer to work with the model that they're mapping large
113  buffers, synchronizing their safe re-use.  (If there's no re-use, then let
114  usbcore do the map/unmap.)  Large periodic transfers make good examples
115  here, since it's cheaper to just synchronize the buffer than to unmap it
116  each time an urb completes and then re-map it on during resubmission.
117
118  These calls all work with initialized urbs:  ``urb->dev``, ``urb->pipe``,
119  ``urb->transfer_buffer``, and ``urb->transfer_buffer_length`` must all be
120  valid when these calls are used (``urb->setup_packet`` must be valid too
121  if urb is a control request)::
122
123	struct urb *usb_buffer_map (struct urb *urb);
124
125	void usb_buffer_dmasync (struct urb *urb);
126
127	void usb_buffer_unmap (struct urb *urb);
128
129  The calls manage ``urb->transfer_dma`` for you, and set
130  ``URB_NO_TRANSFER_DMA_MAP`` so that usbcore won't map or unmap the buffer.
131  They cannot be used for setup_packet buffers in control requests.
132
133Note that several of those interfaces are currently commented out, since
134they don't have current users.  See the source code.  Other than the dmasync
135calls (where the underlying DMA primitives have changed), most of them can
136easily be commented back in if you want to use them.
137