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Linus Torvalds1da177e2005-04-16 15:20:36 -07001In Linux 2.5 kernels (and later), USB device drivers have additional control
2over how DMA may be used to perform I/O operations. The APIs are detailed
3in the kernel usb programming guide (kerneldoc, from the source code).
4
5
6API OVERVIEW
7
8The big picture is that USB drivers can continue to ignore most DMA issues,
Randy Dunlap5872fb92009-01-29 16:28:02 -08009though they still must provide DMA-ready buffers (see
Paul Bolle395cf962011-08-15 02:02:26 +020010Documentation/DMA-API-HOWTO.txt). That's how they've worked through
Randy Dunlap5872fb92009-01-29 16:28:02 -080011the 2.4 (and earlier) kernels.
Linus Torvalds1da177e2005-04-16 15:20:36 -070012
13OR: 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
Alan Stern85bcb5e2010-04-30 16:35:37 -040019 bit saying if it's valid. (Control requests also have "setup_dma", but
20 drivers must not use it.)
Linus Torvalds1da177e2005-04-16 15:20:36 -070021
Alan Stern85bcb5e2010-04-30 16:35:37 -040022- "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
Linus Torvalds1da177e2005-04-16 15:20:36 -070024 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
31ELIMINATING COPIES
32
33It's good to avoid making CPUs copy data needlessly. The costs can add up,
34and effects like cache-trashing can impose subtle penalties.
35
David Brownellfbf54dd2007-07-01 23:33:12 -070036- 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:
Linus Torvalds1da177e2005-04-16 15:20:36 -070045
Daniel Mack997ea582010-04-12 13:17:25 +020046 void *usb_alloc_coherent (struct usb_device *dev, size_t size,
Linus Torvalds1da177e2005-04-16 15:20:36 -070047 int mem_flags, dma_addr_t *dma);
48
Daniel Mack997ea582010-04-12 13:17:25 +020049 void usb_free_coherent (struct usb_device *dev, size_t size,
Linus Torvalds1da177e2005-04-16 15:20:36 -070050 void *addr, dma_addr_t dma);
51
David Brownellfbf54dd2007-07-01 23:33:12 -070052 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
Linus Torvalds1da177e2005-04-16 15:20:36 -070056 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
David Brownellfbf54dd2007-07-01 23:33:12 -070059 systems where the I/O would otherwise thrash an IOMMU mapping. (See
Paul Bolle395cf962011-08-15 02:02:26 +020060 Documentation/DMA-API-HOWTO.txt for definitions of "coherent" and
Randy Dunlap5872fb92009-01-29 16:28:02 -080061 "streaming" DMA mappings.)
Linus Torvalds1da177e2005-04-16 15:20:36 -070062
63 Asking for 1/Nth of a page (as well as asking for N pages) is reasonably
64 space-efficient.
65
David Brownellfbf54dd2007-07-01 23:33:12 -070066 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
Linus Torvalds1da177e2005-04-16 15:20:36 -070072- Devices on some EHCI controllers could handle DMA to/from high memory.
Linus Torvalds1da177e2005-04-16 15:20:36 -070073
David Brownellfbf54dd2007-07-01 23:33:12 -070074 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.
Linus Torvalds1da177e2005-04-16 15:20:36 -070079
David Brownellfbf54dd2007-07-01 23:33:12 -070080 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.
Linus Torvalds1da177e2005-04-16 15:20:36 -070084
85
86WORKING WITH EXISTING BUFFERS
87
88Existing buffers aren't usable for DMA without first being mapped into the
David Brownellfbf54dd2007-07-01 23:33:12 -070089DMA address space of the device. However, most buffers passed to your
90driver can safely be used with such DMA mapping. (See the first section
Paul Bolle395cf962011-08-15 02:02:26 +020091of Documentation/DMA-API-HOWTO.txt, titled "What memory is DMA-able?")
Linus Torvalds1da177e2005-04-16 15:20:36 -070092
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
Alan Stern85bcb5e2010-04-30 16:35:37 -0400127 so that usbcore won't map or unmap the buffer. They cannot be used for
128 setup_packet buffers in control requests.
David Brownellfbf54dd2007-07-01 23:33:12 -0700129
130Note that several of those interfaces are currently commented out, since
131they don't have current users. See the source code. Other than the dmasync
132calls (where the underlying DMA primitives have changed), most of them can
133easily be commented back in if you want to use them.