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1In 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,
9though they still must provide DMA-ready buffers (see DMA-mapping.txt).
10That's how they've worked through the 2.4 (and earlier) kernels.
11
12OR: they can now be DMA-aware.
13
14- New calls enable DMA-aware drivers, letting them allocate dma buffers and
15 manage dma mappings for existing dma-ready buffers (see below).
16
17- URBs have an additional "transfer_dma" field, as well as a transfer_flags
18 bit saying if it's valid. (Control requests also have "setup_dma" and a
19 corresponding transfer_flags bit.)
20
21- "usbcore" will map those DMA addresses, if a DMA-aware driver didn't do
22 it first and set URB_NO_TRANSFER_DMA_MAP or URB_NO_SETUP_DMA_MAP. HCDs
23 don't manage dma mappings for URBs.
24
25- There's a new "generic DMA API", parts of which are usable by USB device
26 drivers. Never use dma_set_mask() on any USB interface or device; that
27 would potentially break all devices sharing that bus.
28
29
30ELIMINATING COPIES
31
32It's good to avoid making CPUs copy data needlessly. The costs can add up,
33and effects like cache-trashing can impose subtle penalties.
34
35- When you're allocating a buffer for DMA purposes anyway, use the buffer
36 primitives. Think of them as kmalloc and kfree that give you the right
37 kind of addresses to store in urb->transfer_buffer and urb->transfer_dma,
38 while guaranteeing that no hidden copies through DMA "bounce" buffers will
39 slow things down. You'd also set URB_NO_TRANSFER_DMA_MAP in
40 urb->transfer_flags:
41
42 void *usb_buffer_alloc (struct usb_device *dev, size_t size,
43 int mem_flags, dma_addr_t *dma);
44
45 void usb_buffer_free (struct usb_device *dev, size_t size,
46 void *addr, dma_addr_t dma);
47
48 For control transfers you can use the buffer primitives or not for each
49 of the transfer buffer and setup buffer independently. Set the flag bits
50 URB_NO_TRANSFER_DMA_MAP and URB_NO_SETUP_DMA_MAP to indicate which
51 buffers you have prepared. For non-control transfers URB_NO_SETUP_DMA_MAP
52 is ignored.
53
54 The memory buffer returned is "dma-coherent"; sometimes you might need to
55 force a consistent memory access ordering by using memory barriers. It's
56 not using a streaming DMA mapping, so it's good for small transfers on
57 systems where the I/O would otherwise tie up an IOMMU mapping. (See
58 Documentation/DMA-mapping.txt for definitions of "coherent" and "streaming"
59 DMA mappings.)
60
61 Asking for 1/Nth of a page (as well as asking for N pages) is reasonably
62 space-efficient.
63
64- Devices on some EHCI controllers could handle DMA to/from high memory.
65 Driver probe() routines can notice this using a generic DMA call, then
66 tell higher level code (network, scsi, etc) about it like this:
67
68 if (dma_supported (&intf->dev, 0xffffffffffffffffULL))
69 net->features |= NETIF_F_HIGHDMA;
70
71 That can eliminate dma bounce buffering of requests that originate (or
72 terminate) in high memory, in cases where the buffers aren't allocated
73 with usb_buffer_alloc() but instead are dma-mapped.
74
75
76WORKING WITH EXISTING BUFFERS
77
78Existing buffers aren't usable for DMA without first being mapped into the
79DMA address space of the device.
80
81- When you're using scatterlists, you can map everything at once. On some
82 systems, this kicks in an IOMMU and turns the scatterlists into single
83 DMA transactions:
84
85 int usb_buffer_map_sg (struct usb_device *dev, unsigned pipe,
86 struct scatterlist *sg, int nents);
87
88 void usb_buffer_dmasync_sg (struct usb_device *dev, unsigned pipe,
89 struct scatterlist *sg, int n_hw_ents);
90
91 void usb_buffer_unmap_sg (struct usb_device *dev, unsigned pipe,
92 struct scatterlist *sg, int n_hw_ents);
93
94 It's probably easier to use the new usb_sg_*() calls, which do the DMA
95 mapping and apply other tweaks to make scatterlist i/o be fast.
96
97- Some drivers may prefer to work with the model that they're mapping large
98 buffers, synchronizing their safe re-use. (If there's no re-use, then let
99 usbcore do the map/unmap.) Large periodic transfers make good examples
100 here, since it's cheaper to just synchronize the buffer than to unmap it
101 each time an urb completes and then re-map it on during resubmission.
102
103 These calls all work with initialized urbs: urb->dev, urb->pipe,
104 urb->transfer_buffer, and urb->transfer_buffer_length must all be
105 valid when these calls are used (urb->setup_packet must be valid too
106 if urb is a control request):
107
108 struct urb *usb_buffer_map (struct urb *urb);
109
110 void usb_buffer_dmasync (struct urb *urb);
111
112 void usb_buffer_unmap (struct urb *urb);
113
114 The calls manage urb->transfer_dma for you, and set URB_NO_TRANSFER_DMA_MAP
115 so that usbcore won't map or unmap the buffer. The same goes for
116 urb->setup_dma and URB_NO_SETUP_DMA_MAP for control requests.