1 USB DMA
2 ~~~~~~~
3
4 In Linux 2.5 kernels (and later), USB device drivers have additional control
5 over how DMA may be used to perform I/O operations. The APIs are detailed
6 in the kernel usb programming guide (kerneldoc, from the source code).
7
8 API overview
9 ============
10
11 The big picture is that USB drivers can continue to ignore most DMA issues,
12 though they still must provide DMA-ready buffers (see
13 Documentation/core-api/dma-api-howto.rst). That's how they've worked through
14 the 2.4 (and earlier) kernels, or they can now be DMA-aware.
15
16 DMA-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
33 Eliminating copies
34 ==================
35
36 It's good to avoid making CPUs copy data needlessly. The costs can add up,
37 and 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
88 Working with existing buffers
89 =============================
90
91 Existing buffers aren't usable for DMA without first being mapped into the
92 DMA address space of the device. However, most buffers passed to your
93 driver can safely be used with such DMA mapping. (See the first section
94 of Documentation/core-api/dma-api-howto.rst, titled "What memory is DMA-able?")
95
96 - When you have the scatterlists which have been mapped for the USB controller,
97 you could use the new ``usb_sg_*()`` calls, which would turn scatterlist
98 into URBs::
99
100 int usb_sg_init(struct usb_sg_request *io, struct usb_device *dev,
101 unsigned pipe, unsigned period, struct scatterlist *sg,
102 int nents, size_t length, gfp_t mem_flags);
103
104 void usb_sg_wait(struct usb_sg_request *io);
105
106 void usb_sg_cancel(struct usb_sg_request *io);
107
108 When the USB controller doesn't support DMA, the ``usb_sg_init()`` would try
109 to submit URBs in PIO way as long as the page in scatterlists is not in the
110 Highmem, which could be very rare in modern architectures.
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