Based on kernel version 4.1. Page generated on 2015-06-28 12:14 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.