Based on kernel version 2.6.26. Page generated on 2008-07-16 21:13 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 DMA-mapping.txt). 10 That's how they've worked through the 2.4 (and earlier) kernels. 11 12 OR: 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 30 ELIMINATING COPIES 31 32 It's good to avoid making CPUs copy data needlessly. The costs can add up, 33 and effects like cache-trashing can impose subtle penalties. 34 35 - If you're doing lots of small data transfers from the same buffer all 36 the time, that can really burn up resources on systems which use an 37 IOMMU to manage the DMA mappings. It can cost MUCH more to set up and 38 tear down the IOMMU mappings with each request than perform the I/O! 39 40 For those specific cases, USB has primitives to allocate less expensive 41 memory. They work like kmalloc and kfree versions that give you the right 42 kind of addresses to store in urb->transfer_buffer and urb->transfer_dma. 43 You'd also set URB_NO_TRANSFER_DMA_MAP in urb->transfer_flags: 44 45 void *usb_buffer_alloc (struct usb_device *dev, size_t size, 46 int mem_flags, dma_addr_t *dma); 47 48 void usb_buffer_free (struct usb_device *dev, size_t size, 49 void *addr, dma_addr_t dma); 50 51 Most drivers should *NOT* be using these primitives; they don't need 52 to use this type of memory ("dma-coherent"), and memory returned from 53 kmalloc() will work just fine. 54 55 For control transfers you can use the buffer primitives or not for each 56 of the transfer buffer and setup buffer independently. Set the flag bits 57 URB_NO_TRANSFER_DMA_MAP and URB_NO_SETUP_DMA_MAP to indicate which 58 buffers you have prepared. For non-control transfers URB_NO_SETUP_DMA_MAP 59 is ignored. 60 61 The memory buffer returned is "dma-coherent"; sometimes you might need to 62 force a consistent memory access ordering by using memory barriers. It's 63 not using a streaming DMA mapping, so it's good for small transfers on 64 systems where the I/O would otherwise thrash an IOMMU mapping. (See 65 Documentation/DMA-mapping.txt for definitions of "coherent" and "streaming" 66 DMA mappings.) 67 68 Asking for 1/Nth of a page (as well as asking for N pages) is reasonably 69 space-efficient. 70 71 On most systems the memory returned will be uncached, because the 72 semantics of dma-coherent memory require either bypassing CPU caches 73 or using cache hardware with bus-snooping support. While x86 hardware 74 has such bus-snooping, many other systems use software to flush cache 75 lines to prevent DMA conflicts. 76 77 - Devices on some EHCI controllers could handle DMA to/from high memory. 78 79 Unfortunately, the current Linux DMA infrastructure doesn't have a sane 80 way to expose these capabilities ... and in any case, HIGHMEM is mostly a 81 design wart specific to x86_32. So your best bet is to ensure you never 82 pass a highmem buffer into a USB driver. That's easy; it's the default 83 behavior. Just don't override it; e.g. with NETIF_F_HIGHDMA. 84 85 This may force your callers to do some bounce buffering, copying from 86 high memory to "normal" DMA memory. If you can come up with a good way 87 to fix this issue (for x86_32 machines with over 1 GByte of memory), 88 feel free to submit patches. 89 90 91 WORKING WITH EXISTING BUFFERS 92 93 Existing buffers aren't usable for DMA without first being mapped into the 94 DMA address space of the device. However, most buffers passed to your 95 driver can safely be used with such DMA mapping. (See the first section 96 of DMA-mapping.txt, titled "What memory is DMA-able?") 97 98 - When you're using scatterlists, you can map everything at once. On some 99 systems, this kicks in an IOMMU and turns the scatterlists into single 100 DMA transactions: 101 102 int usb_buffer_map_sg (struct usb_device *dev, unsigned pipe, 103 struct scatterlist *sg, int nents); 104 105 void usb_buffer_dmasync_sg (struct usb_device *dev, unsigned pipe, 106 struct scatterlist *sg, int n_hw_ents); 107 108 void usb_buffer_unmap_sg (struct usb_device *dev, unsigned pipe, 109 struct scatterlist *sg, int n_hw_ents); 110 111 It's probably easier to use the new usb_sg_*() calls, which do the DMA 112 mapping and apply other tweaks to make scatterlist i/o be fast. 113 114 - Some drivers may prefer to work with the model that they're mapping large 115 buffers, synchronizing their safe re-use. (If there's no re-use, then let 116 usbcore do the map/unmap.) Large periodic transfers make good examples 117 here, since it's cheaper to just synchronize the buffer than to unmap it 118 each time an urb completes and then re-map it on during resubmission. 119 120 These calls all work with initialized urbs: urb->dev, urb->pipe, 121 urb->transfer_buffer, and urb->transfer_buffer_length must all be 122 valid when these calls are used (urb->setup_packet must be valid too 123 if urb is a control request): 124 125 struct urb *usb_buffer_map (struct urb *urb); 126 127 void usb_buffer_dmasync (struct urb *urb); 128 129 void usb_buffer_unmap (struct urb *urb); 130 131 The calls manage urb->transfer_dma for you, and set URB_NO_TRANSFER_DMA_MAP 132 so that usbcore won't map or unmap the buffer. The same goes for 133 urb->setup_dma and URB_NO_SETUP_DMA_MAP for control requests. 134 135 Note that several of those interfaces are currently commented out, since 136 they don't have current users. See the source code. Other than the dmasync 137 calls (where the underlying DMA primitives have changed), most of them can 138 easily be commented back in if you want to use them.