Documentation/usb/dma.txt

   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/PCI/PCI-DMA-mapping.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" and a
  20   corresponding transfer_flags bit.)
  21
  22 - "usbcore" will map those DMA addresses, if a DMA-aware driver didn't do
  23   it first and set URB_NO_TRANSFER_DMA_MAP or URB_NO_SETUP_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_buffer_alloc (struct usb_device *dev, size_t size,
  47                 int mem_flags, dma_addr_t *dma);
  48
  49         void usb_buffer_free (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   For control transfers you can use the buffer primitives or not for each
  57   of the transfer buffer and setup buffer independently.  Set the flag bits
  58   URB_NO_TRANSFER_DMA_MAP and URB_NO_SETUP_DMA_MAP to indicate which
  59   buffers you have prepared.  For non-control transfers URB_NO_SETUP_DMA_MAP
  60   is ignored.
  61
  62   The memory buffer returned is "dma-coherent"; sometimes you might need to
  63   force a consistent memory access ordering by using memory barriers.  It's
  64   not using a streaming DMA mapping, so it's good for small transfers on
  65   systems where the I/O would otherwise thrash an IOMMU mapping.  (See
  66   Documentation/PCI/PCI-DMA-mapping.txt for definitions of "coherent" and
  67   "streaming" DMA mappings.)
  68
  69   Asking for 1/Nth of a page (as well as asking for N pages) is reasonably
  70   space-efficient.
  71
  72   On most systems the memory returned will be uncached, because the
  73   semantics of dma-coherent memory require either bypassing CPU caches
  74   or using cache hardware with bus-snooping support.  While x86 hardware
  75   has such bus-snooping, many other systems use software to flush cache
  76   lines to prevent DMA conflicts.
  77
  78 - Devices on some EHCI controllers could handle DMA to/from high memory.
  79
  80   Unfortunately, the current Linux DMA infrastructure doesn't have a sane
  81   way to expose these capabilities ... and in any case, HIGHMEM is mostly a
  82   design wart specific to x86_32.  So your best bet is to ensure you never
  83   pass a highmem buffer into a USB driver.  That's easy; it's the default
  84   behavior.  Just don't override it; e.g. with NETIF_F_HIGHDMA.
  85
  86   This may force your callers to do some bounce buffering, copying from
  87   high memory to "normal" DMA memory.  If you can come up with a good way
  88   to fix this issue (for x86_32 machines with over 1 GByte of memory),
  89   feel free to submit patches.
  90
  91
  92 WORKING WITH EXISTING BUFFERS
  93
  94 Existing buffers aren't usable for DMA without first being mapped into the
  95 DMA address space of the device.  However, most buffers passed to your
  96 driver can safely be used with such DMA mapping.  (See the first section
  97 of Documentation/PCI/PCI-DMA-mapping.txt, titled "What memory is DMA-able?")
  98
  99 - When you're using scatterlists, you can map everything at once.  On some
 100   systems, this kicks in an IOMMU and turns the scatterlists into single
 101   DMA transactions:
 102
 103         int usb_buffer_map_sg (struct usb_device *dev, unsigned pipe,
 104                 struct scatterlist *sg, int nents);
 105
 106         void usb_buffer_dmasync_sg (struct usb_device *dev, unsigned pipe,
 107                 struct scatterlist *sg, int n_hw_ents);
 108
 109         void usb_buffer_unmap_sg (struct usb_device *dev, unsigned pipe,
 110                 struct scatterlist *sg, int n_hw_ents);
 111
 112   It's probably easier to use the new usb_sg_*() calls, which do the DMA
 113   mapping and apply other tweaks to make scatterlist i/o be fast.
 114
 115 - Some drivers may prefer to work with the model that they're mapping large
 116   buffers, synchronizing their safe re-use.  (If there's no re-use, then let
 117   usbcore do the map/unmap.)  Large periodic transfers make good examples
 118   here, since it's cheaper to just synchronize the buffer than to unmap it
 119   each time an urb completes and then re-map it on during resubmission.
 120
 121   These calls all work with initialized urbs:  urb->dev, urb->pipe,
 122   urb->transfer_buffer, and urb->transfer_buffer_length must all be
 123   valid when these calls are used (urb->setup_packet must be valid too
 124   if urb is a control request):
 125
 126         struct urb *usb_buffer_map (struct urb *urb);
 127
 128         void usb_buffer_dmasync (struct urb *urb);
 129
 130         void usb_buffer_unmap (struct urb *urb);
 131
 132   The calls manage urb->transfer_dma for you, and set URB_NO_TRANSFER_DMA_MAP
 133   so that usbcore won't map or unmap the buffer.  The same goes for
 134   urb->setup_dma and URB_NO_SETUP_DMA_MAP for control requests.
 135
 136 Note that several of those interfaces are currently commented out, since
 137 they don't have current users.  See the source code.  Other than the dmasync
 138 calls (where the underlying DMA primitives have changed), most of them can
 139 easily be commented back in if you want to use them.