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[/] [or1k_soc_on_altera_embedded_dev_kit/] [trunk/] [linux-2.6/] [linux-2.6.24/] [Documentation/] [usb/] [dma.txt] - Blame information for rev 3

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1 3 xianfeng
In Linux 2.5 kernels (and later), USB device drivers have additional control
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over how DMA may be used to perform I/O operations.  The APIs are detailed
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in the kernel usb programming guide (kerneldoc, from the source code).
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API OVERVIEW
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The big picture is that USB drivers can continue to ignore most DMA issues,
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though they still must provide DMA-ready buffers (see DMA-mapping.txt).
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That's how they've worked through the 2.4 (and earlier) kernels.
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OR:  they can now be DMA-aware.
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- New calls enable DMA-aware drivers, letting them allocate dma buffers and
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  manage dma mappings for existing dma-ready buffers (see below).
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- URBs have an additional "transfer_dma" field, as well as a transfer_flags
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  bit saying if it's valid.  (Control requests also have "setup_dma" and a
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  corresponding transfer_flags bit.)
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- "usbcore" will map those DMA addresses, if a DMA-aware driver didn't do
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  it first and set URB_NO_TRANSFER_DMA_MAP or URB_NO_SETUP_DMA_MAP.  HCDs
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  don't manage dma mappings for URBs.
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- There's a new "generic DMA API", parts of which are usable by USB device
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  drivers.  Never use dma_set_mask() on any USB interface or device; that
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  would potentially break all devices sharing that bus.
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ELIMINATING COPIES
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It's good to avoid making CPUs copy data needlessly.  The costs can add up,
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and effects like cache-trashing can impose subtle penalties.
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- If you're doing lots of small data transfers from the same buffer all
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  the time, that can really burn up resources on systems which use an
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  IOMMU to manage the DMA mappings.  It can cost MUCH more to set up and
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  tear down the IOMMU mappings with each request than perform the I/O!
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  For those specific cases, USB has primitives to allocate less expensive
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  memory.  They work like kmalloc and kfree versions that give you the right
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  kind of addresses to store in urb->transfer_buffer and urb->transfer_dma.
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  You'd also set URB_NO_TRANSFER_DMA_MAP in urb->transfer_flags:
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        void *usb_buffer_alloc (struct usb_device *dev, size_t size,
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                int mem_flags, dma_addr_t *dma);
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        void usb_buffer_free (struct usb_device *dev, size_t size,
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                void *addr, dma_addr_t dma);
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  Most drivers should *NOT* be using these primitives; they don't need
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  to use this type of memory ("dma-coherent"), and memory returned from
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  kmalloc() will work just fine.
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  For control transfers you can use the buffer primitives or not for each
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  of the transfer buffer and setup buffer independently.  Set the flag bits
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  URB_NO_TRANSFER_DMA_MAP and URB_NO_SETUP_DMA_MAP to indicate which
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  buffers you have prepared.  For non-control transfers URB_NO_SETUP_DMA_MAP
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  is ignored.
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  The memory buffer returned is "dma-coherent"; sometimes you might need to
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  force a consistent memory access ordering by using memory barriers.  It's
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  not using a streaming DMA mapping, so it's good for small transfers on
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  systems where the I/O would otherwise thrash an IOMMU mapping.  (See
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  Documentation/DMA-mapping.txt for definitions of "coherent" and "streaming"
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  DMA mappings.)
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  Asking for 1/Nth of a page (as well as asking for N pages) is reasonably
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  space-efficient.
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  On most systems the memory returned will be uncached, because the
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  semantics of dma-coherent memory require either bypassing CPU caches
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  or using cache hardware with bus-snooping support.  While x86 hardware
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  has such bus-snooping, many other systems use software to flush cache
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  lines to prevent DMA conflicts.
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- Devices on some EHCI controllers could handle DMA to/from high memory.
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  Unfortunately, the current Linux DMA infrastructure doesn't have a sane
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  way to expose these capabilities ... and in any case, HIGHMEM is mostly a
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  design wart specific to x86_32.  So your best bet is to ensure you never
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  pass a highmem buffer into a USB driver.  That's easy; it's the default
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  behavior.  Just don't override it; e.g. with NETIF_F_HIGHDMA.
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  This may force your callers to do some bounce buffering, copying from
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  high memory to "normal" DMA memory.  If you can come up with a good way
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  to fix this issue (for x86_32 machines with over 1 GByte of memory),
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  feel free to submit patches.
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WORKING WITH EXISTING BUFFERS
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Existing buffers aren't usable for DMA without first being mapped into the
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DMA address space of the device.  However, most buffers passed to your
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driver can safely be used with such DMA mapping.  (See the first section
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of DMA-mapping.txt, titled "What memory is DMA-able?")
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- When you're using scatterlists, you can map everything at once.  On some
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  systems, this kicks in an IOMMU and turns the scatterlists into single
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  DMA transactions:
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        int usb_buffer_map_sg (struct usb_device *dev, unsigned pipe,
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                struct scatterlist *sg, int nents);
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        void usb_buffer_dmasync_sg (struct usb_device *dev, unsigned pipe,
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                struct scatterlist *sg, int n_hw_ents);
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        void usb_buffer_unmap_sg (struct usb_device *dev, unsigned pipe,
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                struct scatterlist *sg, int n_hw_ents);
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  It's probably easier to use the new usb_sg_*() calls, which do the DMA
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  mapping and apply other tweaks to make scatterlist i/o be fast.
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- Some drivers may prefer to work with the model that they're mapping large
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  buffers, synchronizing their safe re-use.  (If there's no re-use, then let
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  usbcore do the map/unmap.)  Large periodic transfers make good examples
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  here, since it's cheaper to just synchronize the buffer than to unmap it
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  each time an urb completes and then re-map it on during resubmission.
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  These calls all work with initialized urbs:  urb->dev, urb->pipe,
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  urb->transfer_buffer, and urb->transfer_buffer_length must all be
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  valid when these calls are used (urb->setup_packet must be valid too
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  if urb is a control request):
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        struct urb *usb_buffer_map (struct urb *urb);
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        void usb_buffer_dmasync (struct urb *urb);
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        void usb_buffer_unmap (struct urb *urb);
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  The calls manage urb->transfer_dma for you, and set URB_NO_TRANSFER_DMA_MAP
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  so that usbcore won't map or unmap the buffer.  The same goes for
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  urb->setup_dma and URB_NO_SETUP_DMA_MAP for control requests.
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Note that several of those interfaces are currently commented out, since
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they don't have current users.  See the source code.  Other than the dmasync
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calls (where the underlying DMA primitives have changed), most of them can
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easily be commented back in if you want to use them.

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