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