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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).
+The big picture is that USB drivers can continue to ignore most DMA issues,
+though they still must provide DMA-ready buffers (see DMA-mapping.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" and a
+ corresponding transfer_flags bit.)
+- "usbcore" will map those DMA addresses, if a DMA-aware driver didn't do
+ 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.
+It's good to avoid making CPUs copy data needlessly. The costs can add up,
+and effects like cache-trashing can impose subtle penalties.
+- When you're allocating a buffer for DMA purposes anyway, use the buffer
+ primitives. Think of them as kmalloc and kfree that give you the right
+ kind of addresses to store in urb->transfer_buffer and urb->transfer_dma,
+ while guaranteeing that no hidden copies through DMA "bounce" buffers will
+ slow things down. You'd also set URB_NO_TRANSFER_DMA_MAP in
+ urb->transfer_flags:
+ void *usb_buffer_alloc (struct usb_device *dev, size_t size,
+ int mem_flags, dma_addr_t *dma);
+ void usb_buffer_free (struct usb_device *dev, size_t size,
+ void *addr, dma_addr_t dma);
+ For control transfers you can use the buffer primitives or not for each
+ of the transfer buffer and setup buffer independently. Set the flag bits
+ buffers you have prepared. For non-control transfers URB_NO_SETUP_DMA_MAP
+ is ignored.
+ 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 tie up an IOMMU mapping. (See
+ Documentation/DMA-mapping.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.
+- Devices on some EHCI controllers could handle DMA to/from high memory.
+ Driver probe() routines can notice this using a generic DMA call, then
+ tell higher level code (network, scsi, etc) about it like this:
+ if (dma_supported (&intf->dev, 0xffffffffffffffffULL))
+ net->features |= NETIF_F_HIGHDMA;
+ That can eliminate dma bounce buffering of requests that originate (or
+ terminate) in high memory, in cases where the buffers aren't allocated
+ with usb_buffer_alloc() but instead are dma-mapped.
+Existing buffers aren't usable for DMA without first being mapped into the
+DMA address space of the device.
+- 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. The same goes for
+ urb->setup_dma and URB_NO_SETUP_DMA_MAP for control requests.