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1USB DMA2~~~~~~~3 4In Linux 2.5 kernels (and later), USB device drivers have additional control5over how DMA may be used to perform I/O operations. The APIs are detailed6in the kernel usb programming guide (kerneldoc, from the source code).7 8API overview9============10 11The big picture is that USB drivers can continue to ignore most DMA issues,12though they still must provide DMA-ready buffers (see13Documentation/core-api/dma-api-howto.rst). That's how they've worked through14the 2.4 (and earlier) kernels, or they can now be DMA-aware.15 16DMA-aware usb drivers:17 18- New calls enable DMA-aware drivers, letting them allocate dma buffers and19 manage dma mappings for existing dma-ready buffers (see below).20 21- URBs have an additional "transfer_dma" field, as well as a transfer_flags22 bit saying if it's valid. (Control requests also have "setup_dma", but23 drivers must not use it.)24 25- "usbcore" will map this DMA address, if a DMA-aware driver didn't do26 it first and set ``URB_NO_TRANSFER_DMA_MAP``. HCDs27 don't manage dma mappings for URBs.28 29- There's a new "generic DMA API", parts of which are usable by USB device30 drivers. Never use dma_set_mask() on any USB interface or device; that31 would potentially break all devices sharing that bus.32 33Eliminating copies34==================35 36It's good to avoid making CPUs copy data needlessly. The costs can add up,37and effects like cache-trashing can impose subtle penalties.38 39- If you're doing lots of small data transfers from the same buffer all40 the time, that can really burn up resources on systems which use an41 IOMMU to manage the DMA mappings. It can cost MUCH more to set up and42 tear down the IOMMU mappings with each request than perform the I/O!43 44 For those specific cases, USB has primitives to allocate less expensive45 memory. They work like kmalloc and kfree versions that give you the right46 kind of addresses to store in urb->transfer_buffer and urb->transfer_dma.47 You'd also set ``URB_NO_TRANSFER_DMA_MAP`` in urb->transfer_flags::48 49 void *usb_alloc_coherent (struct usb_device *dev, size_t size,50 int mem_flags, dma_addr_t *dma);51 52 void usb_free_coherent (struct usb_device *dev, size_t size,53 void *addr, dma_addr_t dma);54 55 Most drivers should **NOT** be using these primitives; they don't need56 to use this type of memory ("dma-coherent"), and memory returned from57 :c:func:`kmalloc` will work just fine.58 59 The memory buffer returned is "dma-coherent"; sometimes you might need to60 force a consistent memory access ordering by using memory barriers. It's61 not using a streaming DMA mapping, so it's good for small transfers on62 systems where the I/O would otherwise thrash an IOMMU mapping. (See63 Documentation/core-api/dma-api-howto.rst for definitions of "coherent" and64 "streaming" DMA mappings.)65 66 Asking for 1/Nth of a page (as well as asking for N pages) is reasonably67 space-efficient.68 69 On most systems the memory returned will be uncached, because the70 semantics of dma-coherent memory require either bypassing CPU caches71 or using cache hardware with bus-snooping support. While x86 hardware72 has such bus-snooping, many other systems use software to flush cache73 lines to prevent DMA conflicts.74 75- Devices on some EHCI controllers could handle DMA to/from high memory.76 77 Unfortunately, the current Linux DMA infrastructure doesn't have a sane78 way to expose these capabilities ... and in any case, HIGHMEM is mostly a79 design wart specific to x86_32. So your best bet is to ensure you never80 pass a highmem buffer into a USB driver. That's easy; it's the default81 behavior. Just don't override it; e.g. with ``NETIF_F_HIGHDMA``.82 83 This may force your callers to do some bounce buffering, copying from84 high memory to "normal" DMA memory. If you can come up with a good way85 to fix this issue (for x86_32 machines with over 1 GByte of memory),86 feel free to submit patches.87 88Working with existing buffers89=============================90 91Existing buffers aren't usable for DMA without first being mapped into the92DMA address space of the device. However, most buffers passed to your93driver can safely be used with such DMA mapping. (See the first section94of Documentation/core-api/dma-api-howto.rst, titled "What memory is DMA-able?")95 96- When you have the scatterlists which have been mapped for the USB controller,97 you could use the new ``usb_sg_*()`` calls, which would turn scatterlist98 into URBs::99 100 int usb_sg_init(struct usb_sg_request *io, struct usb_device *dev,101 unsigned pipe, unsigned period, struct scatterlist *sg,102 int nents, size_t length, gfp_t mem_flags);103 104 void usb_sg_wait(struct usb_sg_request *io);105 106 void usb_sg_cancel(struct usb_sg_request *io);107 108 When the USB controller doesn't support DMA, the ``usb_sg_init()`` would try109 to submit URBs in PIO way as long as the page in scatterlists is not in the110 Highmem, which could be very rare in modern architectures.111