driver-api/usb/dma.rst

1 USB DMA
5 over how DMA may be used to perform I/O operations.  The APIs are detailed
11 The big picture is that USB drivers can continue to ignore most DMA issues,
12 though they still must provide DMA-ready buffers (see
13 Documentation/core-api/dma-api-howto.rst).  That's how they've worked through
14 the 2.4 (and earlier) kernels, or they can now be DMA-aware.
16 DMA-aware usb drivers:
18 - New calls enable DMA-aware drivers, letting them allocate dma buffers and
19   manage dma mappings for existing dma-ready buffers (see below).
21 - URBs have an additional "transfer_dma" field, as well as a transfer_flags
25 - "usbcore" will map this DMA address, if a DMA-aware driver didn't do
27   don't manage dma mappings for URBs.
29 - There's a new "generic DMA API", parts of which are usable by USB device
37 and effects like cache-trashing can impose subtle penalties.
39 - If you're doing lots of small data transfers from the same buffer all
41   IOMMU to manage the DMA mappings.  It can cost MUCH more to set up and
46   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::
50 		int mem_flags, dma_addr_t *dma);
53 		void *addr, dma_addr_t dma);
56   to use this type of memory ("dma-coherent"), and memory returned from
59   The memory buffer returned is "dma-coherent"; sometimes you might need to
61   not using a streaming DMA mapping, so it's good for small transfers on
63   Documentation/core-api/dma-api-howto.rst for definitions of "coherent" and
64   "streaming" DMA mappings.)
67   space-efficient.
70   semantics of dma-coherent memory require either bypassing CPU caches
71   or using cache hardware with bus-snooping support.  While x86 hardware
72   has such bus-snooping, many other systems use software to flush cache
73   lines to prevent DMA conflicts.
75 - Devices on some EHCI controllers could handle DMA to/from high memory.
77   Unfortunately, the current Linux DMA infrastructure doesn't have a sane
84   high memory to "normal" DMA memory.  If you can come up with a good way
91 Existing buffers aren't usable for DMA without first being mapped into the
92 DMA address space of the device.  However, most buffers passed to your
93 driver can safely be used with such DMA mapping.  (See the first section
94 of Documentation/core-api/dma-api-howto.rst, titled "What memory is DMA-able?")
96 - When you're using scatterlists, you can map everything at once.  On some
98   DMA transactions::
109   It's probably easier to use the new ``usb_sg_*()`` calls, which do the DMA
112 - Some drivers may prefer to work with the model that they're mapping large
113   buffers, synchronizing their safe re-use.  (If there's no re-use, then let
116   each time an urb completes and then re-map it on during resubmission.
118   These calls all work with initialized urbs:  ``urb->dev``, ``urb->pipe``,
119   ``urb->transfer_buffer``, and ``urb->transfer_buffer_length`` must all be
120   valid when these calls are used (``urb->setup_packet`` must be valid too
129   The calls manage ``urb->transfer_dma`` for you, and set
135 calls (where the underlying DMA primitives have changed), most of them can