driver-api/usb/dma.rst

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
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
45   memory.  They work like kmalloc and kfree versions that give you the right
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::
55   Most drivers should **NOT** be using these primitives; they don't need
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
63   Documentation/core-api/dma-api-howto.rst for definitions of "coherent" and
67   space-efficient.
69   On most systems the memory returned will be uncached, because the
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
75 - Devices on some EHCI controllers could handle DMA to/from high memory.
92 DMA address space of the device.  However, most buffers passed to your
94 of Documentation/core-api/dma-api-howto.rst, titled "What memory is DMA-able?")
96 - When you have the scatterlists which have been mapped for the USB controller,