usb/dma.txt: convert to ReST and add to driver-api book
This document describe some USB core features. Add it to the driver-api book. Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com> Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Signed-off-by: Jonathan Corbet <corbet@lwn.net>zero-colors
Mauro Carvalho Chehab
Jonathan Corbet
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3db5f406e4
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@ -1,16 +1,19 @@
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USB DMA
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~~~~~~~
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In Linux 2.5 kernels (and later), USB device drivers have additional control
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over how DMA may be used to perform I/O operations. The APIs are detailed
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in the kernel usb programming guide (kerneldoc, from the source code).
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API OVERVIEW
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API overview
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============
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The big picture is that USB drivers can continue to ignore most DMA issues,
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though they still must provide DMA-ready buffers (see
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Documentation/DMA-API-HOWTO.txt). That's how they've worked through
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the 2.4 (and earlier) kernels.
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``Documentation/DMA-API-HOWTO.txt``). That's how they've worked through
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the 2.4 (and earlier) kernels, or they can now be DMA-aware.
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OR: they can now be DMA-aware.
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DMA-aware usb drivers:
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- New calls enable DMA-aware drivers, letting them allocate dma buffers and
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manage dma mappings for existing dma-ready buffers (see below).
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@ -20,15 +23,15 @@ OR: they can now be DMA-aware.
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drivers must not use it.)
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- "usbcore" will map this DMA address, if a DMA-aware driver didn't do
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it first and set URB_NO_TRANSFER_DMA_MAP. HCDs
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it first and set ``URB_NO_TRANSFER_DMA_MAP``. HCDs
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don't manage dma mappings for URBs.
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- There's a new "generic DMA API", parts of which are usable by USB device
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drivers. Never use dma_set_mask() on any USB interface or device; that
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would potentially break all devices sharing that bus.
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ELIMINATING COPIES
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Eliminating copies
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==================
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It's good to avoid making CPUs copy data needlessly. The costs can add up,
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and effects like cache-trashing can impose subtle penalties.
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@ -41,7 +44,7 @@ and effects like cache-trashing can impose subtle penalties.
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For those specific cases, USB has primitives to allocate less expensive
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memory. They work like kmalloc and kfree versions that give you the right
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kind of addresses to store in urb->transfer_buffer and urb->transfer_dma.
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You'd also set URB_NO_TRANSFER_DMA_MAP in urb->transfer_flags:
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You'd also set ``URB_NO_TRANSFER_DMA_MAP`` in urb->transfer_flags::
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void *usb_alloc_coherent (struct usb_device *dev, size_t size,
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int mem_flags, dma_addr_t *dma);
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@ -49,15 +52,15 @@ and effects like cache-trashing can impose subtle penalties.
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void usb_free_coherent (struct usb_device *dev, size_t size,
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void *addr, dma_addr_t dma);
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Most drivers should *NOT* be using these primitives; they don't need
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Most drivers should **NOT** be using these primitives; they don't need
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to use this type of memory ("dma-coherent"), and memory returned from
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kmalloc() will work just fine.
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:c:func:`kmalloc` will work just fine.
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The memory buffer returned is "dma-coherent"; sometimes you might need to
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force a consistent memory access ordering by using memory barriers. It's
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not using a streaming DMA mapping, so it's good for small transfers on
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systems where the I/O would otherwise thrash an IOMMU mapping. (See
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Documentation/DMA-API-HOWTO.txt for definitions of "coherent" and
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``Documentation/DMA-API-HOWTO.txt`` for definitions of "coherent" and
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"streaming" DMA mappings.)
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Asking for 1/Nth of a page (as well as asking for N pages) is reasonably
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@ -75,15 +78,15 @@ and effects like cache-trashing can impose subtle penalties.
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way to expose these capabilities ... and in any case, HIGHMEM is mostly a
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design wart specific to x86_32. So your best bet is to ensure you never
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pass a highmem buffer into a USB driver. That's easy; it's the default
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behavior. Just don't override it; e.g. with NETIF_F_HIGHDMA.
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behavior. Just don't override it; e.g. with ``NETIF_F_HIGHDMA``.
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This may force your callers to do some bounce buffering, copying from
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high memory to "normal" DMA memory. If you can come up with a good way
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to fix this issue (for x86_32 machines with over 1 GByte of memory),
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feel free to submit patches.
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WORKING WITH EXISTING BUFFERS
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Working with existing buffers
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=============================
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Existing buffers aren't usable for DMA without first being mapped into the
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DMA address space of the device. However, most buffers passed to your
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@ -92,7 +95,7 @@ of Documentation/DMA-API-HOWTO.txt, titled "What memory is DMA-able?")
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- When you're using scatterlists, you can map everything at once. On some
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systems, this kicks in an IOMMU and turns the scatterlists into single
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DMA transactions:
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DMA transactions::
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int usb_buffer_map_sg (struct usb_device *dev, unsigned pipe,
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struct scatterlist *sg, int nents);
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@ -103,7 +106,7 @@ of Documentation/DMA-API-HOWTO.txt, titled "What memory is DMA-able?")
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void usb_buffer_unmap_sg (struct usb_device *dev, unsigned pipe,
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struct scatterlist *sg, int n_hw_ents);
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It's probably easier to use the new usb_sg_*() calls, which do the DMA
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It's probably easier to use the new ``usb_sg_*()`` calls, which do the DMA
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mapping and apply other tweaks to make scatterlist i/o be fast.
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- Some drivers may prefer to work with the model that they're mapping large
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@ -112,10 +115,10 @@ of Documentation/DMA-API-HOWTO.txt, titled "What memory is DMA-able?")
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here, since it's cheaper to just synchronize the buffer than to unmap it
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each time an urb completes and then re-map it on during resubmission.
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These calls all work with initialized urbs: urb->dev, urb->pipe,
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urb->transfer_buffer, and urb->transfer_buffer_length must all be
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valid when these calls are used (urb->setup_packet must be valid too
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if urb is a control request):
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These calls all work with initialized urbs: ``urb->dev``, ``urb->pipe``,
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``urb->transfer_buffer``, and ``urb->transfer_buffer_length`` must all be
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valid when these calls are used (``urb->setup_packet`` must be valid too
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if urb is a control request)::
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struct urb *usb_buffer_map (struct urb *urb);
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@ -123,9 +126,9 @@ of Documentation/DMA-API-HOWTO.txt, titled "What memory is DMA-able?")
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void usb_buffer_unmap (struct urb *urb);
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The calls manage urb->transfer_dma for you, and set URB_NO_TRANSFER_DMA_MAP
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so that usbcore won't map or unmap the buffer. They cannot be used for
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setup_packet buffers in control requests.
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The calls manage ``urb->transfer_dma`` for you, and set
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``URB_NO_TRANSFER_DMA_MAP`` so that usbcore won't map or unmap the buffer.
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They cannot be used for setup_packet buffers in control requests.
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Note that several of those interfaces are currently commented out, since
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they don't have current users. See the source code. Other than the dmasync
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@ -9,6 +9,7 @@ Linux USB API
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anchors
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bulk-streams
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callbacks
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dma
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power-management
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writing_usb_driver
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writing_musb_glue_layer
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