137 lines
6.0 KiB
ReStructuredText
137 lines
6.0 KiB
ReStructuredText
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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============
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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/core-api/dma-api-howto.rst). 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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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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- URBs have an additional "transfer_dma" field, as well as a transfer_flags
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bit saying if it's valid. (Control requests also have "setup_dma", but
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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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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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==================
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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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- If you're doing lots of small data transfers from the same buffer all
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the time, that can really burn up resources on systems which use an
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IOMMU to manage the DMA mappings. It can cost MUCH more to set up and
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tear down the IOMMU mappings with each request than perform the I/O!
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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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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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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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to use this type of memory ("dma-coherent"), and memory returned from
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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/core-api/dma-api-howto.rst 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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space-efficient.
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On most systems the memory returned will be uncached, because the
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semantics of dma-coherent memory require either bypassing CPU caches
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or using cache hardware with bus-snooping support. While x86 hardware
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has such bus-snooping, many other systems use software to flush cache
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lines to prevent DMA conflicts.
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- Devices on some EHCI controllers could handle DMA to/from high memory.
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Unfortunately, the current Linux DMA infrastructure doesn't have a sane
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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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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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=============================
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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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driver can safely be used with such DMA mapping. (See the first section
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of Documentation/core-api/dma-api-howto.rst, 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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int usb_buffer_map_sg (struct usb_device *dev, unsigned pipe,
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struct scatterlist *sg, int nents);
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void usb_buffer_dmasync_sg (struct usb_device *dev, unsigned pipe,
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struct scatterlist *sg, int n_hw_ents);
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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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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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buffers, synchronizing their safe re-use. (If there's no re-use, then let
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usbcore do the map/unmap.) Large periodic transfers make good examples
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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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struct urb *usb_buffer_map (struct urb *urb);
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void usb_buffer_dmasync (struct urb *urb);
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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
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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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calls (where the underlying DMA primitives have changed), most of them can
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easily be commented back in if you want to use them.
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