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Diffstat (limited to 'kernel/Documentation/usb/dma.txt')
-rw-r--r-- | kernel/Documentation/usb/dma.txt | 133 |
1 files changed, 133 insertions, 0 deletions
diff --git a/kernel/Documentation/usb/dma.txt b/kernel/Documentation/usb/dma.txt new file mode 100644 index 000000000..444651e70 --- /dev/null +++ b/kernel/Documentation/usb/dma.txt @@ -0,0 +1,133 @@ +In Linux 2.5 kernels (and later), USB device drivers have additional control +over how DMA may be used to perform I/O operations. The APIs are detailed +in the kernel usb programming guide (kerneldoc, from the source code). + + +API OVERVIEW + +The big picture is that USB drivers can continue to ignore most DMA issues, +though they still must provide DMA-ready buffers (see +Documentation/DMA-API-HOWTO.txt). That's how they've worked through +the 2.4 (and earlier) kernels. + +OR: they can now be DMA-aware. + +- New calls enable DMA-aware drivers, letting them allocate dma buffers and + manage dma mappings for existing dma-ready buffers (see below). + +- URBs have an additional "transfer_dma" field, as well as a transfer_flags + bit saying if it's valid. (Control requests also have "setup_dma", but + drivers must not use it.) + +- "usbcore" will map this DMA address, if a DMA-aware driver didn't do + it first and set URB_NO_TRANSFER_DMA_MAP. HCDs + don't manage dma mappings for URBs. + +- There's a new "generic DMA API", parts of which are usable by USB device + drivers. Never use dma_set_mask() on any USB interface or device; that + would potentially break all devices sharing that bus. + + +ELIMINATING COPIES + +It's good to avoid making CPUs copy data needlessly. The costs can add up, +and effects like cache-trashing can impose subtle penalties. + +- If you're doing lots of small data transfers from the same buffer all + the time, that can really burn up resources on systems which use an + IOMMU to manage the DMA mappings. It can cost MUCH more to set up and + tear down the IOMMU mappings with each request than perform the I/O! + + For those specific cases, USB has primitives to allocate less expensive + memory. They work like kmalloc and kfree versions that give you the right + kind of addresses to store in urb->transfer_buffer and urb->transfer_dma. + You'd also set URB_NO_TRANSFER_DMA_MAP in urb->transfer_flags: + + void *usb_alloc_coherent (struct usb_device *dev, size_t size, + int mem_flags, dma_addr_t *dma); + + void usb_free_coherent (struct usb_device *dev, size_t size, + void *addr, dma_addr_t dma); + + Most drivers should *NOT* be using these primitives; they don't need + to use this type of memory ("dma-coherent"), and memory returned from + kmalloc() will work just fine. + + The memory buffer returned is "dma-coherent"; sometimes you might need to + force a consistent memory access ordering by using memory barriers. It's + not using a streaming DMA mapping, so it's good for small transfers on + systems where the I/O would otherwise thrash an IOMMU mapping. (See + Documentation/DMA-API-HOWTO.txt for definitions of "coherent" and + "streaming" DMA mappings.) + + Asking for 1/Nth of a page (as well as asking for N pages) is reasonably + space-efficient. + + On most systems the memory returned will be uncached, because the + semantics of dma-coherent memory require either bypassing CPU caches + or using cache hardware with bus-snooping support. While x86 hardware + has such bus-snooping, many other systems use software to flush cache + lines to prevent DMA conflicts. + +- Devices on some EHCI controllers could handle DMA to/from high memory. + + Unfortunately, the current Linux DMA infrastructure doesn't have a sane + way to expose these capabilities ... and in any case, HIGHMEM is mostly a + design wart specific to x86_32. So your best bet is to ensure you never + pass a highmem buffer into a USB driver. That's easy; it's the default + behavior. Just don't override it; e.g. with NETIF_F_HIGHDMA. + + This may force your callers to do some bounce buffering, copying from + high memory to "normal" DMA memory. If you can come up with a good way + to fix this issue (for x86_32 machines with over 1 GByte of memory), + feel free to submit patches. + + +WORKING WITH EXISTING BUFFERS + +Existing buffers aren't usable for DMA without first being mapped into the +DMA address space of the device. However, most buffers passed to your +driver can safely be used with such DMA mapping. (See the first section +of Documentation/DMA-API-HOWTO.txt, titled "What memory is DMA-able?") + +- When you're using scatterlists, you can map everything at once. On some + systems, this kicks in an IOMMU and turns the scatterlists into single + DMA transactions: + + int usb_buffer_map_sg (struct usb_device *dev, unsigned pipe, + struct scatterlist *sg, int nents); + + void usb_buffer_dmasync_sg (struct usb_device *dev, unsigned pipe, + struct scatterlist *sg, int n_hw_ents); + + void usb_buffer_unmap_sg (struct usb_device *dev, unsigned pipe, + struct scatterlist *sg, int n_hw_ents); + + It's probably easier to use the new usb_sg_*() calls, which do the DMA + mapping and apply other tweaks to make scatterlist i/o be fast. + +- Some drivers may prefer to work with the model that they're mapping large + buffers, synchronizing their safe re-use. (If there's no re-use, then let + usbcore do the map/unmap.) Large periodic transfers make good examples + here, since it's cheaper to just synchronize the buffer than to unmap it + each time an urb completes and then re-map it on during resubmission. + + These calls all work with initialized urbs: urb->dev, urb->pipe, + urb->transfer_buffer, and urb->transfer_buffer_length must all be + valid when these calls are used (urb->setup_packet must be valid too + if urb is a control request): + + struct urb *usb_buffer_map (struct urb *urb); + + void usb_buffer_dmasync (struct urb *urb); + + void usb_buffer_unmap (struct urb *urb); + + The calls manage urb->transfer_dma for you, and set URB_NO_TRANSFER_DMA_MAP + so that usbcore won't map or unmap the buffer. They cannot be used for + setup_packet buffers in control requests. + +Note that several of those interfaces are currently commented out, since +they don't have current users. See the source code. Other than the dmasync +calls (where the underlying DMA primitives have changed), most of them can +easily be commented back in if you want to use them. |