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 DMA-mapping.txt).
-That's how they've worked through the 2.4 (and earlier) kernels.
+though they still must provide DMA-ready buffers (see
+Documentation/PCI/PCI-DMA-mapping.txt). That's how they've worked through
+the 2.4 (and earlier) kernels.
OR: they can now be DMA-aware.
It's good to avoid making CPUs copy data needlessly. The costs can add up,
and effects like cache-trashing can impose subtle penalties.
-- When you're allocating a buffer for DMA purposes anyway, use the buffer
- primitives. Think of them as kmalloc and kfree that give you the right
- kind of addresses to store in urb->transfer_buffer and urb->transfer_dma,
- while guaranteeing that no hidden copies through DMA "bounce" buffers will
- slow things down. You'd also set URB_NO_TRANSFER_DMA_MAP in
- urb->transfer_flags:
+- 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_buffer_alloc (struct usb_device *dev, size_t size,
int mem_flags, dma_addr_t *dma);
void usb_buffer_free (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.
+
For control transfers you can use the buffer primitives or not for each
of the transfer buffer and setup buffer independently. Set the flag bits
URB_NO_TRANSFER_DMA_MAP and URB_NO_SETUP_DMA_MAP to indicate which
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 tie up an IOMMU mapping. (See
- Documentation/DMA-mapping.txt for definitions of "coherent" and "streaming"
- DMA mappings.)
+ systems where the I/O would otherwise thrash an IOMMU mapping. (See
+ Documentation/PCI/PCI-DMA-mapping.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.
- Driver probe() routines can notice this using a generic DMA call, then
- tell higher level code (network, scsi, etc) about it like this:
- if (dma_supported (&intf->dev, 0xffffffffffffffffULL))
- net->features |= NETIF_F_HIGHDMA;
+ 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.
- That can eliminate dma bounce buffering of requests that originate (or
- terminate) in high memory, in cases where the buffers aren't allocated
- with usb_buffer_alloc() but instead are dma-mapped.
+ 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.
+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/PCI/PCI-DMA-mapping.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
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. The same goes for
urb->setup_dma and URB_NO_SETUP_DMA_MAP for 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.
git://ftp.safe.ca / safe / jmp / linux-2.6 / blobdiff