140 lines
6.2 KiB
Plaintext
140 lines
6.2 KiB
Plaintext
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/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.
|
|
|
|
- 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" and a
|
|
corresponding transfer_flags bit.)
|
|
|
|
- "usbcore" will map those DMA addresses, if a DMA-aware driver didn't do
|
|
it first and set URB_NO_TRANSFER_DMA_MAP or URB_NO_SETUP_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_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
|
|
buffers you have prepared. For non-control transfers URB_NO_SETUP_DMA_MAP
|
|
is ignored.
|
|
|
|
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/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.
|
|
|
|
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/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
|
|
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. 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.
|