diff options
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/usb/dma.txt | 52 |
1 files changed, 37 insertions, 15 deletions
diff --git a/Documentation/usb/dma.txt b/Documentation/usb/dma.txt index 62844aeba69..e8b50b7de9d 100644 --- a/Documentation/usb/dma.txt +++ b/Documentation/usb/dma.txt @@ -32,12 +32,15 @@ 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. -- 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); @@ -45,6 +48,10 @@ and effects like cache-trashing can impose subtle penalties. 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 @@ -54,29 +61,39 @@ and effects like cache-trashing can impose subtle penalties. 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 + systems where the I/O would otherwise thrash an IOMMU mapping. (See Documentation/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 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 @@ -114,3 +131,8 @@ DMA address space of the device. 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. |