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-rw-r--r--Documentation/usb/dma.txt18
1 files changed, 6 insertions, 12 deletions
diff --git a/Documentation/usb/dma.txt b/Documentation/usb/dma.txt
index a37e59cf2786..84ef865237db 100644
--- a/Documentation/usb/dma.txt
+++ b/Documentation/usb/dma.txt
@@ -16,11 +16,11 @@ OR: they can now be DMA-aware.
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.)
+ bit saying if it's valid. (Control requests also have "setup_dma", but
+ drivers must not use it.)
-- "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
+- "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
@@ -53,12 +53,6 @@ and effects like cache-trashing can impose subtle penalties.
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
@@ -130,8 +124,8 @@ of Documentation/PCI/PCI-DMA-mapping.txt, titled "What memory is DMA-able?")
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.
+ 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