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<HTML
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>USB-ethernet Data Transfers</TITLE
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REL="UP"
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TITLE="eCos Support for Developing USB-ethernet Peripherals"
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HREF="io-usb-slave-eth.html"><LINK
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REL="PREVIOUS"
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TITLE="Initializing the USB-ethernet Package"
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TITLE="USB-ethernet State Handling"
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WIDTH="100%"></DIV
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><H1
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><A
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NAME="USBSETH-DATA">USB-ethernet Data Transfers</H1
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><DIV
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CLASS="REFNAMEDIV"
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><A
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NAME="AEN17445"
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></A
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><H2
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>Name</H2
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>USB-ethernet Data Transfers -- Exchanging ethernet packets with the USB host</DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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><A
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NAME="AEN17448"><H2
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>Synopsis</H2
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><DIV
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CLASS="FUNCSYNOPSIS"
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><A
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><PRE
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CLASS="FUNCSYNOPSISINFO"
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>#include <cyg/io/usb/usbs_eth.h></PRE
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void usbs_eth_start_rx</CODE
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>(usbs_eth* usbseth, unsigned char* buffer, void (*)(usbs_eth*, void*, int) complete_fn, void* complete_data);</CODE
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></P
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><P
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><CODE
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><CODE
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CLASS="FUNCDEF"
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>void usbs_eth_start_tx</CODE
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>(usbs_eth* usbseth, unsigned char* buffer, void (*)(usbs_eth*, void*, int) complete_fn, void* complete_data);</CODE
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></P
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><P
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></P
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NAME="AEN17473"
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></A
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><H2
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>Description</H2
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><P
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>The USB-ethernet package provides two main modes of operation. In the
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first mode it provides a <A
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HREF="usbseth-netdev.html"
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>network device
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driver</A
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> for use by a TCP/IP stack running inside the USB
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peripheral. All incoming ethernet packages should be passed up the
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TCP/IP stack, and only the stack will generate outgoing packets. Apart
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from <A
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HREF="usbseth-init.html"
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>initialization</A
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> and possibly
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certain <A
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HREF="usbseth-control.html"
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>control operations</A
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>,
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higher-level code will not interact with the USB-ethernet package
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directly.</P
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><P
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>In the second mode there is no TCP/IP stack running inside the USB
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peripheral. For example, a simple USB-ethernet converter has an
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ethernet chip and a USB port: ethernet packets received by the
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ethernet chip need to be forwarded to the USB host, and ethernet
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packets sent by the USB host need to be sent out of the ethernet chip.
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<TT
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CLASS="FUNCTION"
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>usbs_eth_start_rx</TT
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> and
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<TT
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CLASS="FUNCTION"
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>usbs_eth_start_tx</TT
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> allow for this lower-level
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access to the USB-ethernet package.</P
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><P
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>The two modes of operation are mutually exclusive. If the network
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device driver mode is enabled then application code should communicate
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at the TCP/IP level, and not by using the lower-level functions.
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Instead, it is the network device driver that will make use of these
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functions, and it assumes that it has exclusive access. The package
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does not perform any locking.</P
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><P
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>The transmit and receive functions work in much the same way. The
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first argument identifies the <SPAN
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CLASS="STRUCTNAME"
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>usbs_eth</SPAN
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>
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structure that should be used. For the majority of applications this
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will be <TT
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CLASS="LITERAL"
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>usbs_eth0</TT
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>. The second argument specifies
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the location of the ethernet packet; outgoing for
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<TT
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CLASS="FUNCTION"
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>usbs_eth_start_tx</TT
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> and incoming for
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<TT
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CLASS="FUNCTION"
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>usbs_eth_start_rx</TT
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>. This buffer should correspond
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to the <A
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HREF="usbseth-protocol.html"
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>protocol</A
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>:</P
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><P
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></P
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><OL
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TYPE="1"
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><LI
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><P
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>Outgoing packets can consist of up to 1516 bytes, consisting of a
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two-byte header specific to USB-ethernet followed by a standard
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ethernet frame (a header with 6-byte destination address, 6-byte
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source address and a further two bytes, followed by a payload of
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up to 1500 bytes). The two-byte USB-ethernet header consists simply of
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the size of the ethernet frame, i.e. the size of the rest of the
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packet not including the USB-ethernet header, with the least
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significant byte first.</P
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></LI
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><LI
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><P
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>For incoming packets the supplied buffer should usually be at least
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1516 bytes. There may be special circumstances in which a smaller
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buffer might be safe; for example, if the host-side device driver is
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modified to support only smaller packets. Once the packet has been
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received the buffer will contain a two-byte header specific to
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USB-ethernet, followed by a normal ethernet frame. The header
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gives the size of the ethernet frame, excluding the header, with the
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least significant byte first.</P
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></LI
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></OL
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><P
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>Both <TT
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CLASS="FUNCTION"
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>usbs_eth_start_tx</TT
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> and
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<TT
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CLASS="FUNCTION"
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>usbs_eth_start_rx</TT
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> are asynchronous: the transfer
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is started and, some time later, a completion function will be invoked.
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The third and fourth arguments to both
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<TT
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CLASS="FUNCTION"
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>usbs_eth_start_tx</TT
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> and
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<TT
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CLASS="FUNCTION"
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>usbs_eth_start_rx</TT
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> supply the completion function
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and an argument to that function respectively. The completion function
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will be invoked with three arguments: a pointer to the
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<SPAN
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CLASS="STRUCTNAME"
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>usbs_eth</SPAN
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> data structure, usually
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<TT
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CLASS="LITERAL"
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>usbs_eth0</TT
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>; the supplied completion data ; and a
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return code field. A negative value indicates that an error occurred,
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for example <TT
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CLASS="LITERAL"
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>-EPIPE</TT
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> if the connection between USB
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host and peripheral has been broken, or <TT
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CLASS="LITERAL"
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>-EAGAIN</TT
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> if
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an endpoint has been halted. A positive value indicates the total size
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of the transfer, which should correspond to the size in the
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USB-ethernet header plus an additional two bytes for the header
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itself.</P
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><P
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>If the data transfer is succesful then the completion function will
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typically be invoked in DSR context rather than in thread context,
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although this depends on the implementation of the underlying USB
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device driver. Therefore the completion function is restricted in what
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it can do; in particular, it must not make any calls that will or may
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block such as locking a mutex or allocating memory. The kernel
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documentation should be consulted for more details of DSR's and
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interrupt handling generally. Note that if the transfer finishes
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quickly then the completion function may be invoked before
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<TT
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CLASS="FUNCTION"
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>usbs_eth_start_rx</TT
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> or
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<TT
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CLASS="FUNCTION"
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>usbs_eth_start_tx</TT
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> returns. This is especially
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likely to happen if the current thread is descheduled after starting
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the data transfer but before returning from these functions.</P
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><P
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>For transmit operations, it is possible for
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<TT
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CLASS="FUNCTION"
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>usbs_eth_start_tx</TT
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> to invoke the completion
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function immediately. If there is no current connection between host
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and target then the transmit will fail immediately with
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<TT
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CLASS="LITERAL"
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>-EPIPE</TT
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>. In addition the USB-ethernet package will
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check the destination MAC address and make sure that the ethernet
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frame really is intended for the host: either it must be for the
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address specified in the initialization call <A
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HREF="usbseth-init.html"
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><TT
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CLASS="FUNCTION"
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>usbs_eth_init</TT
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></A
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>, or
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it must be a broadcast packet, or the host must have enabled
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promiscuous mode. </P
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ALIGN="left"
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VALIGN="top"
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>Initializing the USB-ethernet Package</TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="io-usb-slave-eth.html"
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ACCESSKEY="U"
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>Up</A
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></TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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>USB-ethernet State Handling</TD
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></TR
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