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<HTML
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>Data Endpoints</TITLE
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TITLE="eCos Reference Manual"
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TITLE="eCos USB Slave Support"
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TITLE="Control Endpoints"
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TITLE="Writing a USB Device Driver"
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>eCos Reference Manual</TH
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><A
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><H1
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><A
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NAME="USBS-DATA">Data Endpoints</H1
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><DIV
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CLASS="REFNAMEDIV"
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><A
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NAME="AEN16660"
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></A
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><H2
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>Name</H2
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>Data Endpoints&nbsp;--&nbsp;Data endpoint data structures</DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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><A
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NAME="AEN16663"><H2
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>Synopsis</H2
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><TABLE
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BGCOLOR="#E0E0F0"
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WIDTH="70%"
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><TR
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><TD
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><PRE
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CLASS="SYNOPSIS"
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>#include &lt;cyg/io/usb/usbs.h&gt;
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typedef struct usbs_rx_endpoint {
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    void                 (*start_rx_fn)(struct usbs_rx_endpoint*);
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    void                 (*set_halted_fn)(struct usbs_rx_endpoint*, cyg_bool);
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    void                 (*complete_fn)(void*, int);
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    void*                complete_data;
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    unsigned char*       buffer;
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    int                  buffer_size;
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    cyg_bool             halted;
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} usbs_rx_endpoint;
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typedef struct usbs_tx_endpoint {
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    void                 (*start_tx_fn)(struct usbs_tx_endpoint*);
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    void                 (*set_halted_fn)(struct usbs_tx_endpoint*, cyg_bool);
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    void                 (*complete_fn)(void*, int);
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    void*                complete_data;
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    const unsigned char* buffer;
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    int                  buffer_size;
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    cyg_bool             halted;
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} usbs_tx_endpoint;</PRE
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></TD
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></TABLE
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></DIV
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><DIV
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CLASS="REFSECT1"
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><A
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NAME="AEN16665"
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></A
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><H2
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>Receive and Transmit Data Structures</H2
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><P
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>In addition to a single <SPAN
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CLASS="STRUCTNAME"
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>usbs_control_endpoint</SPAN
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>
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data structure per USB slave device, the USB device driver should also
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provide receive and transmit data structures corresponding to the
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other endpoints. The names of these are determined by the device
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driver. For example, the SA1110 USB device driver package provides
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<TT
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CLASS="LITERAL"
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>usbs_sa11x0_ep1</TT
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> for receives and
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<TT
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CLASS="LITERAL"
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>usbs_sa11x0_ep2</TT
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> for transmits.</P
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><P
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>Unlike control endpoints, the common USB slave package does provide a
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number of utility routines to manipulate data endpoints. For example
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<A
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HREF="usbs-start-rx.html"
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><TT
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CLASS="FUNCTION"
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>usbs_start_rx_buffer</TT
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></A
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>
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can be used to receive data from the host into a buffer. In addition
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the USB device driver can provide devtab entries such as
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<TT
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CLASS="LITERAL"
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>/dev/usbs1r</TT
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> and <TT
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CLASS="LITERAL"
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>/dev/usbs2w</TT
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>, so
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higher-level code can <TT
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CLASS="FUNCTION"
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>open</TT
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> these devices and then
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perform blocking <TT
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CLASS="FUNCTION"
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>read</TT
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> and
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<TT
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CLASS="FUNCTION"
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>write</TT
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> operations.</P
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><P
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>However, the operation of data endpoints and the various
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endpoint-related functions is relatively straightforward. First
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consider a <SPAN
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CLASS="STRUCTNAME"
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>usbs_rx_endpoint</SPAN
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> structure. The
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device driver will provide the members
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<TT
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CLASS="STRUCTFIELD"
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><I
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>start_rx_fn</I
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></TT
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> and
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<TT
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CLASS="STRUCTFIELD"
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><I
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>set_halted_fn</I
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></TT
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>, and it will maintain the
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<TT
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CLASS="STRUCTFIELD"
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><I
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>halted</I
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></TT
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> field. To receive data, higher-level
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code sets the <TT
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CLASS="STRUCTFIELD"
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><I
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>buffer</I
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></TT
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>,
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<TT
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CLASS="STRUCTFIELD"
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><I
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>buffer_size</I
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></TT
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>,
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<TT
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CLASS="STRUCTFIELD"
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><I
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>complete_fn</I
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></TT
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> and optionally the
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<TT
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CLASS="STRUCTFIELD"
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><I
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>complete_data</I
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></TT
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> fields. Next the
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<TT
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CLASS="STRUCTFIELD"
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><I
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>start_rx_fn</I
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></TT
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> member should be called. When
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the transfer has finished the device driver will invoke the completion
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function, using <TT
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CLASS="STRUCTFIELD"
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><I
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>complete_data</I
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></TT
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> as the first
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argument and a size field for the second argument. A negative size
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indicates an error of some sort: <TT
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CLASS="LITERAL"
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>-EGAIN</TT
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> indicates
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that the endpoint has been halted, usually at the request of the host;
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<TT
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CLASS="LITERAL"
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>-EPIPE</TT
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> indicates that the connection between the
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host and the peripheral has been broken. Certain device drivers may
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generate other error codes.</P
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><P
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>If higher-level code needs to halt or unhalt an endpoint then it can
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invoke the <TT
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CLASS="STRUCTFIELD"
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><I
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>set_halted_fn</I
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></TT
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> member. When an
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endpoint is halted, invoking <TT
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CLASS="STRUCTFIELD"
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><I
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>start_rx_fn</I
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></TT
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>
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wit <TT
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CLASS="STRUCTFIELD"
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><I
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>buffer_size</I
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></TT
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> set to 0 indicates that
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higher-level code wants to block until the endpoint is no longer
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halted; at that point the completion function will be invoked.</P
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><P
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>USB device drivers are allowed to assume that higher-level protocols
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ensure that host and peripheral agree on the amount of data that will
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be transferred, or at least on an upper bound. Therefore there is no
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need for the device driver to maintain its own buffers, and copy
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operations are avoided. If the host sends more data than expected then
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the resulting behaviour is undefined.</P
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><P
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>Transmit endpoints work in essentially the same way as receive
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endpoints. Higher-level code should set the
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<TT
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CLASS="STRUCTFIELD"
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><I
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>buffer</I
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></TT
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> and
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<TT
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CLASS="STRUCTFIELD"
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><I
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>buffer_size</I
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></TT
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> fields to point at the data to
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be transferred, then call <TT
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CLASS="STRUCTFIELD"
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><I
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>start_tx_fn</I
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></TT
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>, and
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the device driver will invoked the completion function when the
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transfer has completed.</P
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><P
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>USB device drivers are not expected to perform any locking. If at any
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time there are two concurrent receive operations for a given endpoint,
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or two concurrent transmit operations, then the resulting behaviour is
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undefined. It is the responsibility of higher-level code to perform
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any synchronisation that may be necessary. In practice, conflicts are
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unlikely because typically a given endpoint will only be accessed
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sequentially by just one part of the overall system.</P
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