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<HTML
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><HEAD
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><TITLE
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>Sending Data to the Host</TITLE
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><meta name="MSSmartTagsPreventParsing" content="TRUE">
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.64
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"><LINK
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REL="HOME"
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TITLE="eCos USB Slave Support"
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HREF="io-usb-slave.html"><LINK
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REL="PREVIOUS"
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TITLE="Receiving Data from the Host"
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HREF="usbs-start-rx.html"><LINK
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REL="NEXT"
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TITLE="Halted Endpoints"
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HREF="usbs-halt.html"></HEAD
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><BODY
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CLASS="REFENTRY"
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BGCOLOR="#FFFFFF"
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><TH
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COLSPAN="3"
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ALIGN="center"
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>eCos USB Slave Support</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="usbs-start-rx.html"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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></TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="usbs-halt.html"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><H1
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><A
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NAME="USBS-START-TX"
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>Sending Data to the Host</A
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></H1
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><DIV
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CLASS="REFNAMEDIV"
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><A
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NAME="AEN349"
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></A
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><H2
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>Name</H2
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><TT
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CLASS="FUNCTION"
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>usbs_start_tx_buffer</TT
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> -- Sending Data to the Host</DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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><A
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NAME="AEN353"
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></A
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><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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NAME="AEN354"
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></A
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><P
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></P
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><TABLE
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BORDER="0"
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="FUNCSYNOPSISINFO"
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>#include <cyg/io/usb/usbs.h></PRE
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></TD
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></TR
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></TABLE
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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_start_tx_buffer</CODE
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>(usbs_tx_endpoint* ep, const unsigned char* buffer, int length, void (*)(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_start_tx</CODE
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>(usbs_tx_endpoint* ep);</CODE
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></P
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><P
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></P
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></DIV
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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="AEN374"
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></A
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><H2
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><TT
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CLASS="FUNCTION"
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>Description</TT
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></H2
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><P
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><TT
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CLASS="FUNCTION"
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>usbs_start_tx_buffer</TT
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> is a USB-specific function
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to transfer data from peripheral to host. It can be used for bulk,
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interrupt or isochronous transfers, but not for control messages;
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instead those involve manipulating the <A
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HREF="usbs-control.html"
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><SPAN
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CLASS="STRUCTNAME"
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>usbs_control_endpoint</SPAN
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></A
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>
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data structure directly. The function takes five arguments:</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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>The first argument identifies the specific endpoint that should be
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used. Different USB devices will support different sets of endpoints
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and the device driver will provide appropriate data structures. The
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device driver's documentation should be consulted for details of which
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endpoints are available.</P
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></LI
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><LI
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><P
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>The <TT
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CLASS="PARAMETER"
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><I
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>buffer</I
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></TT
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> and <TT
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CLASS="PARAMETER"
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><I
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>length</I
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></TT
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>
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arguments control the actual transfer. USB device drivers are not
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allowed to modify the buffer during the transfer, so the data can
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reside in read-only memory. The transfer will be for all the data
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specified, and it is the responsibility of higher-level code to make
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sure that the host is expecting this amount of data. For isochronous
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transfers the USB specification imposes an upper bound of 1023 bytes,
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but a smaller limit may be set in the <A
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HREF="usbs-enum.html#AEN142"
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>enumeration data</A
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>. Interrupt
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transfers have an upper bound of 64 bytes or less, as per the
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enumeration data. Bulk transfers are more complicated because they can
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involve multiple 64-byte packets plus a terminating packet of less
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than 64 bytes, so the basic USB specification does not impose an upper
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limit on the total transfer size. Instead it is left to higher-level
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protocols to specify an appropriate upper bound. If the peripheral
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attempts to send more data than the host is willing to accept then the
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resulting behaviour is undefined and may well depend on the specific
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host operating system being used.</P
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><P
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>For bulk transfers, the USB device driver or the underlying hardware
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will automatically split the transfer up into the appropriate number
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of full-size 64-byte packets plus a single terminating packet, which
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may be 0 bytes.</P
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></LI
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><LI
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><P
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><TT
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CLASS="FUNCTION"
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>usbs_start_tx_buffer</TT
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> is non-blocking. It merely
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starts the transmit operation, and does not wait for completion. At
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some later point the USB device driver will invoke the completion
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function parameter with two arguments: the completion data defined by
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the last parameter, and a result field. This result will be either an
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error code < <TT
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CLASS="LITERAL"
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>0</TT
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>, or the amount of data
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transferred which should correspond to the
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<TT
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CLASS="PARAMETER"
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><I
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>length</I
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></TT
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> argument. The most likely errors are
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<TT
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CLASS="LITERAL"
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>-EPIPE</TT
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> to indicate that the connection between the
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host and the target has been broken, and <TT
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CLASS="LITERAL"
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>-EAGAIN</TT
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>
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for when the endpoint has been <A
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HREF="usbs-halt.html"
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>halted</A
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>. Specific USB device drivers may
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define additional error conditions.</P
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></LI
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></OL
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><P
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>The normal sequence of events is that the USB device driver will
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update the appropriate hardware registers. At some point after that
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the host will attempt to fetch data by transmitting an IN token. Since
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a transmit operation is now in progress the peripheral can send a
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packet of data, and the host will generate an ACK. At this point the
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USB hardware will generate an interrupt, and the device driver will
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service this interrupt and arrange for a DSR to be called. Isochronous
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and interrupt transfers involve just a single packet. However, bulk
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transfers may involve multiple packets so the device driver has to
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check whether there is more data to send and set things up for the
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next packet. When the device driver DSR detects a complete transfer it
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will inform higher-level code by invoking the supplied completion
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function.</P
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><P
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>This means that the completion function will normally be invoked by a
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DSR and not in thread context - although some USB device drivers may
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have a different implementation. Therefore the completion function is
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restricted in what it can do, in particular it must not make any
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calls that will or may block such as locking a mutex or allocating
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memory. The kernel documentation should be consulted for more details
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of DSR's and interrupt handling generally.</P
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><P
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>It is possible that the completion function will be invoked before
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<TT
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CLASS="FUNCTION"
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>usbs_start_tx_buffer</TT
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> returns. Such an event would
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be unusual because the transfer cannot happen until the next time the
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host tries to fetch data from this peripheral, but it may happen if,
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for example, another interrupt happens and a higher priority thread is
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scheduled to run. Also, if the endpoint is currently halted then the
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completion function will be invoked immediately with
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<TT
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CLASS="LITERAL"
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>-EAGAIN</TT
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>: typically this will happen in the current
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thread rather than in a separate DSR. The completion function is
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allowed to start another transfer immediately by calling
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<TT
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CLASS="FUNCTION"
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>usbs_start_tx_buffer</TT
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> again.</P
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><P
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>USB device drivers are not expected to perform any locking. It is the
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responsibility of higher-level code to ensure that there is only one
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transmit operation for a given endpoint in progress at any one time.
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If there are concurrent calls to
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<TT
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CLASS="FUNCTION"
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>usbs_start_tx_buffer</TT
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> then the resulting behaviour
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is undefined. For typical USB applications this does not present any
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problems because only piece of code will access a given endpoint at
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any particular time.</P
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><P
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>The following code fragment illustrates a very simple use of
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<TT
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CLASS="FUNCTION"
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>usbs_start_tx_buffer</TT
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> to implement a blocking
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transmit, using a semaphore to synchronise between the foreground
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thread and the DSR. For a simple example like this no completion data
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is needed.</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>static int error_code = 0;
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static cyg_sem_t completion_wait;
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static void
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completion_fn(void* data, int result)
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{
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error_code = result;
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cyg_semaphore_post(&completion_wait);
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}
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int
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blocking_transmit(usbs_tx_endpoint* ep, const unsigned char* buf, int len)
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{
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error_code = 0;
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usbs_start_tx_buffer(ep, buf, len, &completion_fn, NULL);
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cyg_semaphore_wait(&completion_wait);
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return error_code;
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}</PRE
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></TD
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></TR
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></TABLE
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><P
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>There is also a utility function <TT
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| 343 |
|
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CLASS="FUNCTION"
|
| 344 |
|
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>usbs_start</TT
|
| 345 |
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>. This
|
| 346 |
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can be used by code that wants to manipulate <A
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| 347 |
|
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HREF="usbs-data.html"
|
| 348 |
|
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>data endpoints</A
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| 349 |
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> directly, specifically the
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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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>,
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<TT
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CLASS="STRUCTFIELD"
|
| 358 |
|
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><I
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>complete_data</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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| 364 |
|
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><I
|
| 365 |
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>buffer</I
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| 366 |
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></TT
|
| 367 |
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> and
|
| 368 |
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<TT
|
| 369 |
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CLASS="STRUCTFIELD"
|
| 370 |
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><I
|
| 371 |
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>buffer_size</I
|
| 372 |
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></TT
|
| 373 |
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> fields.
|
| 374 |
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<TT
|
| 375 |
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CLASS="FUNCTION"
|
| 376 |
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|
>usbs_start_tx</TT
|
| 377 |
|
|
> just calls a function supplied by
|
| 378 |
|
|
the device driver.</P
|
| 379 |
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|
></DIV
|
| 380 |
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><DIV
|
| 381 |
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CLASS="NAVFOOTER"
|
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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WIDTH="100%"
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BORDER="0"
|
| 387 |
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CELLPADDING="0"
|
| 388 |
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CELLSPACING="0"
|
| 389 |
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><TR
|
| 390 |
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><TD
|
| 391 |
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WIDTH="33%"
|
| 392 |
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ALIGN="left"
|
| 393 |
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VALIGN="top"
|
| 394 |
|
|
><A
|
| 395 |
|
|
HREF="usbs-start-rx.html"
|
| 396 |
|
|
>Prev</A
|
| 397 |
|
|
></TD
|
| 398 |
|
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><TD
|
| 399 |
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WIDTH="34%"
|
| 400 |
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ALIGN="center"
|
| 401 |
|
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VALIGN="top"
|
| 402 |
|
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><A
|
| 403 |
|
|
HREF="io-usb-slave.html"
|
| 404 |
|
|
>Home</A
|
| 405 |
|
|
></TD
|
| 406 |
|
|
><TD
|
| 407 |
|
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WIDTH="33%"
|
| 408 |
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ALIGN="right"
|
| 409 |
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|
VALIGN="top"
|
| 410 |
|
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><A
|
| 411 |
|
|
HREF="usbs-halt.html"
|
| 412 |
|
|
>Next</A
|
| 413 |
|
|
></TD
|
| 414 |
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|
></TR
|
| 415 |
|
|
><TR
|
| 416 |
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><TD
|
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WIDTH="33%"
|
| 418 |
|
|
ALIGN="left"
|
| 419 |
|
|
VALIGN="top"
|
| 420 |
|
|
>Receiving Data from the Host</TD
|
| 421 |
|
|
><TD
|
| 422 |
|
|
WIDTH="34%"
|
| 423 |
|
|
ALIGN="center"
|
| 424 |
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VALIGN="top"
|
| 425 |
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> </TD
|
| 426 |
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><TD
|
| 427 |
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WIDTH="33%"
|
| 428 |
|
|
ALIGN="right"
|
| 429 |
|
|
VALIGN="top"
|
| 430 |
|
|
>Halted Endpoints</TD
|
| 431 |
|
|
></TR
|
| 432 |
|
|
></TABLE
|
| 433 |
|
|
></DIV
|
| 434 |
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></BODY
|
| 435 |
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></HTML
|
| 436 |
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>
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