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<HTML
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><HEAD
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><TITLE
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>Receiving Data from the Host</TITLE
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"><LINK
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REL="HOME"
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TITLE="eCos Reference Manual"
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HREF="ecos-ref.html"><LINK
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REL="UP"
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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="Devtab Entries"
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HREF="usbs-devtab.html"><LINK
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REL="NEXT"
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TITLE="Sending Data to the Host"
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HREF="usbs-start-tx.html"></HEAD
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><BODY
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>eCos Reference Manual</TH
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><TR
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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-devtab.html"
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ACCESSKEY="P"
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>Prev</A
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VALIGN="bottom"
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HREF="usbs-start-tx.html"
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ACCESSKEY="N"
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>Next</A
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></TABLE
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WIDTH="100%"></DIV
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><H1
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><A
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NAME="USBS-START-RX">Receiving Data from the Host</H1
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><DIV
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CLASS="REFNAMEDIV"
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><A
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NAME="AEN16314"
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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_rx_buffer</TT
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> -- Receiving Data from the Host</DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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><A
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NAME="AEN16318"><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="AEN16319"><P
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></P
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><TABLE
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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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></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_rx_buffer</CODE
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>(usbs_rx_endpoint* ep, 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_rx</CODE
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>(usbs_rx_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="AEN16339"
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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_rx_buffer</TT
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> is a USB-specific function
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to accept a transfer from host to peripheral. 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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expected to perform any buffering or to support partial transfers, so
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the length specified should correspond to the maximum transfer that is
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currently possible and the buffer should be at least this large. For
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isochronous transfers the USB specification imposes an upper bound of
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1023 bytes, and a smaller limit may be set in the <A
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HREF="usbs-enum.html#AEN16179"
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>enumeration data</A
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>. Interrupt
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transfers are similarly straightforward with an upper bound of 64
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bytes, or less as per the enumeration data. Bulk transfers are more
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complicated because they can involve multiple 64-byte packets plus a
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terminating packet of less than 64 bytes, so there is no predefined
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limit on the transfer size. Instead it is left to higher-level
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protocols to specify an appropriate upper bound.</P
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><P
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>One technique that may work for bulk transfers is to exploit the fact
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that such transfers happen in 64-byte packets: it may be possible to
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receive an initial 64 bytes, corresponding to the first packet in the
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transfer; these 64 bytes can then be examined to determine the total
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transfer size, and the remaining data can be transferred in another
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receive operation. This technique is not guaranteed to work with all
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USB hardware. Also, if the delay between accepting the first packet and
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the remainder of the transfer is excessive then this could cause
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timeout problems for the host-side software. For these reasons this
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technique should be avoided.</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_rx_buffer</TT
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> is non-blocking. It merely
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starts the receive 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. A result >=
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<TT
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CLASS="LITERAL"
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>0</TT
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> indicates a successful transfer of that many
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bytes, which may be less than the upper bound imposed by 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. A result <
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<TT
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CLASS="LITERAL"
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>0</TT
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> indicates an error. 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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specify 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 send data by transmitting an OUT token
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followed by a data packet, and since a receive operation is now in
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progress the data will be accepted and ACK'd. If there were no receive
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operation then the peripheral would instead generate a NAK. The USB
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hardware will generate an interrupt once the whole packet has been
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received, and the USB device driver will service this interrupt and
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arrange for a DSR to be called. Isochronous and interrupt transfers
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involve just a single packet. However, bulk transfers may involve
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multiple packets so the device driver has to check whether the packet
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was a full 64 bytes or whether it was a terminating packet of less
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than this. 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_rx_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 send data to this peripheral, but it may happen if for
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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_rx_buffer</TT
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> again.</P
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><P
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| 297 |
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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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receive operation for a given endpoint in progress at any one time. If
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| 300 |
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there are concurrent calls to
|
| 301 |
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<TT
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CLASS="FUNCTION"
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>usbs_start_rx_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 one piece of code will access a given endpoint
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at 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_rx_buffer</TT
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> to implement a blocking
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receive, 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="5"
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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="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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| 334 |
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int
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blocking_receive(usbs_rx_endpoint* ep, unsigned char* buf, int len)
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| 337 |
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{
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| 338 |
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error_code = 0;
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| 339 |
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usbs_start_rx_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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| 348 |
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CLASS="FUNCTION"
|
| 349 |
|
|
>usbs_start_rx</TT
|
| 350 |
|
|
>. This
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| 351 |
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|
can be used by code that wants to manipulate <A
|
| 352 |
|
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HREF="usbs-data.html"
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| 353 |
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>data endpoints</A
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| 354 |
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> directly, specifically the
|
| 355 |
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<TT
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CLASS="STRUCTFIELD"
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| 357 |
|
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><I
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| 358 |
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>complete_fn</I
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| 359 |
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></TT
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| 360 |
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>,
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<TT
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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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><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.
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<TT
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CLASS="FUNCTION"
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>usbs_start_tx</TT
|
| 382 |
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> just invokes a function
|
| 383 |
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supplied by the device driver.</P
|
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></DIV
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><DIV
|
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CLASS="NAVFOOTER"
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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WIDTH="100%"
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CELLSPACING="0"
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
|
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><A
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HREF="usbs-devtab.html"
|
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ACCESSKEY="P"
|
| 403 |
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>Prev</A
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></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="ecos-ref.html"
|
| 411 |
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ACCESSKEY="H"
|
| 412 |
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>Home</A
|
| 413 |
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></TD
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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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><A
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HREF="usbs-start-tx.html"
|
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ACCESSKEY="N"
|
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>Next</A
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></TD
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></TR
|
| 424 |
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><TR
|
| 425 |
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><TD
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WIDTH="33%"
|
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ALIGN="left"
|
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VALIGN="top"
|
| 429 |
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>Devtab Entries</TD
|
| 430 |
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><TD
|
| 431 |
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WIDTH="34%"
|
| 432 |
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ALIGN="center"
|
| 433 |
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VALIGN="top"
|
| 434 |
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><A
|
| 435 |
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|
HREF="io-usb-slave.html"
|
| 436 |
|
|
ACCESSKEY="U"
|
| 437 |
|
|
>Up</A
|
| 438 |
|
|
></TD
|
| 439 |
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><TD
|
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WIDTH="33%"
|
| 441 |
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ALIGN="right"
|
| 442 |
|
|
VALIGN="top"
|
| 443 |
|
|
>Sending Data to the Host</TD
|
| 444 |
|
|
></TR
|
| 445 |
|
|
></TABLE
|
| 446 |
|
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></DIV
|
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></BODY
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></HTML
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>
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