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<!-- Copyright (C) 2003 Red Hat, Inc.                                -->
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>Halted Endpoints</TITLE
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><H1
><A
NAME="USBS-HALT">Halted Endpoints</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN16457"
></A
><H2
>Name</H2
>Halted Endpoints&nbsp;--&nbsp;Support for Halting and Halted Endpoints</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN16460"><H2
>Synopsis</H2
><DIV
CLASS="FUNCSYNOPSIS"
><A
NAME="AEN16461"><P
></P
><TABLE
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><TR
><TD
><PRE
CLASS="FUNCSYNOPSISINFO"
>#include &lt;cyg/io/usb/usbs.h&gt;</PRE
></TD
></TR
></TABLE
><P
><CODE
><CODE
CLASS="FUNCDEF"
>cyg_bool usbs_rx_endpoint_halted</CODE
>(usbs_rx_endpoint* ep);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void usbs_set_rx_endpoint_halted</CODE
>(usbs_rx_endpoint* ep, cyg_bool new_state);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void usbs_start_rx_endpoint_wait</CODE
>(usbs_rx_endpoint* ep, void (*)(void*, int) complete_fn, void * complete_data);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>cyg_bool
usbs_tx_endpoint_halted</CODE
>(usbs_tx_endpoint* ep);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void usbs_set_tx_endpoint_halted</CODE
>(usbs_tx_endpoint* ep, cyg_bool new_state);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void usbs_start_tx_endpoint_wait</CODE
>(usbs_tx_endpoint* ep, void (*)(void*, int) complete_fn, void * complete_data);</CODE
></P
><P
></P
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN16505"
></A
><H2
><TT
CLASS="FUNCTION"
>Description</TT
></H2
><P
>Normal USB traffic involves straightforward handshakes, with either an
<TT
CLASS="LITERAL"
>ACK</TT
> to indicate that a packet was transferred
without errors, or a <TT
CLASS="LITERAL"
>NAK</TT
> if an error occurred, or
if a peripheral is currently unable to process another packet from the
host, or has no packet to send to the host. There is a third form of
handshake, a <TT
CLASS="LITERAL"
>STALL</TT
>, which indicates that the
endpoint is currently <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>halted</I
></SPAN
>.</P
><P
>When an endpoint is halted it means that the host-side code needs to
take some sort of recovery action before communication over that
endpoint can resume. The exact circumstances under which this can
happen are not defined by the USB specification, but one example would
be a protocol violation if say the peripheral attempted to transmit
more data to the host than was permitted by the protocol in use. The
host can use the standard control messages get-status, set-feature and
clear-feature to examine and manipulate the halted status of a given
endpoint. There are USB-specific functions which can be used inside
the peripheral to achieve the same effect. Once an endpoint has been
halted the host can then interact with the peripheral using class or
vendor control messages to perform appropriate recovery, and then the
halted condition can be cleared.</P
><P
>Halting an endpoint does not constitute a device state change, and
there is no mechanism by which higher-level code can be informed
immediately. However, any ongoing receive or transmit operations will
be aborted with an <TT
CLASS="LITERAL"
>-EAGAIN</TT
> error, and any new
receives or transmits will fail immediately with the same error.</P
><P
>There are six functions to support halted endpoints, one set for
receive endpoints and another for transmit endpoints, with both sets
behaving in essentially the same way. The first,
<TT
CLASS="FUNCTION"
>usbs_rx_endpoint_halted</TT
>, can be used to determine
whether or not an endpoint is currently halted: it takes a single
argument that identifies the endpoint of interest. The second
function, <TT
CLASS="FUNCTION"
>usbs_set_rx_endpoint_halted</TT
>, can be
used to change the halted condition of an endpoint: it takes two
arguments; one to identify the endpoint and another to specify the new
state. The last function
<TT
CLASS="FUNCTION"
>usbs_start_rx_endpoint_wait</TT
> operates in much the
same way as <TT
CLASS="FUNCTION"
>usbs_start_rx_buffer</TT
>: when the
endpoint is no longer halted the device driver will invoke the
supplied completion function with a status of 0. The completion
function has the same signature as that for a transfer operation.
Often it will be possible to use a single completion function and have
the foreground code invoke either
<TT
CLASS="FUNCTION"
>usbs_start_rx_buffer</TT
> or
<TT
CLASS="FUNCTION"
>usbs_start_rx_endpoint_wait</TT
> depending on the
current state of the endpoint.</P
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