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>Sending Data to the Host</TITLE
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><H1
><A
NAME="USBS-START-TX">Sending Data to the Host</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN16386"
></A
><H2
>Name</H2
><TT
CLASS="FUNCTION"
>usbs_start_tx_buffer</TT
> -- Sending Data to the Host</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN16390"><H2
>Synopsis</H2
><DIV
CLASS="FUNCSYNOPSIS"
><A
NAME="AEN16391"><P
></P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
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><TR
><TD
><PRE
CLASS="FUNCSYNOPSISINFO"
>#include <cyg/io/usb/usbs.h></PRE
></TD
></TR
></TABLE
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void usbs_start_tx_buffer</CODE
>(usbs_tx_endpoint* ep, const unsigned char* buffer, int length, void (*)(void*,int) complete_fn, void * complete_data);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void usbs_start_tx</CODE
>(usbs_tx_endpoint* ep);</CODE
></P
><P
></P
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN16411"
></A
><H2
><TT
CLASS="FUNCTION"
>Description</TT
></H2
><P
><TT
CLASS="FUNCTION"
>usbs_start_tx_buffer</TT
> is a USB-specific function
to transfer data from peripheral to host. It can be used for bulk,
interrupt or isochronous transfers, but not for control messages;
instead those involve manipulating the <A
HREF="usbs-control.html"
><SPAN
CLASS="STRUCTNAME"
>usbs_control_endpoint</SPAN
></A
>
data structure directly. The function takes five arguments:</P
><P
></P
><OL
TYPE="1"
><LI
><P
>The first argument identifies the specific endpoint that should be
used. Different USB devices will support different sets of endpoints
and the device driver will provide appropriate data structures. The
device driver's documentation should be consulted for details of which
endpoints are available.</P
></LI
><LI
><P
>The <TT
CLASS="PARAMETER"
><I
>buffer</I
></TT
> and <TT
CLASS="PARAMETER"
><I
>length</I
></TT
>
arguments control the actual transfer. USB device drivers are not
allowed to modify the buffer during the transfer, so the data can
reside in read-only memory. The transfer will be for all the data
specified, and it is the responsibility of higher-level code to make
sure that the host is expecting this amount of data. For isochronous
transfers the USB specification imposes an upper bound of 1023 bytes,
but a smaller limit may be set in the <A
HREF="usbs-enum.html#AEN16179"
>enumeration data</A
>. Interrupt
transfers have an upper bound of 64 bytes or less, as per the
enumeration data. Bulk transfers are more complicated because they can
involve multiple 64-byte packets plus a terminating packet of less
than 64 bytes, so the basic USB specification does not impose an upper
limit on the total transfer size. Instead it is left to higher-level
protocols to specify an appropriate upper bound. If the peripheral
attempts to send more data than the host is willing to accept then the
resulting behaviour is undefined and may well depend on the specific
host operating system being used.</P
><P
>For bulk transfers, the USB device driver or the underlying hardware
will automatically split the transfer up into the appropriate number
of full-size 64-byte packets plus a single terminating packet, which
may be 0 bytes.</P
></LI
><LI
><P
><TT
CLASS="FUNCTION"
>usbs_start_tx_buffer</TT
> is non-blocking. It merely
starts the transmit operation, and does not wait for completion. At
some later point the USB device driver will invoke the completion
function parameter with two arguments: the completion data defined by
the last parameter, and a result field. This result will be either an
error code < <TT
CLASS="LITERAL"
>0</TT
>, or the amount of data
transferred which should correspond to the
<TT
CLASS="PARAMETER"
><I
>length</I
></TT
> argument. The most likely errors are
<TT
CLASS="LITERAL"
>-EPIPE</TT
> to indicate that the connection between the
host and the target has been broken, and <TT
CLASS="LITERAL"
>-EAGAIN</TT
>
for when the endpoint has been <A
HREF="usbs-halt.html"
>halted</A
>. Specific USB device drivers may
define additional error conditions.</P
></LI
></OL
><P
>The normal sequence of events is that the USB device driver will
update the appropriate hardware registers. At some point after that
the host will attempt to fetch data by transmitting an IN token. Since
a transmit operation is now in progress the peripheral can send a
packet of data, and the host will generate an ACK. At this point the
USB hardware will generate an interrupt, and the device driver will
service this interrupt and arrange for a DSR to be called. Isochronous
and interrupt transfers involve just a single packet. However, bulk
transfers may involve multiple packets so the device driver has to
check whether there is more data to send and set things up for the
next packet. When the device driver DSR detects a complete transfer it
will inform higher-level code by invoking the supplied completion
function.</P
><P
>This means that the completion function will normally be invoked by a
DSR and not in thread context - although some USB device drivers may
have a different implementation. Therefore the completion function is
restricted in what it can do, in particular it must not make any
calls that will or may block such as locking a mutex or allocating
memory. The kernel documentation should be consulted for more details
of DSR's and interrupt handling generally.</P
><P
>It is possible that the completion function will be invoked before
<TT
CLASS="FUNCTION"
>usbs_start_tx_buffer</TT
> returns. Such an event would
be unusual because the transfer cannot happen until the next time the
host tries to fetch data from this peripheral, but it may happen if,
for example, another interrupt happens and a higher priority thread is
scheduled to run. Also, if the endpoint is currently halted then the
completion function will be invoked immediately with
<TT
CLASS="LITERAL"
>-EAGAIN</TT
>: typically this will happen in the current
thread rather than in a separate DSR. The completion function is
allowed to start another transfer immediately by calling
<TT
CLASS="FUNCTION"
>usbs_start_tx_buffer</TT
> again.</P
><P
>USB device drivers are not expected to perform any locking. It is the
responsibility of higher-level code to ensure that there is only one
transmit operation for a given endpoint in progress at any one time.
If there are concurrent calls to
<TT
CLASS="FUNCTION"
>usbs_start_tx_buffer</TT
> then the resulting behaviour
is undefined. For typical USB applications this does not present any
problems because only piece of code will access a given endpoint at
any particular time.</P
><P
>The following code fragment illustrates a very simple use of
<TT
CLASS="FUNCTION"
>usbs_start_tx_buffer</TT
> to implement a blocking
transmit, using a semaphore to synchronise between the foreground
thread and the DSR. For a simple example like this no completion data
is needed.</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>static int error_code = 0;
static cyg_sem_t completion_wait;
static void
completion_fn(void* data, int result)
{
error_code = result;
cyg_semaphore_post(&completion_wait);
}
int
blocking_transmit(usbs_tx_endpoint* ep, const unsigned char* buf, int len)
{
error_code = 0;
usbs_start_tx_buffer(ep, buf, len, &completion_fn, NULL);
cyg_semaphore_wait(&completion_wait);
return error_code;
}</PRE
></TD
></TR
></TABLE
><P
>There is also a utility function <TT
CLASS="FUNCTION"
>usbs_start</TT
>. This
can be used by code that wants to manipulate <A
HREF="usbs-data.html"
>data endpoints</A
> directly, specifically the
<TT
CLASS="STRUCTFIELD"
><I
>complete_fn</I
></TT
>,
<TT
CLASS="STRUCTFIELD"
><I
>complete_data</I
></TT
>,
<TT
CLASS="STRUCTFIELD"
><I
>buffer</I
></TT
> and
<TT
CLASS="STRUCTFIELD"
><I
>buffer_size</I
></TT
> fields.
<TT
CLASS="FUNCTION"
>usbs_start_tx</TT
> just calls a function supplied by
the device driver.</P
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