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>Receiving Data from the Host</TITLE
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><H1
><A
NAME="USBS-START-RX">Receiving Data from the Host</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN16314"
></A
><H2
>Name</H2
><TT
CLASS="FUNCTION"
>usbs_start_rx_buffer</TT
> -- Receiving Data from the Host</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN16318"><H2
>Synopsis</H2
><DIV
CLASS="FUNCSYNOPSIS"
><A
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></P
><TABLE
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><TD
><PRE
CLASS="FUNCSYNOPSISINFO"
>#include <cyg/io/usb/usbs.h></PRE
></TD
></TR
></TABLE
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void usbs_start_rx_buffer</CODE
>(usbs_rx_endpoint* ep, unsigned char* buffer, int length, void (*)(void*,int) complete_fn, void * complete_data);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void usbs_start_rx</CODE
>(usbs_rx_endpoint* ep);</CODE
></P
><P
></P
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN16339"
></A
><H2
><TT
CLASS="FUNCTION"
>Description</TT
></H2
><P
><TT
CLASS="FUNCTION"
>usbs_start_rx_buffer</TT
> is a USB-specific function
to accept a transfer from host to peripheral. 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
expected to perform any buffering or to support partial transfers, so
the length specified should correspond to the maximum transfer that is
currently possible and the buffer should be at least this large. For
isochronous transfers the USB specification imposes an upper bound of
1023 bytes, and a smaller limit may be set in the <A
HREF="usbs-enum.html#AEN16179"
>enumeration data</A
>. Interrupt
transfers are similarly straightforward with 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 there is no predefined
limit on the transfer size. Instead it is left to higher-level
protocols to specify an appropriate upper bound.</P
><P
>One technique that may work for bulk transfers is to exploit the fact
that such transfers happen in 64-byte packets: it may be possible to
receive an initial 64 bytes, corresponding to the first packet in the
transfer; these 64 bytes can then be examined to determine the total
transfer size, and the remaining data can be transferred in another
receive operation. This technique is not guaranteed to work with all
USB hardware. Also, if the delay between accepting the first packet and
the remainder of the transfer is excessive then this could cause
timeout problems for the host-side software. For these reasons this
technique should be avoided.</P
></LI
><LI
><P
><TT
CLASS="FUNCTION"
>usbs_start_rx_buffer</TT
> is non-blocking. It merely
starts the receive 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. A result >=
<TT
CLASS="LITERAL"
>0</TT
> indicates a successful transfer of that many
bytes, which may be less than the upper bound imposed by the
<TT
CLASS="PARAMETER"
><I
>length</I
></TT
> argument. A result <
<TT
CLASS="LITERAL"
>0</TT
> indicates an error. 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
specify 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 send data by transmitting an OUT token
followed by a data packet, and since a receive operation is now in
progress the data will be accepted and ACK'd. If there were no receive
operation then the peripheral would instead generate a NAK. The USB
hardware will generate an interrupt once the whole packet has been
received, and the USB 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 the packet
was a full 64 bytes or whether it was a terminating packet of less
than this. 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_rx_buffer</TT
> returns. Such an event would
be unusual because the transfer cannot happen until the next time the
host tries to send data to 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_rx_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
receive operation for a given endpoint in progress at any one time. If
there are concurrent calls to
<TT
CLASS="FUNCTION"
>usbs_start_rx_buffer</TT
> then the resulting behaviour
is undefined. For typical USB applications this does not present any
problems, because only one 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_rx_buffer</TT
> to implement a blocking
receive, 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_receive(usbs_rx_endpoint* ep, unsigned char* buf, int len)
{
error_code = 0;
usbs_start_rx_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_rx</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 invokes a function
supplied by the device driver.</P
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