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><A
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><A
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NAME="USBS-START-RX">Receiving Data from the Host</H1
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NAME="USBS-START-RX">Receiving Data from the Host</H1
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><DIV
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><DIV
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CLASS="REFNAMEDIV"
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CLASS="REFNAMEDIV"
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><A
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><A
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NAME="AEN16314"
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NAME="AEN16314"
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></A
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></A
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><H2
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><H2
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>Name</H2
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>Name</H2
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><TT
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><TT
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CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>usbs_start_rx_buffer</TT
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>usbs_start_rx_buffer</TT
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> -- Receiving Data from the Host</DIV
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> -- Receiving Data from the Host</DIV
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><DIV
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><DIV
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CLASS="REFSYNOPSISDIV"
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CLASS="REFSYNOPSISDIV"
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><A
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><A
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NAME="AEN16318"><H2
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NAME="AEN16318"><H2
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>Synopsis</H2
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>Synopsis</H2
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><DIV
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><DIV
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CLASS="FUNCSYNOPSIS"
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CLASS="FUNCSYNOPSIS"
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><A
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><A
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NAME="AEN16319"><P
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NAME="AEN16319"><P
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></P
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></P
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><TABLE
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WIDTH="70%"
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><TR
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><TR
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><TD
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><TD
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><PRE
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><PRE
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CLASS="FUNCSYNOPSISINFO"
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CLASS="FUNCSYNOPSISINFO"
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>#include <cyg/io/usb/usbs.h></PRE
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>#include <cyg/io/usb/usbs.h></PRE
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></TD
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></TD
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></TR
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></TR
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></TABLE
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></TABLE
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><P
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><P
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><CODE
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><CODE
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><CODE
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><CODE
|
CLASS="FUNCDEF"
|
CLASS="FUNCDEF"
|
>void usbs_start_rx_buffer</CODE
|
>void usbs_start_rx_buffer</CODE
|
>(usbs_rx_endpoint* ep, unsigned char* buffer, int length, void (*)(void*,int) complete_fn, void * complete_data);</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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><P
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><P
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><CODE
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><CODE
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><CODE
|
><CODE
|
CLASS="FUNCDEF"
|
CLASS="FUNCDEF"
|
>void usbs_start_rx</CODE
|
>void usbs_start_rx</CODE
|
>(usbs_rx_endpoint* ep);</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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><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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></DIV
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><DIV
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><DIV
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CLASS="REFSECT1"
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CLASS="REFSECT1"
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><A
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><A
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NAME="AEN16339"
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NAME="AEN16339"
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></A
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></A
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><H2
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><H2
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><TT
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><TT
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CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>Description</TT
|
>Description</TT
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></H2
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></H2
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><P
|
><P
|
><TT
|
><TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>usbs_start_rx_buffer</TT
|
>usbs_start_rx_buffer</TT
|
> is a USB-specific function
|
> is a USB-specific function
|
to accept a transfer from host to peripheral. It can be used for bulk,
|
to accept a transfer from host to peripheral. It can be used for bulk,
|
interrupt or isochronous transfers, but not for control messages.
|
interrupt or isochronous transfers, but not for control messages.
|
Instead those involve manipulating the <A
|
Instead those involve manipulating the <A
|
HREF="usbs-control.html"
|
HREF="usbs-control.html"
|
><SPAN
|
><SPAN
|
CLASS="STRUCTNAME"
|
CLASS="STRUCTNAME"
|
>usbs_control_endpoint</SPAN
|
>usbs_control_endpoint</SPAN
|
></A
|
></A
|
>
|
>
|
data structure directly. The function takes five arguments:</P
|
data structure directly. The function takes five arguments:</P
|
><P
|
><P
|
></P
|
></P
|
><OL
|
><OL
|
TYPE="1"
|
TYPE="1"
|
><LI
|
><LI
|
><P
|
><P
|
>The first argument identifies the specific endpoint that should be
|
>The first argument identifies the specific endpoint that should be
|
used. Different USB devices will support different sets of endpoints
|
used. Different USB devices will support different sets of endpoints
|
and the device driver will provide appropriate data structures. The
|
and the device driver will provide appropriate data structures. The
|
device driver's documentation should be consulted for details of which
|
device driver's documentation should be consulted for details of which
|
endpoints are available.</P
|
endpoints are available.</P
|
></LI
|
></LI
|
><LI
|
><LI
|
><P
|
><P
|
>The <TT
|
>The <TT
|
CLASS="PARAMETER"
|
CLASS="PARAMETER"
|
><I
|
><I
|
>buffer</I
|
>buffer</I
|
></TT
|
></TT
|
> and <TT
|
> and <TT
|
CLASS="PARAMETER"
|
CLASS="PARAMETER"
|
><I
|
><I
|
>length</I
|
>length</I
|
></TT
|
></TT
|
>
|
>
|
arguments control the actual transfer. USB device drivers are not
|
arguments control the actual transfer. USB device drivers are not
|
expected to perform any buffering or to support partial transfers, so
|
expected to perform any buffering or to support partial transfers, so
|
the length specified should correspond to the maximum transfer that is
|
the length specified should correspond to the maximum transfer that is
|
currently possible and the buffer should be at least this large. For
|
currently possible and the buffer should be at least this large. For
|
isochronous transfers the USB specification imposes an upper bound of
|
isochronous transfers the USB specification imposes an upper bound of
|
1023 bytes, and a smaller limit may be set in the <A
|
1023 bytes, and a smaller limit may be set in the <A
|
HREF="usbs-enum.html#AEN16179"
|
HREF="usbs-enum.html#AEN16179"
|
>enumeration data</A
|
>enumeration data</A
|
>. Interrupt
|
>. Interrupt
|
transfers are similarly straightforward with an upper bound of 64
|
transfers are similarly straightforward with an upper bound of 64
|
bytes, or less as per the enumeration data. Bulk transfers are more
|
bytes, or less as per the enumeration data. Bulk transfers are more
|
complicated because they can involve multiple 64-byte packets plus a
|
complicated because they can involve multiple 64-byte packets plus a
|
terminating packet of less than 64 bytes, so there is no predefined
|
terminating packet of less than 64 bytes, so there is no predefined
|
limit on the transfer size. Instead it is left to higher-level
|
limit on the transfer size. Instead it is left to higher-level
|
protocols to specify an appropriate upper bound.</P
|
protocols to specify an appropriate upper bound.</P
|
><P
|
><P
|
>One technique that may work for bulk transfers is to exploit the fact
|
>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
|
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
|
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; these 64 bytes can then be examined to determine the total
|
transfer size, and the remaining data can be transferred in another
|
transfer size, and the remaining data can be transferred in another
|
receive operation. This technique is not guaranteed to work with all
|
receive operation. This technique is not guaranteed to work with all
|
USB hardware. Also, if the delay between accepting the first packet and
|
USB hardware. Also, if the delay between accepting the first packet and
|
the remainder of the transfer is excessive then this could cause
|
the remainder of the transfer is excessive then this could cause
|
timeout problems for the host-side software. For these reasons this
|
timeout problems for the host-side software. For these reasons this
|
technique should be avoided.</P
|
technique should be avoided.</P
|
></LI
|
></LI
|
><LI
|
><LI
|
><P
|
><P
|
><TT
|
><TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>usbs_start_rx_buffer</TT
|
>usbs_start_rx_buffer</TT
|
> is non-blocking. It merely
|
> is non-blocking. It merely
|
starts the receive operation, and does not wait for completion. At
|
starts the receive operation, and does not wait for completion. At
|
some later point the USB device driver will invoke the completion
|
some later point the USB device driver will invoke the completion
|
function parameter with two arguments: the completion data defined by
|
function parameter with two arguments: the completion data defined by
|
the last parameter and a result field. A result >=
|
the last parameter and a result field. A result >=
|
<TT
|
<TT
|
CLASS="LITERAL"
|
CLASS="LITERAL"
|
>0</TT
|
>0</TT
|
> indicates a successful transfer of that many
|
> indicates a successful transfer of that many
|
bytes, which may be less than the upper bound imposed by the
|
bytes, which may be less than the upper bound imposed by the
|
<TT
|
<TT
|
CLASS="PARAMETER"
|
CLASS="PARAMETER"
|
><I
|
><I
|
>length</I
|
>length</I
|
></TT
|
></TT
|
> argument. A result <
|
> argument. A result <
|
<TT
|
<TT
|
CLASS="LITERAL"
|
CLASS="LITERAL"
|
>0</TT
|
>0</TT
|
> indicates an error. The most likely errors are
|
> indicates an error. The most likely errors are
|
<TT
|
<TT
|
CLASS="LITERAL"
|
CLASS="LITERAL"
|
>-EPIPE</TT
|
>-EPIPE</TT
|
> to indicate that the connection between the
|
> to indicate that the connection between the
|
host and the target has been broken, and <TT
|
host and the target has been broken, and <TT
|
CLASS="LITERAL"
|
CLASS="LITERAL"
|
>-EAGAIN</TT
|
>-EAGAIN</TT
|
>
|
>
|
for when the endpoint has been <A
|
for when the endpoint has been <A
|
HREF="usbs-halt.html"
|
HREF="usbs-halt.html"
|
>halted</A
|
>halted</A
|
>. Specific USB device drivers may
|
>. Specific USB device drivers may
|
specify additional error conditions.</P
|
specify additional error conditions.</P
|
></LI
|
></LI
|
></OL
|
></OL
|
><P
|
><P
|
>The normal sequence of events is that the USB device driver will
|
>The normal sequence of events is that the USB device driver will
|
update the appropriate hardware registers. At some point after that
|
update the appropriate hardware registers. At some point after that
|
the host will attempt to send data by transmitting an OUT token
|
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
|
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
|
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
|
operation then the peripheral would instead generate a NAK. The USB
|
hardware will generate an interrupt once the whole packet has been
|
hardware will generate an interrupt once the whole packet has been
|
received, and the USB device driver will service this interrupt and
|
received, and the USB device driver will service this interrupt and
|
arrange for a DSR to be called. Isochronous and interrupt transfers
|
arrange for a DSR to be called. Isochronous and interrupt transfers
|
involve just a single packet. However, bulk transfers may involve
|
involve just a single packet. However, bulk transfers may involve
|
multiple packets so the device driver has to check whether the packet
|
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
|
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
|
than this. When the device driver DSR detects a complete transfer it
|
will inform higher-level code by invoking the supplied completion
|
will inform higher-level code by invoking the supplied completion
|
function.</P
|
function.</P
|
><P
|
><P
|
>This means that the completion function will normally be invoked by a
|
>This means that the completion function will normally be invoked by a
|
DSR and not in thread context - although some USB device drivers may
|
DSR and not in thread context - although some USB device drivers may
|
have a different implementation. Therefore the completion function is
|
have a different implementation. Therefore the completion function is
|
restricted in what it can do. In particular it must not make any
|
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
|
calls that will or may block such as locking a mutex or allocating
|
memory. The kernel documentation should be consulted for more details
|
memory. The kernel documentation should be consulted for more details
|
of DSR's and interrupt handling generally.</P
|
of DSR's and interrupt handling generally.</P
|
><P
|
><P
|
>It is possible that the completion function will be invoked before
|
>It is possible that the completion function will be invoked before
|
<TT
|
<TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>usbs_start_rx_buffer</TT
|
>usbs_start_rx_buffer</TT
|
> returns. Such an event would
|
> returns. Such an event would
|
be unusual because the transfer cannot happen until the next time the
|
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
|
host tries to send data to this peripheral, but it may happen if for
|
example another interrupt happens and a higher priority thread is
|
example another interrupt happens and a higher priority thread is
|
scheduled to run. Also, if the endpoint is currently halted then the
|
scheduled to run. Also, if the endpoint is currently halted then the
|
completion function will be invoked immediately with
|
completion function will be invoked immediately with
|
<TT
|
<TT
|
CLASS="LITERAL"
|
CLASS="LITERAL"
|
>-EAGAIN</TT
|
>-EAGAIN</TT
|
>: typically this will happen in the current
|
>: typically this will happen in the current
|
thread rather than in a separate DSR. The completion function is
|
thread rather than in a separate DSR. The completion function is
|
allowed to start another transfer immediately by calling
|
allowed to start another transfer immediately by calling
|
<TT
|
<TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>usbs_start_rx_buffer</TT
|
>usbs_start_rx_buffer</TT
|
> again.</P
|
> again.</P
|
><P
|
><P
|
>USB device drivers are not expected to perform any locking. It is the
|
>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
|
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
|
receive operation for a given endpoint in progress at any one time. If
|
there are concurrent calls to
|
there are concurrent calls to
|
<TT
|
<TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>usbs_start_rx_buffer</TT
|
>usbs_start_rx_buffer</TT
|
> then the resulting behaviour
|
> then the resulting behaviour
|
is undefined. For typical USB applications this does not present any
|
is undefined. For typical USB applications this does not present any
|
problems, because only one piece of code will access a given endpoint
|
problems, because only one piece of code will access a given endpoint
|
at any particular time.</P
|
at any particular time.</P
|
><P
|
><P
|
>The following code fragment illustrates a very simple use of
|
>The following code fragment illustrates a very simple use of
|
<TT
|
<TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>usbs_start_rx_buffer</TT
|
>usbs_start_rx_buffer</TT
|
> to implement a blocking
|
> to implement a blocking
|
receive, using a semaphore to synchronise between the foreground
|
receive, using a semaphore to synchronise between the foreground
|
thread and the DSR. For a simple example like this no completion data
|
thread and the DSR. For a simple example like this no completion data
|
is needed.</P
|
is needed.</P
|
><TABLE
|
><TABLE
|
BORDER="5"
|
BORDER="5"
|
BGCOLOR="#E0E0F0"
|
BGCOLOR="#E0E0F0"
|
WIDTH="70%"
|
WIDTH="70%"
|
><TR
|
><TR
|
><TD
|
><TD
|
><PRE
|
><PRE
|
CLASS="PROGRAMLISTING"
|
CLASS="PROGRAMLISTING"
|
>static int error_code = 0;
|
>static int error_code = 0;
|
static cyg_sem_t completion_wait;
|
static cyg_sem_t completion_wait;
|
|
|
static void
|
static void
|
completion_fn(void* data, int result)
|
completion_fn(void* data, int result)
|
{
|
{
|
error_code = result;
|
error_code = result;
|
cyg_semaphore_post(&completion_wait);
|
cyg_semaphore_post(&completion_wait);
|
}
|
}
|
|
|
int
|
int
|
blocking_receive(usbs_rx_endpoint* ep, unsigned char* buf, int len)
|
blocking_receive(usbs_rx_endpoint* ep, unsigned char* buf, int len)
|
{
|
{
|
error_code = 0;
|
error_code = 0;
|
usbs_start_rx_buffer(ep, buf, len, &completion_fn, NULL);
|
usbs_start_rx_buffer(ep, buf, len, &completion_fn, NULL);
|
cyg_semaphore_wait(&completion_wait);
|
cyg_semaphore_wait(&completion_wait);
|
return error_code;
|
return error_code;
|
}</PRE
|
}</PRE
|
></TD
|
></TD
|
></TR
|
></TR
|
></TABLE
|
></TABLE
|
><P
|
><P
|
>There is also a utility function <TT
|
>There is also a utility function <TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>usbs_start_rx</TT
|
>usbs_start_rx</TT
|
>. This
|
>. This
|
can be used by code that wants to manipulate <A
|
can be used by code that wants to manipulate <A
|
HREF="usbs-data.html"
|
HREF="usbs-data.html"
|
>data endpoints</A
|
>data endpoints</A
|
> directly, specifically the
|
> directly, specifically the
|
<TT
|
<TT
|
CLASS="STRUCTFIELD"
|
CLASS="STRUCTFIELD"
|
><I
|
><I
|
>complete_fn</I
|
>complete_fn</I
|
></TT
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></TT
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>,
|
>,
|
<TT
|
<TT
|
CLASS="STRUCTFIELD"
|
CLASS="STRUCTFIELD"
|
><I
|
><I
|
>complete_data</I
|
>complete_data</I
|
></TT
|
></TT
|
>,
|
>,
|
<TT
|
<TT
|
CLASS="STRUCTFIELD"
|
CLASS="STRUCTFIELD"
|
><I
|
><I
|
>buffer</I
|
>buffer</I
|
></TT
|
></TT
|
> and
|
> and
|
<TT
|
<TT
|
CLASS="STRUCTFIELD"
|
CLASS="STRUCTFIELD"
|
><I
|
><I
|
>buffer_size</I
|
>buffer_size</I
|
></TT
|
></TT
|
> fields.
|
> fields.
|
<TT
|
<TT
|
CLASS="FUNCTION"
|
CLASS="FUNCTION"
|
>usbs_start_tx</TT
|
>usbs_start_tx</TT
|
> just invokes a function
|
> just invokes a function
|
supplied by the device driver.</P
|
supplied by the device driver.</P
|
></DIV
|
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|
><DIV
|
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|
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