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<!-- Copyright (C) 2003 Red Hat, Inc.                                -->
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><H1
><A
NAME="USBS-CONTROL">Control Endpoints</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN16526"
></A
><H2
>Name</H2
>Control Endpoints&nbsp;--&nbsp;Control endpoint data structure</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN16529"><H2
>Synopsis</H2
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="SYNOPSIS"
>#include &lt;cyg/io/usb/usbs.h&gt;
 
typedef struct usbs_control_endpoint {
    *hellip;
} usbs_control_endpoint;</PRE
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN16531"
></A
><H2
><TT
CLASS="LITERAL"
>usbs_control_endpoint</TT
> Data Structure</H2
><P
>The device driver for a USB slave device should supply one
<SPAN
CLASS="STRUCTNAME"
>usbs_control_endpoint</SPAN
> data structure per USB
device. This corresponds to endpoint 0 which will be used for all
control message interaction between the host and that device. The data
structure is also used for internal management purposes, for example
to keep track of the current state. In a typical USB peripheral there
will only be one such data structure in the entire system, but if
there are multiple USB slave ports, allowing the peripheral to be
connected to multiple hosts, then there will be a separate data
structure for each one. The name or names of the data structures are
determined by the device drivers. For example, the SA11x0 USB device
driver package provides <TT
CLASS="LITERAL"
>usbs_sa11x0_ep0</TT
>.</P
><P
>The operations on a control endpoint do not fit cleanly into a
conventional open/read/write I/O model. For example, when the host
sends a control message to the USB peripheral this may be one of four
types: standard, class, vendor and reserved. Some or all of the
standard control messages will be handled automatically by the common
USB slave package or by the device driver itself. Other standard
control messages and the other types of control messages may be
handled by a class-specific package or by application code. Although
it would be possible to have devtab entries such as
<TT
CLASS="LITERAL"
>/dev/usbs_ep0/standard</TT
> and
<TT
CLASS="LITERAL"
>/dev/usbs_ep0/class</TT
>, and then support read and
write operations on these devtab entries, this would add significant
overhead and code complexity. Instead, all of the fields in the
control endpoint data structure are public and can be manipulated
directly by higher level code if and when required. </P
><P
>Control endpoints involve a number of callback functions, with
higher-level code installing suitable function pointers in the control
endpoint data structure. For example, if the peripheral involves
vendor-specific control messages then a suitable handler for these
messages should be installed. Although the exact details depend on the
device driver, typically these callback functions will be invoked at
DSR level rather than thread level. Therefore, only certain eCos
functions can be invoked; specifically, those functions that are
guaranteed not to block. If a potentially blocking function such as a
semaphore wait or a mutex lock operation is invoked from inside the
callback then the resulting behaviour is undefined, and the system as
a whole may fail. In addition, if one of the callback functions
involves significant processing effort then this may adversely affect
the system's real time characteristics. The eCos kernel documentation
should be consulted for more details of DSR handling.</P
><DIV
CLASS="REFSECT2"
><A
NAME="AEN16541"
></A
><H3
>Initialization</H3
><P
>The <SPAN
CLASS="STRUCTNAME"
>usbs_control_endpoint</SPAN
> data structure
contains the following fields related to initialization.</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>typedef struct usbs_control_endpoint {
    &#8230;
    const usbs_enumeration_data* enumeration_data;
    void                         (*start_fn)(usbs_control_endpoint*);
    &#8230;
};</PRE
></TD
></TR
></TABLE
><P
>It is the responsibility of higher-level code, usually the
application, to define the USB enumeration data. This needs to be
installed in the control endpoint data structure early on during
system startup, before the USB device is actually started and any
interaction with the host is possible. Details of the enumeration data
are supplied in the section <A
HREF="usbs-enum.html"
>USB Enumeration
Data</A
>. Typically, the enumeration data is constant for a given
peripheral, although it can be constructed dynamically if necessary.
However, the enumeration data cannot change while the peripheral is
connected to a host: the peripheral cannot easily claim to be a
keyboard one second and a printer the next.</P
><P
>The <TT
CLASS="STRUCTFIELD"
><I
>start_fn</I
></TT
> member is normally accessed
via the utility <A
HREF="usbs-start.html"
><TT
CLASS="FUNCTION"
>usbs_start</TT
></A
> rather
than directly. It is provided by the device driver and should be
invoked once the system is fully initialized and interaction with the
host is possible. A typical implementation would change the USB data
pins from tristated to active. If the peripheral is already plugged
into a host then the latter should detect this change and start
interacting with the peripheral, including requesting the enumeration
data.</P
></DIV
><DIV
CLASS="REFSECT2"
><A
NAME="AEN16552"
></A
><H3
>State</H3
><P
>There are three <SPAN
CLASS="STRUCTNAME"
>usbs_control_endpoint</SPAN
> fields
related to the current state of a USB slave device, plus some state
constants and an enumeration of the possible state changes:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>typedef struct usbs_control_endpoint {
    &#8230;
    int     state;
    void    (*state_change_fn)(struct usbs_control_endpoint*, void*,
                               usbs_state_change, int);
    void*   state_change_data;
    &#8230;
};
 
#define USBS_STATE_DETACHED             0x01
#define USBS_STATE_ATTACHED             0x02
#define USBS_STATE_POWERED              0x03
#define USBS_STATE_DEFAULT              0x04
#define USBS_STATE_ADDRESSED            0x05
#define USBS_STATE_CONFIGURED           0x06
#define USBS_STATE_MASK                 0x7F
#define USBS_STATE_SUSPENDED            (1 &lt;&lt; 7)
 
typedef enum {
    USBS_STATE_CHANGE_DETACHED          = 1,
    USBS_STATE_CHANGE_ATTACHED          = 2,
    USBS_STATE_CHANGE_POWERED           = 3,
    USBS_STATE_CHANGE_RESET             = 4,
    USBS_STATE_CHANGE_ADDRESSED         = 5,
    USBS_STATE_CHANGE_CONFIGURED        = 6,
    USBS_STATE_CHANGE_DECONFIGURED      = 7,
    USBS_STATE_CHANGE_SUSPENDED         = 8,
    USBS_STATE_CHANGE_RESUMED           = 9
} usbs_state_change;</PRE
></TD
></TR
></TABLE
><P
>The USB standard defines a number of states for a given USB
peripheral. The initial state is <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>detached</I
></SPAN
>, where
the peripheral is either not connected to a host at all or, from the
host's perspective, the peripheral has not started up yet because the
relevant pins are tristated. The peripheral then moves via
intermediate <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>attached</I
></SPAN
> and
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>powered</I
></SPAN
> states to its default or
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>reset</I
></SPAN
> state, at which point the host and
peripheral can actually start exchanging data. The first message is
from host to peripheral and provides a unique 7-bit address within the
local USB network, resulting in a state change to
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>addressed</I
></SPAN
>. The host then requests enumeration
data and performs other initialization. If everything succeeds the
host sends a standard set-configuration control message, after which
the peripheral is <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>configured</I
></SPAN
> and expected to be
up and running. Note that some USB device drivers may be unable to
distinguish between the <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>detached</I
></SPAN
>,
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>attached</I
></SPAN
> and <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>powered</I
></SPAN
> states
but generally this is not important to higher-level code.</P
><P
>A USB host should generate at least one token every millisecond. If a
peripheral fails to detect any USB traffic for a period of time then
typically this indicates that the host has entered a power-saving
mode, and the peripheral should do the same if possible. This
corresponds to the <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>suspended</I
></SPAN
> bit. The actual
state is a combination of <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>suspended</I
></SPAN
> and the
previous state, for example <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>configured</I
></SPAN
> and
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>suspended</I
></SPAN
> rather than just
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>suspended</I
></SPAN
>. When the peripheral subsequently
detects USB traffic it would switch back to the
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>configured</I
></SPAN
> state.</P
><P
>The USB device driver and the common USB slave package will maintain
the current state in the control endpoint's
<TT
CLASS="STRUCTFIELD"
><I
>state</I
></TT
> field. There should be no need for
any other code to change this field, but it can be examined whenever
appropriate. In addition whenever a state change occurs the generic
code can invoke a state change callback function. By default, no such
callback function will be installed. Some class-specific packages such
as the USB-ethernet package will install a suitable function to keep
track of whether or not the host-peripheral connection is up, that is
whether or not ethernet packets can be exchanged. Application code can
also update this field. If multiple parties want to be informed of
state changes, for example both a class-specific package and
application code, then typically the application code will install its
state change handler after the class-specific package and is
responsible for chaining into the package's handler.</P
><P
>The state change callback function is invoked with four arguments. The
first identifies the control endpoint. The second is an arbitrary
pointer: higher-level code can fill in the
<TT
CLASS="STRUCTFIELD"
><I
>state_change_data</I
></TT
> field to set this. The
third argument specifies the state change that has occurred, and the
last argument supplies the previous state (the new state is readily
available from the control endpoint structure).</P
><P
>eCos does not provide any utility functions for updating or examining
the <TT
CLASS="STRUCTFIELD"
><I
>state_change_fn</I
></TT
> or
<TT
CLASS="STRUCTFIELD"
><I
>state_change_data</I
></TT
> fields. Instead, it is
expected that the fields in the
<SPAN
CLASS="STRUCTNAME"
>usbs_control_endpoint</SPAN
> data structure will be
manipulated directly. Any utility functions would do just this, but
at the cost of increased code and cpu overheads.</P
></DIV
><DIV
CLASS="REFSECT2"
><A
NAME="AEN16582"
></A
><H3
>Standard Control Messages</H3
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>typedef struct usbs_control_endpoint {
    &#8230;
    unsigned char       control_buffer[8];
    usbs_control_return (*standard_control_fn)(struct usbs_control_endpoint*, void*);
    void*               standard_control_data;
    &#8230;
} usbs_control_endpoint;
 
typedef enum {
    USBS_CONTROL_RETURN_HANDLED = 0,
    USBS_CONTROL_RETURN_UNKNOWN = 1,
    USBS_CONTROL_RETURN_STALL   = 2
} usbs_control_return;
 
extern usbs_control_return usbs_handle_standard_control(struct usbs_control_endpoint*);</PRE
></TD
></TR
></TABLE
><P
>When a USB peripheral is connected to the host it must always respond
to control messages sent to endpoint 0. Control messages always
consist of an initial eight-byte header, containing fields such as a
request type. This may be followed by a further data transfer, either
from host to peripheral or from peripheral to host. The way this is
handled is described in the <A
HREF="usbs-control.html#AEN16615"
>Buffer Management</A
> section below.</P
><P
>The USB device driver will always accept the initial eight-byte
header, storing it in the <TT
CLASS="STRUCTFIELD"
><I
>control_buffer</I
></TT
>
field. Then it determines the request type: standard, class, vendor,
or reserved. The way in which the last three of these are processed is
described in the section <A
HREF="usbs-control.html#AEN16607"
>Other
Control Messages</A
>. Some
standard control messages will be handled by the device driver itself;
typically the <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>set-address</I
></SPAN
> request and the
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>get-status</I
></SPAN
>, <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>set-feature</I
></SPAN
> and
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>clear-feature</I
></SPAN
> requests when applied to endpoints.</P
><P
>If a standard control message cannot be handled by the device driver
itself, the driver checks the
<TT
CLASS="STRUCTFIELD"
><I
>standard_control_fn</I
></TT
> field in the control
endpoint data structure. If higher-level code has installed a suitable
callback function then this will be invoked with two argument, the
control endpoint data structure itself and the
<TT
CLASS="STRUCTFIELD"
><I
>standard_control_data</I
></TT
> field. The latter
allows the higher level code to associate arbitrary data with the
control endpoint. The callback function can return one of three
values: <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>HANDLED</I
></SPAN
> to indicate that the request has
been processed; <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>UNKNOWN</I
></SPAN
> if the message should be
handled by the default code; or <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>STALL</I
></SPAN
> to indicate
an error condition. If higher level code has not installed a callback
function or if the callback function has returned
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>UNKNOWN</I
></SPAN
> then the device driver will invoke a
default handler, <TT
CLASS="FUNCTION"
>usbs_handle_standard_control</TT
>
provided by the common USB slave package.</P
><P
>The default handler can cope with all of the standard control messages
for a simple USB peripheral. However, if the peripheral involves
multiple configurations, multiple interfaces in a configuration, or
alternate settings for an interface, then this cannot be handled by
generic code. For example, a multimedia peripheral may support various
alternate settings for a given data source with different bandwidth
requirements, and the host can select a setting that takes into
account the current load. Clearly higher-level code needs to be aware
when the host changes the current setting, so that it can adjust the
rate at which data is fed to or retrieved from the host. Therefore the
higher-level code needs to install its own standard control callback
and process appropriate messages, rather than leaving these to the
default handler.</P
><P
>The default handler will take care of the
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>get-descriptor</I
></SPAN
> request used to obtain the
enumeration data. It has support for string descriptors but ignores
language encoding issues. If language encoding is important for the
peripheral then this will have to be handled by an
application-specific standard control handler.</P
><P
>The header file <TT
CLASS="FILENAME"
>&lt;cyg/io/usb/usb.h&gt;</TT
> defines various
constants related to control messages, for example the function codes
corresponding to the standard request types. This header file is
provided by the common USB package, not by the USB slave package,
since the information is also relevant to USB hosts.</P
></DIV
><DIV
CLASS="REFSECT2"
><A
NAME="AEN16607"
></A
><H3
>Other Control Messages</H3
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>typedef struct usbs_control_endpoint {
    &#8230;
    usbs_control_return (*class_control_fn)(struct usbs_control_endpoint*, void*);
    void*               class_control_data;
    usbs_control_return (*vendor_control_fn)(struct usbs_control_endpoint*, void*);
    void*               vendor_control_data;
    usbs_control_return (*reserved_control_fn)(struct usbs_control_endpoint*, void*);
    void*               reserved_control_data;
    &#8230;
} usbs_control_endpoint;</PRE
></TD
></TR
></TABLE
><P
>Non-standard control messages always have to be processed by
higher-level code. This could be class-specific packages. For example,
the USB-ethernet package will handle requests for getting the MAC
address and for enabling or disabling promiscuous mode. In all cases
the device driver will store the initial request in the
<TT
CLASS="STRUCTFIELD"
><I
>control_buffer</I
></TT
> field, check for an
appropriate handler, and invoke it with details of the control
endpoint and any handler-specific data that has been installed
alongside the handler itself. The handler should return either
<TT
CLASS="LITERAL"
>USBS_CONTROL_RETURN_HANDLED</TT
> to report success or
<TT
CLASS="LITERAL"
>USBS_CONTROL_RETURN_STALL</TT
> to report failure. The
device driver will report this to the host.</P
><P
>If there are multiple parties interested in a particular type of
control messages, it is the responsibility of application code to
install an appropriate handler and process the requests appropriately. </P
></DIV
><DIV
CLASS="REFSECT2"
><A
NAME="AEN16615"
></A
><H3
>Buffer Management</H3
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>typedef struct usbs_control_endpoint {
    &#8230;
    unsigned char*      buffer;
    int                 buffer_size;
    void                (*fill_buffer_fn)(struct usbs_control_endpoint*);
    void*               fill_data;
    int                 fill_index;
    usbs_control_return (*complete_fn)(struct usbs_control_endpoint*, int);
    &#8230;
} usbs_control_endpoint;</PRE
></TD
></TR
></TABLE
><P
>Many USB control messages involve transferring more data than just the
initial eight-byte header. The header indicates the direction of the
transfer, OUT for host to peripheral or IN for peripheral to host.
It also specifies a length field, which is exact for an OUT transfer
or an upper bound for an IN transfer. Control message handlers can
manipulate six fields within the control endpoint data structure to
ensure that the transfer happens correctly.</P
><P
>For an OUT transfer, the handler should examine the length field in
the header and provide a single buffer for all the data. A
class-specific protocol would typically impose an upper bound on the
amount of data, allowing the buffer to be allocated statically.
The handler should update the <TT
CLASS="STRUCTFIELD"
><I
>buffer</I
></TT
> and
<TT
CLASS="STRUCTFIELD"
><I
>complete_fn</I
></TT
> fields. When all the data has
been transferred the completion callback will be invoked, and its
return value determines the response sent back to the host. The USB
standard allows for a new control message to be sent before the
current transfer has completed, effectively cancelling the current
operation. When this happens the completion function will also be
invoked. The second argument to the completion function specifies what
has happened, with a value of 0 indicating success and an error code
such as <TT
CLASS="LITERAL"
>-EPIPE</TT
> or <TT
CLASS="LITERAL"
>-EIO</TT
>
indicating that the current transfer has been cancelled.</P
><P
>IN transfers are a little bit more complicated. The required
information, for example the enumeration data, may not be in a single
contiguous buffer. Instead a mechanism is provided by which the buffer
can be refilled, thus allowing the transfer to move from one record to
the next. Essentially, the transfer operates as follows:</P
><P
></P
><OL
TYPE="1"
><LI
><P
>When the host requests another chunk of data (typically eight bytes),
the USB device driver will examine the
<TT
CLASS="STRUCTFIELD"
><I
>buffer_size</I
></TT
> field. If non-zero then
<TT
CLASS="STRUCTFIELD"
><I
>buffer</I
></TT
> contains at least one more byte of
data, and then <TT
CLASS="STRUCTFIELD"
><I
>buffer_size</I
></TT
> is decremented.</P
></LI
><LI
><P
>When <TT
CLASS="STRUCTFIELD"
><I
>buffer_size</I
></TT
> has dropped to 0, the
<TT
CLASS="STRUCTFIELD"
><I
>fill_buffer_fn</I
></TT
> field will be examined. If
non-null it will be invoked to refill the buffer.</P
></LI
><LI
><P
>The <TT
CLASS="STRUCTFIELD"
><I
>fill_data</I
></TT
> and
<TT
CLASS="STRUCTFIELD"
><I
>fill_index</I
></TT
> fields are not used by the
device driver. Instead these fields are available to the refill
function to keep track of the current state of the transfer.</P
></LI
><LI
><P
>When <TT
CLASS="STRUCTFIELD"
><I
>buffer_size</I
></TT
> is 0 and
<TT
CLASS="STRUCTFIELD"
><I
>fill_buffer_fn</I
></TT
> is NULL, no more data is
available and the transfer has completed.</P
></LI
><LI
><P
>Optionally a completion function can be installed. This will be
invoked with 0 if the transfer completes successfully, or with an
error code if the transfer is cancelled because of another control
messsage. </P
></LI
></OL
><P
>If the requested data is contiguous then the only fields that need
to be manipulated are <TT
CLASS="STRUCTFIELD"
><I
>buffer</I
></TT
> and
<TT
CLASS="STRUCTFIELD"
><I
>buffer_size</I
></TT
>, and optionally
<TT
CLASS="STRUCTFIELD"
><I
>complete_fn</I
></TT
>. If the requested data is not
contiguous then the initial control message handler should update
<TT
CLASS="STRUCTFIELD"
><I
>fill_buffer_fn</I
></TT
> and some or all of the other
fields, as required. An example of this is the handling of the
standard <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>get-descriptor</I
></SPAN
> control message by
<TT
CLASS="FUNCTION"
>usbs_handle_standard_control</TT
>.</P
></DIV
><DIV
CLASS="REFSECT2"
><A
NAME="AEN16652"
></A
><H3
>Polling Support</H3
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>typedef struct usbs_control_endpoint {
    void                (*poll_fn)(struct usbs_control_endpoint*);
    int                 interrupt_vector;
    &#8230;
} usbs_control_endpoint;</PRE
></TD
></TR
></TABLE
><P
>In nearly all circumstances USB I/O should be interrupt-driven.
However, there are special environments such as RedBoot where polled
operation may be appropriate. If the device driver can operate in
polled mode then it will provide a suitable function via the
<TT
CLASS="STRUCTFIELD"
><I
>poll_fn</I
></TT
> field, and higher-level code can
invoke this regularly. This polling function will take care of all
endpoints associated with the device, not just the control endpoint.
If the USB hardware involves a single interrupt vector then this will
be identified in the data structure as well.</P
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