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><H1
><A
NAME="SYNTH-NEW-TARGET">Writing New Devices - target</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN18180"
></A
><H2
>Name</H2
>Writing New Devices -- extending the synthetic target, target-side</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN18183"><H2
>Synopsis</H2
><DIV
CLASS="FUNCSYNOPSIS"
><A
NAME="AEN18184"><P
></P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="FUNCSYNOPSISINFO"
>#include <cyg/hal/hal_io.h>
</PRE
></TD
></TR
></TABLE
><P
><CODE
><CODE
CLASS="FUNCDEF"
>int synth_auxiliary_instantiate</CODE
>(const char* package, const char* version, const char* device, const char* instance, const char* data);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>void synth_auxiliary_xchgmsg</CODE
>(int device_id, int request, int arg1, int arg2, const unsigned char* txdata, int txlen, int* reply, unsigned char* rxdata, int* rxlen, int max_rxlen);</CODE
></P
><P
></P
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="SYNTH-NEW-TARGET-DESCRIPTION"
></A
><H2
>Description</H2
><P
>In some ways writing a device driver for the synthetic target is very
similar to writing one for a real target. Obviously it has to provide
the standard interface for that class of device, so for example an
ethernet device has to provide <TT
CLASS="FUNCTION"
>can_send</TT
>,
<TT
CLASS="FUNCTION"
>send</TT
>, <TT
CLASS="FUNCTION"
>recv</TT
> and similar
functions. Many devices will involve interrupts, so the driver
contains ISR and DSR functions and will call
<TT
CLASS="FUNCTION"
>cyg_drv_interrupt_create</TT
>,
<TT
CLASS="FUNCTION"
>cyg_drv_interrupt_acknowledge</TT
>, and related
functions.
</P
><P
>In other ways writing a device driver for the synthetic target is very
different. Usually the driver will not have any direct access to the
underlying hardware. In fact for some devices the I/O may not involve
real hardware, instead everything is emulated by widgets on the
graphical display. Therefore the driver cannot just peek and poke
device registers, instead it must interact with host-side code by
exchanging message. The synthetic target HAL provides a function
<TT
CLASS="FUNCTION"
>synth_auxiliary_xchgmsg</TT
> for this purpose.
</P
><P
>Initialization of a synthetic target device driver is also very
different. On real targets the device hardware already exists when the
driver's initialization routine runs. On the synthetic target it is
first necessary to instantiate the device inside the I/O auxiliary, by
a call to <TT
CLASS="FUNCTION"
>synth_auxiliary_instantiate</TT
>. That
function performs a special message exchange with the I/O auxiliary,
causing it to load a Tcl script for the desired type of device and run
an instantiation procedure within that script.
</P
><P
>Use of the I/O auxiliary is optional: if the user does not specify
<TT
CLASS="OPTION"
>--io</TT
> on the command line then the auxiliary will not
be started and hence most I/O operations will not be possible. Device
drivers should allow for this possibility, for example by just
discarding any data that gets written. The HAL exports a flag
<TT
CLASS="VARNAME"
>synth_auxiliary_running</TT
> which should be checked.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="SYNTH-NEW-TARGET-INSTANTIATE"
></A
><H2
>Instantiating a Device</H2
><P
>Device instantiation should happen during the C++ prioritized static
constructor phase of system initialization, before control switches to
<TT
CLASS="FUNCTION"
>cyg_user_start</TT
> and general application code. This
ensures that there is a clearly defined point at which the I/O
auxiliary knows that all required devices have been loaded. It can
then perform various consistency checks and clean-ups, run the user's
<TT
CLASS="FILENAME"
>mainrc.tcl</TT
> script, and make the main window
visible.
</P
><P
>For standard devices generic eCos I/O code will call the device
initialization routines at the right time, iterating through the
<TT
CLASS="VARNAME"
>DEVTAB</TT
> table in a static constructor. The same
holds for network devices and file systems. For more custom devices
code like the following can be used:
</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>#include <cyg/infra/cyg_type.h>
class mydev_init {
public:
mydev_init() {
…
}
};
static mydev_init mydev_init_object CYGBLD_ATTRIB_INIT_PRI(CYG_INIT_IO);</PRE
></TD
></TR
></TABLE
><P
>Some care has to be taken because the object
<TT
CLASS="VARNAME"
>mydev_init_object</TT
> will typically not be referenced
by other code, and hence may get eliminated at link-time. If the code
is part of an eCos package then problems can be avoided by putting the
relevant file in <TT
CLASS="FILENAME"
>libextras.a</TT
>:
</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>cdl_package CYGPKG_DEVS_MINE {
…
compile -library=libextras.a init.cxx
}</PRE
></TD
></TR
></TABLE
><P
>For devices inside application code the same can be achieved by
linking the relevant module as a <TT
CLASS="FILENAME"
>.o</TT
> file rather
than putting it in a <TT
CLASS="FILENAME"
>.a</TT
> library.
</P
><P
>In the device initialization routine the main operation is a call to
<TT
CLASS="FUNCTION"
>synth_auxiliary_instantiate</TT
>. This takes five
arguments, all of which should be strings:
</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="VARNAME"
>package</TT
></DT
><DD
><P
>For device drivers which are eCos packages this should be a directory
path relative to the eCos repository, for example
<TT
CLASS="LITERAL"
>devs/eth/synth/ecosynth</TT
>. This will allow the I/O
auxiliary to find the various host-side support files for this package
within the install tree. If the device is application-specific and not
part of an eCos package then a NULL pointer can be used, causing the
I/O auxiliary to search for the support files in the current directory
and then in <TT
CLASS="FILENAME"
>~/.ecos/synth</TT
>
instead.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>version</TT
></DT
><DD
><P
>For eCos packages this argument should be the version of the package
that is being used, for example <TT
CLASS="LITERAL"
>current</TT
>. A simple
way to get this version is to use the
<TT
CLASS="FUNCTION"
>SYNTH_MAKESTRING</TT
> macro on the package name.
If the device is application-specific then a NULL pointer should be
used.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>device</TT
></DT
><DD
><P
>This argument specifies the type of device being instantiated, for
example <TT
CLASS="LITERAL"
>ethernet</TT
>. More specifically the I/O
auxiliary will append a <TT
CLASS="FILENAME"
>.tcl</TT
> suffix, giving
the name of a Tcl script that will handle all I/O requests for the
device. If the application requires several instances of a type
of device then the script will only be loaded once, but the script
will contain an instantiation procedure that will be called for each
device instance.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>instance</TT
></DT
><DD
><P
>If it is possible to have multiple instances of a device then this
argument identifies the particular instance, for example
<TT
CLASS="LITERAL"
>eth0</TT
> or <TT
CLASS="LITERAL"
>eth1</TT
>. Otherwise a NULL
pointer can be used.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>data</TT
></DT
><DD
><P
>This argument can be used to pass additional initialization data from
eCos to the host-side support. This is useful for devices where eCos
configury must control certain aspects of the device, rather than
host-side configury such as the target definition file, because eCos
has compile-time dependencies on some or all of the relevant options.
An example might be an emulated frame buffer where eCos has been
statically configured for a particular screen size, orientation and
depth. There is no fixed format for this string, it will be
interpreted only by the device-specific host-side Tcl script. However
the string length should be limited to a couple of hundred bytes to
avoid possible buffer overflow problems.
</P
></DD
></DL
></DIV
><P
>Typical usage would look like:
</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
> if (!synth_auxiliary_running) {
return;
}
id = synth_auxiliary_instantiate("devs/eth/synth/ecosynth",
SYNTH_MAKESTRING(CYGPKG_DEVS_ETH_ECOSYNTH),
"ethernet",
"eth0",
(const char*) 0);</PRE
></TD
></TR
></TABLE
><P
>The return value will be a device identifier which can be used for
subsequent calls to <TT
CLASS="FUNCTION"
>synth_auxiliary_xchgmsg</TT
>. If
the device could not be instantiated then <TT
CLASS="LITERAL"
>-1</TT
> will
be returned. It is the responsibility of the host-side software to
issue suitable diagnostics explaining what went wrong, so normally the
target-side code should fail silently.
</P
><P
>Once the desired device has been instantiated, often it will be
necessary to do some additional initialization by a message exchange.
For example an ethernet device might need information from the
host-side about the MAC address, the <A
HREF="synth-new-target.html#SYNTH-NEW-TARGET-INTERRUPTS"
>interrupt vector</A
>, and
whether or not multicasting is supported.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="SYNTH-NEW-TARGET-XCHGMSG"
></A
><H2
>Communicating with a Device</H2
><P
>Once a device has been instantiated it is possible to perform I/O by
sending messages to the appropriate Tcl script running inside the
auxiliary, and optionally getting back replies. I/O operations are
always initiated by the eCos target-side, it is not possible for the
host-side software to initiate data transfers. However the host-side
can raise interrupts, and the interrupt handler inside the target can
then exchange one or more messages with the host.
</P
><P
>There is a single function to perform I/O operations,
<TT
CLASS="FUNCTION"
>synth_auxiliary_xchgmsg</TT
>. This takes the following
arguments:
</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="VARNAME"
>device_id</TT
></DT
><DD
><P
>This should be one of the identifiers returned by a previous
call to <TT
CLASS="FUNCTION"
>synth_auxiliary_instantiate</TT
>, specifying the
particular device which should perform some I/O.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>request</TT
></DT
><DD
><P
>Request are just signed 32-bit integers that identify the particular
I/O operation being requested. There is no fixed set of codes, instead
each type of device can define its own.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>arg1</TT
>, <TT
CLASS="VARNAME"
>arg2</TT
></DT
><DD
><P
>For some requests it is convenient to pass one or two additional
parameters alongside the request code. For example an ethernet device
could define a multicast-all request, with <TT
CLASS="VARNAME"
>arg1</TT
>
controlling whether this mode should be enabled or disabled. Both
<TT
CLASS="VARNAME"
>arg1</TT
> and <TT
CLASS="VARNAME"
>arg2</TT
> should be signed
32-bit integers, and their values are interpreted only by the
device-specific Tcl script.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>txdata</TT
>, <TT
CLASS="VARNAME"
>txlen</TT
></DT
><DD
><P
>Some I/O operations may involve sending additional data, for example
an ethernet packet. Alternatively a control operation may require many
more parameters than can easily be encoded in <TT
CLASS="VARNAME"
>arg1</TT
>
and <TT
CLASS="VARNAME"
>arg2</TT
>, so those parameters have to be placed in
a suitable buffer and extracted at the other end.
<TT
CLASS="VARNAME"
>txdata</TT
> is an arbitrary buffer of
<TT
CLASS="VARNAME"
>txlen</TT
> bytes that should be sent to the host-side.
There is no specific upper bound on the number of bytes that can be
sent, but usually it is a good idea to allocate the transmit buffer
statically and keep transfers down to at most several kilobytes.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>reply</TT
></DT
><DD
><P
>If the host-side is expected to send a reply message then
<TT
CLASS="VARNAME"
>reply</TT
> should be a pointer to an integer variable
and will be updated with a reply code, a simple 32-bit integer. The
synthetic target HAL code assumes that the host-side and target-side
agree on the protocol being used: if the host-side will not send a
reply to this message then the <TT
CLASS="VARNAME"
>reply</TT
> argument
should be a NULL pointer; otherwise the host-side must always send
a reply code and the <TT
CLASS="VARNAME"
>reply</TT
> argument must be valid.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>rxdata</TT
>, <TT
CLASS="VARNAME"
>rxlen</TT
></DT
><DD
><P
>Some operations may involve additional data coming from the host-side,
for example an incoming ethernet packet. <TT
CLASS="VARNAME"
>rxdata</TT
>
should be a suitably-sized buffer, and <TT
CLASS="VARNAME"
>rxlen</TT
> a
pointer to an integer variable that will end up containing the number
of bytes that were actually received. These arguments will only be
used if the host-side is expected to send a reply and hence the
<TT
CLASS="VARNAME"
>reply</TT
> argument was not NULL.
</P
></DD
><DT
><TT
CLASS="VARNAME"
>max_rxlen</TT
></DT
><DD
><P
>If a reply to this message is expected and that reply may involve
additional data, <TT
CLASS="VARNAME"
>max_rxlen</TT
> limits the size of that
reply. In other words, it corresponds to the size of the
<TT
CLASS="VARNAME"
>rxdata</TT
> buffer.
</P
></DD
></DL
></DIV
><P
>Most I/O operations involve only some of the arguments. For example
transmitting an ethernet packet would use the
<TT
CLASS="VARNAME"
>request</TT
>, <TT
CLASS="VARNAME"
>txdata</TT
> and
<TT
CLASS="VARNAME"
>txlen</TT
> fields (in addition to
<TT
CLASS="VARNAME"
>device_id</TT
> which is always required), but would not
involve <TT
CLASS="VARNAME"
>arg1</TT
> or <TT
CLASS="VARNAME"
>arg2</TT
> and no
reply would be expected. Receiving an ethernet packet would involve
<TT
CLASS="VARNAME"
>request</TT
>, <TT
CLASS="VARNAME"
>rxdata</TT
>,
<TT
CLASS="VARNAME"
>rxlen</TT
> and <TT
CLASS="VARNAME"
>max_rxlen</TT
>; in addition
<TT
CLASS="VARNAME"
>reply</TT
> is needed to get any reply from the host-side
at all, and could be used to indicate whether or not any more packets
are buffered up. A control operation such as enabling multicast mode
would involve <TT
CLASS="VARNAME"
>request</TT
> and <TT
CLASS="VARNAME"
>arg1</TT
>,
but none of the remaining arguments.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="SYNTH-NEW-TARGET-INTERRUPTS"
></A
><H2
>Interrupt Handling</H2
><P
>Interrupt handling in the synthetic target is much the same as on a
real target. An interrupt object is created using
<TT
CLASS="FUNCTION"
>cyg_drv_interrupt_create</TT
>, attached, and unmasked.
The emulated device - in other words the Tcl script running inside the
I/O auxiliary - can raise an interrupt. Subject to interrupts being
disabled and the appropriate vector being masked, the system will
invoke the specified ISR function. The synthetic target HAL
implementation does have some limitations: there is no support for
nested interrupts, interrupt priorities, or a separate interrupt
stack. Supporting those might be appropriate when targetting a
simulator that attempts to model real hardware accurately, but not for
the simple emulation provided by the synthetic target.
</P
><P
>Of course the actual implementation of the ISR and DSR functions will
be rather different for a synthetic target device driver. For real
hardware the device driver will interact with the device by reading
and writing device registers, managing DMA engines, and the like. A
synthetic target driver will instead call
<TT
CLASS="FUNCTION"
>synth_auxiliary_xchgmsg</TT
> to perform the I/O
operations.
</P
><P
>There is one other significant difference between interrupt handling
on the synthetic target and on real hardware. Usually the eCos code
will know which interrupt vectors are used for which devices. That
information is fixed when the target hardware is designed. With the
synthetic target interrupt vectors are assigned to devices on the host
side, either via the target definition file or dynamically when the
device is instantiated. Therefore the initialization code for a
target-side device driver will need to request interrupt vector
information from the host-side, via a message exchange. Such interrupt
vectors will be in the range 1 to 31 inclusive, with interrupt 0 being
reserved for the real-time clock.
</P
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