56.3. Writing C++ device support software
This section describes how to write C++ software support for new data taking devices and how to integrate that support into the system. Device support modules are built into shared images. Tcl provides the load command which loads a shared object into an application and invokes an initialization function. This provicdes a plugin scheme that the framework leverages to support externally written device drivers.
A device driver therefore consists of a C++ program that provides a new device class and an initialization function that associates this class with a Tcl command in the interpreter that reads the configuration file. Since each time the configuration file is read, a new interpreter is used, this also means that the current version of the device driver shared image is loaded at the start of each run.
The device driver support package is supplied as a driver development kit that consists of a template driver and a Makefile. These are two files in the vmusbdriver directory of the NSCLDAQ installation. The example below shows how to create a new directory and prepare it for driver development. In the example we are assuming that the environment variable DAQROOT points to the top level of the installation directory.
Example 56-4. Obtaning the VM-USB device driver development kit
mkdir mydriver
cd mydriver
cp $DAQROOT/vmusbdriver/drivertemplate.cpp .
cp $DAQROOT/vmusbdriver/Makefile .
The template driver is a complete example that builds a
marker driver which inserts constant word into the event.
You can do a make to build the driver if you like.
The template driver then defines a command changeme
for the configuration file interpreter and a configuration parameter
-value which allows you to set the value of the
markrer.
The script fragment below shows how to load the driver, create and configure a module instance using it. The fragment assumes that the driver shared object libtemplatedriver.so is in the same directory as the DAQ configuration script but that that directory my not be the current working directory when the configuration script is sourced.
Example 56-5. Using a user written VMUSB driver
set here [file nativename [file dirname [info script]]]
load [file join $here libtemplatedriver.so]
changeme cdreate testing -value -0x1234
The work done by the set here... command builds the full path to the directory the driver is in. This is necessary beause the load command normally only uses directories that are in the dynamic loader search path to look for shared objects. Note finally that once the driver is loaded, it registers the changeme command with the interpreter and that this command operates exactly like any other driver command.
The next sections will examine the driver elements in detail. Before doing that, let's take a broad brush overview look at the driver template.
The template consists of two sections. The first section is the
definition and implementation of a class which derives from
CReadoutHardware the base class for
all DAQ device support. The methods of this class define
configuration parameters, initialize the module as the run
is starting and provides the appropriate commands to the
VME list that is being generated for the stack this module
is an element of. Finally virtual duplication
(clone) is also defined.
The second section is an initialization function that the Tcl load command automatically locates and calls. This function creates an instance of the driver which is cloned for each device instance the user creates. It also associates a Tcl command with the device driver so that the DAQ configuration script can create and manipulate new instances.
While the driver template is heavily commmented, and modification points indicated, the next few sectinos are a guided tour of the driver in detail, pointing out what needs to be modified to make the driver work with a specific device.
56.3.1. The driver onAttach method
Each driver instance has a configuration database attached to it when it is created. The configuration database holds configuration parameter definitions and their current values. The framework takes care of managing the values for you, however you must define the set of configuration parameters supported by your driver.
The template driver's code is (comments removed for brevity:
void
CTemplateDriver::onAttach(CReadoutModule& configuration)
{
m_pConfiguration = &configuration; 
m_pConfiguration->addIntegerParameter("-base"); 
m_pConfiguration->addIntegerParameter("-id", 0, 0xffff, 0); 
}
In the discussion below, the numbers refer to the same numbers in the example above.
-
The method is passed a reference to its instance
configuration database. This will be used here,
to establish configuration parameters, in the
Initializemethod to know how to set up the module and inaddReadoutListto know how to read the module. This line saves a pointer to the configuration database for this instance in member data where it can be accessed in those other methods.
-
Most if not all VME modules must be addressed relative
to some base address that is set via jumpers or switches
on the module itself. Therefore the template driver
provides a definition for a
-baseoption to hold this value. The specific version ofaddIntegerParameterused only requires that the value passed to-basebe a valid integer. No constraint on the range is imposed. -
Since the template driver inserts a marker
the
idparameter is defined to provide the value of the marker. The VM-USB only supports 16 bit markers, therefore the version ofaddIntegerParameterconstrains the range of values to be in the range [0..0xffff]. If a constraint is specified, and a daq configuration script violates it, the configuration file interpreter outputs an error message and refuses to start the run. Using constraints allows error checking to be done by the configuration subsystem without intervention by user code.
Constraint checking comes from the
CConfigurableObjectclass. See CConfigurableObject(3vmusb) for pre-defined constraints. That manpage also shows you how to create your own constraints if the pre-defined ones don't work for you.
56.3.2. The driver Initialize method
When a run is starting, each stack invokes the
Initialize method for each element in
its -modules list. Each driver is supposed
to query its configuration and do any initialization demanded
by the configuration. For example the adc
command queries the set of pedestal values and programs them
into its module (using the -base of course)
at this time.
The Initialize method is passed a
reference to a CVMUSB object. Methods
on that object allow you to perform single or block VME
operations. You can also create and stock a
CVMUSBReadoutList with several VME
operations and ask the controller to execute that list in
immediate mode.
See CVMUSB(3vmusb) and CVMUSBReadoutList for reference information about those two classes.
The template driver is a marker and does not perform any
VME operations. Since, however your driver will most likely
need the -base parameter value, it shows
how to obtain that from the configuration database:
Example 56-6. The template driver Initialize method
void
CTemplateDriver::Initialize(CVMUSB& controller)
{
uint32_t base = m_pConfiguration->getUnsignedParameter("-base");
}
The configuration database stores all parameter values as
strings after validating them however it also provides a rich
set of member function to convert the string to some other
format. Since the -base parameter can take values
greater than 0x80000000 it must be converted
and treated as an unsigned integer.
getUnsignedParameter converts the
value of the configuration parameter given to an unsigned integer.
56.3.3. The driver addReadoutList method
The addReadoutList method is called
by stacks containing a driver instance when the stack is
building its list of VME operations to download into the VM-USB.
addReadoutList is passed a
CVMUSBReadoutList object and is expected
to add entries to that object.
The template ddriver fetches the -base
and -id option values and adds a marker
instruction to the stack with the value of the
-id option.
Example 56-7. Template Driver addReadoutList method
void
CTemplateDriver::addReadoutList(CVMUSBReadoutList& list)
{
uint32_t base = m_pConfiguration->getUnsignedParameter("-base");
int id = m_pConfiguration->getIntegerParameter("-id");
list.addMarker(id);
}
-
The
-idoption is an integer in the range [0 .. 0xffff]. This line fetches its current value from the configuration database. -
This line adds a marker instruction to the
stack. The value of the marker to be
inserted in the event is the value of the
-idconfiguration parameter.
56.3.4. Driver initialization xxxx_init
The driver will build to a shared object of the name
libxxxx.so where you will choose
xxxx when you edit the driver Makefile.
When the load command loads this library,
it will look for a function named
Xxxx_Init (note the capitalization).
and call it with a pointer to the running Tcl Interptreter.
You must make sure the initialization entry point name is
correct for the driver name. For exmample:
libmyvmedriver.so requires an initialization
function entry point of
Myvmedriver_Init.
Let's pick apart the template driver's implementation of its initialization function.
Example 56-8.
The VMUSB driver Xxxx_Init
function.
extern "C" {
int Templatedriver_Init(Tcl_Interp* pInterp)
{
Tcl_PkgProvide(pInterp, "Templatedriver", "1.0");
CUserCommand::addDriver("changeme", new CTemplateDriver); 
return TCL_OK; 
}
} 
- The Tcl load command will be looking for a specific function name to call to initialize the library it just loaded. C++ decorates or mangles function names adding information about the return type and the type of parameters expected by the function. This is how it implements function/operator overloading.
Using the extern "C" block shown tells the GNU C++ compiler to use C language call methods which disable this function name mangling. Without this, the load command would not find the initialization function.
- As described above, the initialization function name must be precisely chosen to match both the library and the package name (see below). The function name used here must be modified to match your changes to the Makefile. The initialization function here is correct for the package TemplateDriver and the library file libtemplatedriver.so.
-
This line also allows you to use the
Tcl package require command to
load the driver if you have created a
pkgIndex.tcl file using e.g.
pkg_mkIndex and added the diretory
the driver lives in to the Tcl package load path
(
auto_path) or theTCLLIBPATHenvironment variable. The package name must match the part of the function name prior to _Init, as it is used to located the name of the package initialzation function by package require
- This line associates the tcl command changeme with the driver by creating a prototype instance of the driver object that will be cloned to produce driver instances. Normally you would change the name of the command to be a meaningful command name for your driver.
This is part of an implementation of the prototype pattern. For more about the prototype pattern see e.g. http://en.wikipedia.org/wiki/Prototype_pattern
- The load or package require command expects the initialization function to return TCL_OK on success or TCL_ERROR if it is not able to successfully initialize. This line indicates a successful installation/initialization of the library.
- Ends the extern "C" { block.