1. TWOPASS

LinuxCNC 2.5 supports TWOPASS processing of hal configuration files that can help in the modularization and readability of hal files. (Hal files are specified in an LinuxCNC ini file in the HAL stanza as [HAL]HALFILE=filename).

Normally, a set of one or more hal configuration files must use a single, unique loadrt line to load a kernel module that may handle multiple instances of a component. For example, if you use a two input AND gate component (and2) in three different places in your setup, you would need to have a single line somewhere to specify:

   loadrt and2 count=3

resulting in components and2.0, and2.1, and2.2.

Configurations are more readable if you specify with the names=option for components where it is supported, e.g.,:

loadrt and2 names=aa,ab,ac

resulting in components aa,ab,ac.

It can be a maintenance problem to keep track of the components and their names since when you add (or remove) a component, you must find and update the single loadrt directive applicable to the component.

TWOPASS processing is enabled by including an ini file parameter in the [HAL] section:

[HAL]

TWOPASS = anystring

Where "anystring" can be any non-null string. With this setting, you can have multiple specifications like:

loadrt and2 names=aa
...
loadrt and2 names=ab,ac
...
loadrt and2 names=ad

These commands can appear in different HALFILES. The HALFILES are processed in the order of their appearance in the ini file.

The TWOPASS option can be specified with options to add output for debugging (verbose) and to prevent deletion of temporary files (nodelete). The options are separated with commas.

Example:

[HAL]

TWOPASS = on,verbose,nodelete

With TWOPASS processing, all [HAL]HALFILES are first read and multiple appearances of loadrt directives for each module are accumulated. User components (loadusr) are loaded in order but no other hal commands are executed in the initial pass.

Note
 User components should use the wait (-W) option to ensure the component is ready before other commands are executed.

After the initial pass, the realtime modules are loaded (loadrt) automatically with a number equal to the total number when using the count= option or with all of the individual names specified when using the names= option.

A second pass is then made to execute all of the other hal instructions specified in the HALFILES. The addf commands that associate a component’s functions with thread execution are executed in the order of appearance with other commands during this second pass.

While you can use either the count= or names= options, they are mutually exclusive — only one type can be specified for a given module.

TWOPASS processing is most effective when using the names= option. This option allows you to provide unique names that are mnemonic or otherwise relevant to the configuration. For example, if you use a derivative component to estimate the velocities and accelerations on each (x,y,z) coordinate, using the count= method will give arcane component names like ddt.0, ddt.1, ddt.2, etc.

Alternatively, using the names= option like:

loadrt ddt names=xvel,yvel,zvel
...
loadrt ddt names=xaccel,yaccel,zaccel

results in components sensibly named xvel,yvel,zvel, xaccel,yaccel,zaccel.

Many comps supplied with the distribution are created with the halcompile utility and support the names= option. These include the common logic components that are the glue of many hal configurations.

User-created comps that use the halcompile utility automatically support the names= option as well. In addition to comps generated with the halcompile utility, numerous other comps support the names=option. Comps that support names= option include: at_pid, encoder, encoder_ratio, pid, siggen, and sim_encoder.

2. Post GUI

Some GUIs support halfiles that are processed after the GUI is started in order to connect hal pins that are created by the GUI. When using a postgui halfile with TWOPASS processing, include all loadrt items for components added by postgui halfiles in a separate halfile that is processed before the GUI. The addf commands can also be included in the file. Example:

[HAL]
TWOPASS = on
HALFILE = file_1.hal
...
HALFILE = file_n.hal
HALFILE = file_with_all_loads_for_postgui.hal
...
POSTGUI_HALFILE = the_postgui_file.hal

3. Excluding .hal files

TWOPASS processing converts .hal files to equivalent .tcl files and uses haltcl to find loadrt and addf commands in order to accumulate and consolidate their usage. Loadrt parameters that conform to the simple names= (or count=) parameters accepted by the HAL Component Generator (halcompile) are expected. More complex parameter items included in specialized hal components may not be handled properly.

A .hal file may be excluded from TWOPASS processing by including a magic comment line anywhere in the .hal file. The magic comment line must begin with the string: #NOTWOPASS. Files specified with this magic comment are sourced by halcmd using the -k (keep going if failure) and -v (verbose) options.

This exclusion provision can be used to isolate problems or for loading any special hal component that does not require or benefit from TWOPASS processing.

Ordinarily, the loadrt ordering of realtime components is not critical, but loadrt ordering for special components can be enforced by placing the such loadrt directives in an excluded file.

Note
 While the order of loadrt directives is not usually critical, ordering of addf directives is often very important for proper operation of servo loop components.

Excluded .hal file example:

$ cat twopass_excluded.hal
# The following magic comment causes this file to
# be excluded from twopass processing:
# NOTWOPASS

# debugging component with complex options:
loadrt mycomponent parm1="abc def" parm2=ghi
show pin mycomponent

# ordering special components
loadrt component_1
loadrt component_2
Note
 Case and whitespace within the magic comment are ignored. The loading of components that use names= or count= parameters (typically built by halcompile) should not be used in excluded files as that would eliminate the benefits of TWOPASS processing. The hal commands that create signals (net) and commands that establish execution order (addf) should not be placed in excluded files. This is especially true for addf commands since their ordering may be important.

4. Examples

Examples of TWOPASS usage for a simulator are included in the directories:

configs/sim/axis/twopass/
configs/sim/axis/simtcl/