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[[cha:hal-twopass]]
= HAL TWOPASS

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== TWOPASS

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

Normally, a set of one or more LinuxCNC 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:

[source,{hal}]
----
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.,:

[source,{hal}]
----
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:

[source,{ini}]
----
[HAL]

TWOPASS = anystring
----

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

[source,{hal}]
----
loadrt and2 names=aa
...
loadrt and2 names=ab,ac
...
loadrt and2 names=ad
----

These commands can appear in different HAL files. The HAL files
are processed in the order of their appearance in the INI file,
in multiple HALFILE assignments.

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
[source,{ini}]
----
[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 LinuxCNC 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 LinuxCNC
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:

[source,{hal}]
----
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 LinuxCNC 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.

Two-step processing occurs before the GUI is loaded. When using a
[HAL]POSTGUI_HALFILE, it is convenient to place all the [HAL]POSTGUI_HALFILE
loadrt declarations for the necessary components in a preloaded HAL file.

.Example of a HAL section when using a POSTGUI_HALFILE
[source,{ini}]
----
[HAL]

TWOPASS = on
HALFILE = core_sim.hal
HALFILE = sim_spindle_encoder.hal
HALFILE = axis_manualtoolchange.hal
HALFILE = simulated_home.hal
HALFILE = load_for_postgui.hal <- loadrt lines for components in postgui.hal

POSTGUI_HALFILE = postgui.hal
HALUI = halui
----

== Post GUI

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

.Example
[source,{ini}]
----
[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
----

== 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 LinuxCNC 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 LinuxCNC 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 LinuxCNC 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.


== Examples

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

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

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