Property calculated -- Used if the current option's value is not user-modifiable,
but is calculated using a suitable CDL expression.
Synopsis
cdl_option <name> {
calculated <expression>
…
}
Description
In some cases it is useful to have a configuration option whose value
cannot be modified directly by the user. This can be achieved using a
calculated, which takes a CDL expression as argument (see the Section called Ordinary Expressions in Chapter 3 for a description of expression
syntax). The configuration system evaluates the expression when the
current package is loaded and whenever there is a change to any other
option referenced in the expression. The result depends on the
option's flavor:
flavor none
Options with this flavor have no value, so the calculated
property is not applicable.
flavor bool
If the expression evaluates to a non-zero result the option is
enabled, otherwise it is disabled.
flavor booldata
If the result of evaluating the expression is zero then the option is
disabled, otherwise the option is enabled and its value is the result.
flavor data
The value of the option is the result of evaluating the expression.
There are a number of valid uses for calculated options, and there are
also many cases where some other CDL facility would be more
appropriate. Valid uses of calculated options include the following:
On some target hardware a particular feature may be user-configurable,
while on other targets it is fixed. For example some processors can
operate in either big-endian or little-endian mode, while other
processors do not provide any choice. It is possible to have an
option CYGARC_BIGENDIAN which is calculated in
some architectural HAL packages but user-modifiable in others.
Calculated options can provide an alternative way for one package to
affect the behavior of another one. Suppose a package may provide two
possible implementations, a preferred one involving self-modifying
code and a slower alternative. If the system involves a ROM bootstrap
then the slower alternative must be used, but it would be
inappropriate to modify the startup option in every HAL to impose
constraints on this package. Instead it is possible to have a
calculated option whose value is
{ CYG_HAL_STARTUP == "ROM" },
and which has appropriate consequences. Arguably this is a spurious
example, and it should be a user choice whether or not to use
self-modifying code with a default_value based on
CYG_HAL_STARTUP, but that is for the component
writer to decide.
Sometimes it should be possible to perform a particular test at
compile-time, for example by using a C preprocessor
#if construct. However the preprocessor has only
limited functionality, for example it cannot perform string
comparisons. CDL expressions are more powerful.
Occasionally a particular sub-expression may occur multiple times in
a CDL script. If the sub-expression is sufficiently complex then it
may be worthwhile to have a calculated option whose value is the
sub-expression, and then reference that calculated option in the
appropriate places.
Alternatives to using calculated options include the following:
CDL interfaces are a form of
calculated option intended as an abstraction mechanism. An interface
can be used to express the concept of any
scheduler, as opposed to a specific one such as the bitmap scheduler.
If a calculated option would serve only to add additional information
to a configuration header file, it may be possible to achieve the same
effect using a define_proc
property or one of the other properties related to header file
generation.
Tip: If the first entry in a calculated expression is a negative
number, for example calculated -1 then this
can be misinterpreted as an option instead of as part of the
expression. Currently the calculated property does not take any
options, but this may change in future. Option processing halts at the
sequence --, so the desired value can be expressed
safely using
calculated -- -1
Warning
Some of the CDL scripts in current eCos releases make excessive use
of calculated options. This is partly because the recommended
alternatives were not always available at the time the scripts were
written. It is also partly because there is still some missing
functionality, for example define_proc properties cannot yet access
the configuration data so it may be necessary to use calculated
properties to access the data and perform the desired manipulation via
a CDL expression. New scripts should use calculated options only in
accordance with the guidelines.
Note: For options with the booldata flavor the current CDL syntax does not
allow the enabled flag and the value to be calculated separately.
Functionality to permit this may be added in a future release.
Note: It has been suggested that having options which are not
user-modifiable is potentially confusing, and that a top-level
cdl_constant command should be added to the
language instead of or in addition to the calculated property. Such
a change is under consideration. However because the value of a
calculated option can depend on other options, it is not necessarily
constant.
Example
# A constant on some target hardware, perhaps user-modifiable on other
# targets.
cdl_option CYGNUM_HAL_RTC_PERIOD {
display "Real-time clock period"
flavor data
calculated 12500
}
