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><A
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><A
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NAME="LANGUAGE.INTERFACE">Interfaces</H1
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NAME="LANGUAGE.INTERFACE">Interfaces</H1
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><P
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><P
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>For many configurability requirements, options provide sufficient
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>For many configurability requirements, options provide sufficient
|
expressive power. However there are times when a higher level of
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expressive power. However there are times when a higher level of
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abstraction is appropriate. As an example, suppose that some package
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abstraction is appropriate. As an example, suppose that some package
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relies on the presence of code that implements the standard kernel
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relies on the presence of code that implements the standard kernel
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scheduling interface. However the requirement is no more stringent
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scheduling interface. However the requirement is no more stringent
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than this, so the constraint can be satisfied by the mlqueue
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than this, so the constraint can be satisfied by the mlqueue
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scheduler, the bitmap scheduler, or any additional schedulers that may
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scheduler, the bitmap scheduler, or any additional schedulers that may
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get implemented in future. A first attempt at expressing the
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get implemented in future. A first attempt at expressing the
|
dependency might be:</P
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dependency might be:</P
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><TABLE
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><PRE
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><PRE
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CLASS="PROGRAMLISTING"
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CLASS="PROGRAMLISTING"
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> requires CYGSEM_KERNEL_SCHED_MLQUEUE || CYGSEM_KERNEL_SCHED_BITMAP</PRE
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> requires CYGSEM_KERNEL_SCHED_MLQUEUE || CYGSEM_KERNEL_SCHED_BITMAP</PRE
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></TD
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></TD
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></TABLE
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><P
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><P
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>This constraint will work with the current release, but it is limited.
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>This constraint will work with the current release, but it is limited.
|
Suppose there is a new release of the kernel which adds another
|
Suppose there is a new release of the kernel which adds another
|
scheduler such as a deadline scheduler, or suppose that there is a new
|
scheduler such as a deadline scheduler, or suppose that there is a new
|
third party package which adds such a scheduler. The package
|
third party package which adds such a scheduler. The package
|
containing the limited constraint would now have to be updated and
|
containing the limited constraint would now have to be updated and
|
another release made, with possible knock-on effects.</P
|
another release made, with possible knock-on effects.</P
|
><P
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><P
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><SPAN
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><SPAN
|
CLASS="APPLICATION"
|
CLASS="APPLICATION"
|
>CDL</SPAN
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>CDL</SPAN
|
> interfaces provide an abstraction mechanism: constraints can be
|
> interfaces provide an abstraction mechanism: constraints can be
|
expressed in terms of an abstract concept, for example
|
expressed in terms of an abstract concept, for example
|
“scheduler”, rather than specific implementations such as
|
“scheduler”, rather than specific implementations such as
|
<TT
|
<TT
|
CLASS="VARNAME"
|
CLASS="VARNAME"
|
>CYGSEM_KERNEL_SCHED_MLQUEUE</TT
|
>CYGSEM_KERNEL_SCHED_MLQUEUE</TT
|
> and
|
> and
|
<TT
|
<TT
|
CLASS="VARNAME"
|
CLASS="VARNAME"
|
>CYGSEM_KERNEL_SCHED_BITMAP</TT
|
>CYGSEM_KERNEL_SCHED_BITMAP</TT
|
>. Basically an interface
|
>. Basically an interface
|
is a calculated configuration option:</P
|
is a calculated configuration option:</P
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><TR
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><TD
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><TD
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><PRE
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><PRE
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CLASS="PROGRAMLISTING"
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CLASS="PROGRAMLISTING"
|
>cdl_interface CYGINT_KERNEL_SCHEDULER {
|
>cdl_interface CYGINT_KERNEL_SCHEDULER {
|
display "Number of schedulers in this configuration"
|
display "Number of schedulers in this configuration"
|
…
|
…
|
}</PRE
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}</PRE
|
></TD
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></TD
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></TR
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></TR
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></TABLE
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></TABLE
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><P
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><P
|
>The individual schedulers can then implement this interface:</P
|
>The individual schedulers can then implement this interface:</P
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><TABLE
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><TD
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><TD
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><PRE
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><PRE
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CLASS="PROGRAMLISTING"
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CLASS="PROGRAMLISTING"
|
>cdl_option CYGSEM_KERNEL_SCHED_MLQUEUE {
|
>cdl_option CYGSEM_KERNEL_SCHED_MLQUEUE {
|
display "Multi-level queue scheduler"
|
display "Multi-level queue scheduler"
|
default_value 1
|
default_value 1
|
implements CYGINT_KERNEL_SCHEDULER
|
implements CYGINT_KERNEL_SCHEDULER
|
…
|
…
|
}
|
}
|
|
|
cdl_option CYGSEM_KERNEL_SCHED_BITMAP {
|
cdl_option CYGSEM_KERNEL_SCHED_BITMAP {
|
display "Bitmap scheduler"
|
display "Bitmap scheduler"
|
default_value 0
|
default_value 0
|
implements CYGINT_KERNEL_SCHEDULER
|
implements CYGINT_KERNEL_SCHEDULER
|
…
|
…
|
}</PRE
|
}</PRE
|
></TD
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></TD
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></TR
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></TR
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></TABLE
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></TABLE
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><P
|
><P
|
>Future schedulers can also implement this interface. The value of an
|
>Future schedulers can also implement this interface. The value of an
|
interface, for the purposes of expression evaluation, is the number of
|
interface, for the purposes of expression evaluation, is the number of
|
active and enabled options which implement this interface. Packages
|
active and enabled options which implement this interface. Packages
|
which rely on the presence of a scheduler can impose constraints such
|
which rely on the presence of a scheduler can impose constraints such
|
as:</P
|
as:</P
|
><TABLE
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><TR
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><TD
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><TD
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><PRE
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><PRE
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CLASS="PROGRAMLISTING"
|
CLASS="PROGRAMLISTING"
|
> requires CYGINT_KERNEL_SCHEDULER</PRE
|
> requires CYGINT_KERNEL_SCHEDULER</PRE
|
></TD
|
></TD
|
></TR
|
></TR
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></TABLE
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></TABLE
|
><P
|
><P
|
>If none of the schedulers are enabled, or if the kernel package is not
|
>If none of the schedulers are enabled, or if the kernel package is not
|
loaded, then <TT
|
loaded, then <TT
|
CLASS="VARNAME"
|
CLASS="VARNAME"
|
>CYGINT_KERNEL_SCHEDULER</TT
|
>CYGINT_KERNEL_SCHEDULER</TT
|
> will evaluate
|
> will evaluate
|
to <TT
|
to <TT
|
CLASS="LITERAL"
|
CLASS="LITERAL"
|
>0</TT
|
>0</TT
|
>. If at least one scheduler is active and
|
>. If at least one scheduler is active and
|
enabled then the constraint will be satisfied.</P
|
enabled then the constraint will be satisfied.</P
|
><P
|
><P
|
>Because interfaces have a calculated value determined by the
|
>Because interfaces have a calculated value determined by the
|
implementors, the <SPAN
|
implementors, the <SPAN
|
CLASS="PROPERTY"
|
CLASS="PROPERTY"
|
>default_value</SPAN
|
>default_value</SPAN
|
> and <SPAN
|
> and <SPAN
|
CLASS="PROPERTY"
|
CLASS="PROPERTY"
|
>calculated</SPAN
|
>calculated</SPAN
|
> properties are not
|
> properties are not
|
applicable and should not appear in the body of a <TT
|
applicable and should not appear in the body of a <TT
|
CLASS="LITERAL"
|
CLASS="LITERAL"
|
>cdl_interface</TT
|
>cdl_interface</TT
|
>
|
>
|
command. Interfaces have the <TT
|
command. Interfaces have the <TT
|
CLASS="LITERAL"
|
CLASS="LITERAL"
|
>data</TT
|
>data</TT
|
> flavor by
|
> flavor by
|
default, but the <TT
|
default, but the <TT
|
CLASS="LITERAL"
|
CLASS="LITERAL"
|
>bool</TT
|
>bool</TT
|
> and
|
> and
|
<TT
|
<TT
|
CLASS="LITERAL"
|
CLASS="LITERAL"
|
>booldata</TT
|
>booldata</TT
|
> flavors may be specified instead. A
|
> flavors may be specified instead. A
|
<TT
|
<TT
|
CLASS="LITERAL"
|
CLASS="LITERAL"
|
>bool</TT
|
>bool</TT
|
> interface is disabled if there are no active
|
> interface is disabled if there are no active
|
and enabled implementors, otherwise it is enabled. A
|
and enabled implementors, otherwise it is enabled. A
|
<TT
|
<TT
|
CLASS="LITERAL"
|
CLASS="LITERAL"
|
>booldata</TT
|
>booldata</TT
|
> interface is disabled if there are no
|
> interface is disabled if there are no
|
active and enabled implementors, otherwise it is enabled and has a
|
active and enabled implementors, otherwise it is enabled and has a
|
value corresponding to the number of these implementors. Other
|
value corresponding to the number of these implementors. Other
|
properties such as <SPAN
|
properties such as <SPAN
|
CLASS="PROPERTY"
|
CLASS="PROPERTY"
|
>requires</SPAN
|
>requires</SPAN
|
> and <SPAN
|
> and <SPAN
|
CLASS="PROPERTY"
|
CLASS="PROPERTY"
|
>compile</SPAN
|
>compile</SPAN
|
> can be used as normal.</P
|
> can be used as normal.</P
|
><P
|
><P
|
>Some component writers will not want to use interfaces in this way.
|
>Some component writers will not want to use interfaces in this way.
|
The reasoning is that their code will only have been tested with the
|
The reasoning is that their code will only have been tested with the
|
existing schedulers, so the <SPAN
|
existing schedulers, so the <SPAN
|
CLASS="PROPERTY"
|
CLASS="PROPERTY"
|
>requires</SPAN
|
>requires</SPAN
|
> constraint needs to be
|
> constraint needs to be
|
expressed in terms of those schedulers; it is possible that the
|
expressed in terms of those schedulers; it is possible that the
|
component will still work with a new scheduler, but there are no
|
component will still work with a new scheduler, but there are no
|
guarantees. Other component writers may take a more optimistic view
|
guarantees. Other component writers may take a more optimistic view
|
and assume that their code will work with any scheduler until proven
|
and assume that their code will work with any scheduler until proven
|
otherwise. It is up to individual component writers to decide which
|
otherwise. It is up to individual component writers to decide which
|
approach is most appropriate in any given case.</P
|
approach is most appropriate in any given case.</P
|
><P
|
><P
|
>One common use for interfaces is to describe the hardware
|
>One common use for interfaces is to describe the hardware
|
functionality provided by a given target. For example the <SPAN
|
functionality provided by a given target. For example the <SPAN
|
CLASS="APPLICATION"
|
CLASS="APPLICATION"
|
>CDL</SPAN
|
>CDL</SPAN
|
>
|
>
|
scripts for a TCP/IP package might want to know whether or not the
|
scripts for a TCP/IP package might want to know whether or not the
|
target hardware has an ethernet interface. Generally it is not
|
target hardware has an ethernet interface. Generally it is not
|
necessary for the TCP/IP stack to know exactly which ethernet hardware
|
necessary for the TCP/IP stack to know exactly which ethernet hardware
|
is present, since there should be a device driver which implements the
|
is present, since there should be a device driver which implements the
|
appropriate functionality. In <SPAN
|
appropriate functionality. In <SPAN
|
CLASS="APPLICATION"
|
CLASS="APPLICATION"
|
>CDL</SPAN
|
>CDL</SPAN
|
> terms the device drivers should
|
> terms the device drivers should
|
implement an interface <TT
|
implement an interface <TT
|
CLASS="VARNAME"
|
CLASS="VARNAME"
|
>CYGHWR_NET_DRIVERS</TT
|
>CYGHWR_NET_DRIVERS</TT
|
>, and the
|
>, and the
|
<SPAN
|
<SPAN
|
CLASS="APPLICATION"
|
CLASS="APPLICATION"
|
>CDL</SPAN
|
>CDL</SPAN
|
> scripts for the TCP/IP stack can use this in appropriate
|
> scripts for the TCP/IP stack can use this in appropriate
|
expressions. </P
|
expressions. </P
|
><DIV
|
><DIV
|
CLASS="NOTE"
|
CLASS="NOTE"
|
><BLOCKQUOTE
|
><BLOCKQUOTE
|
CLASS="NOTE"
|
CLASS="NOTE"
|
><P
|
><P
|
><B
|
><B
|
>Note: </B
|
>Note: </B
|
>Using the term <SPAN
|
>Using the term <SPAN
|
CLASS="emphasis"
|
CLASS="emphasis"
|
><I
|
><I
|
CLASS="EMPHASIS"
|
CLASS="EMPHASIS"
|
>interface</I
|
>interface</I
|
></SPAN
|
></SPAN
|
> for this concept is
|
> for this concept is
|
sometimes confusing, since the term has various other meanings as
|
sometimes confusing, since the term has various other meanings as
|
well. In practice, it is often correct. If there is a configuration
|
well. In practice, it is often correct. If there is a configuration
|
option that implements a given <SPAN
|
option that implements a given <SPAN
|
CLASS="APPLICATION"
|
CLASS="APPLICATION"
|
>CDL</SPAN
|
>CDL</SPAN
|
> interface, then usually this
|
> interface, then usually this
|
option will enable some code that provides a particular interface at
|
option will enable some code that provides a particular interface at
|
the C or C++ level. For example an ethernet device driver implements
|
the C or C++ level. For example an ethernet device driver implements
|
the <SPAN
|
the <SPAN
|
CLASS="APPLICATION"
|
CLASS="APPLICATION"
|
>CDL</SPAN
|
>CDL</SPAN
|
> interface <TT
|
> interface <TT
|
CLASS="VARNAME"
|
CLASS="VARNAME"
|
>CYGHWR_NET_DRIVERS</TT
|
>CYGHWR_NET_DRIVERS</TT
|
>, and also
|
>, and also
|
implements a set of C functions that can be used by the TCP/IP stack.
|
implements a set of C functions that can be used by the TCP/IP stack.
|
Similarly <TT
|
Similarly <TT
|
CLASS="VARNAME"
|
CLASS="VARNAME"
|
>CYGSEM_KERNEL_SCHED_MLQUEUE</TT
|
>CYGSEM_KERNEL_SCHED_MLQUEUE</TT
|
> implements
|
> implements
|
the <SPAN
|
the <SPAN
|
CLASS="APPLICATION"
|
CLASS="APPLICATION"
|
>CDL</SPAN
|
>CDL</SPAN
|
> interface <TT
|
> interface <TT
|
CLASS="VARNAME"
|
CLASS="VARNAME"
|
>CYGINT_KERNEL_SCHEDULER</TT
|
>CYGINT_KERNEL_SCHEDULER</TT
|
> and
|
> and
|
also provides the appropriate scheduling functions.</P
|
also provides the appropriate scheduling functions.</P
|
></BLOCKQUOTE
|
></BLOCKQUOTE
|
></DIV
|
></DIV
|
></DIV
|
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>Values and Expressions</TD
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>Values and Expressions</TD
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