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>Degrees of Configurability</TITLE
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> Component Writer's Guide</TH
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WIDTH="80%"
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>Chapter 1. Overview</TD
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NAME="OVERVIEW.DEGRESS">Degrees of Configurability</H1
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><P
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>Components can support configurability in varying degrees. It is not
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necessary to have any configuration options at all, and the only user
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choice is whether or not to load a particular package. Alternatively
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it is possible to implement highly-configurable code. As an example
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consider a typical facility that is provided by many real-time
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kernels, mutex locks. The possible configuration options include:</P
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><P
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></P
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TYPE="1"
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><LI
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><P
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>If no part of the application and no other component requires mutexes
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then there is no point in having the mutex code compiled into a
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library at all. This saves having to compile the code. In addition
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there will never be any need for the user to configure the detailed
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behavior of mutexes. Therefore the presence of mutexes is a
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configuration option in itself.</P
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></LI
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><LI
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><P
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>Even if the application does make use of mutexes directly or
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indirectly, this does not mean that all mutex functions have to be
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included. The minimum functionality consists of lock and unlock
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functions. However there are variants of the locking primitive such as
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try-lock and try-with-timeout which may or may not be needed.</P
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><P
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>Generally it will be harmless to compile the try-lock function even if
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it is not actually required, because the function will get eliminated
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at link-time. Some users might take the view that the try-lock
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function should never get compiled in unless it is actually needed, to
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reduce compile-time and disk usage. Other users might argue that there
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are very few valid uses for a try-lock function and it should not be
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compiled by default to discourage incorrect uses. The presence of a
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try-lock function is a possible configuration option, although it may
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be sensible to default it to true.</P
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><P
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>The try-with-timeout variant is more complicated because it adds a
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dependency: the mutex code will now rely on some other component to
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provide a timer facility. To make things worse the presence of this
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timer might impact other components, for example it may now be
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necessary to guard against timer interrupts, and thus have an
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insidious effect on code size. The presence of a lock-with-timeout
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function is clearly a sensible configuration option, but the default
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value is less obvious. If the option is enabled by default then the
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final application image may end up with code that is not actually
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essential. If the option is disabled by default then users will have
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to enable the option somehow in order to use the function, implying
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more effort on the part of the user. One possible approach is to
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calculate the default value based on whether or not a timer component
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is present anyway.</P
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></LI
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><LI
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><P
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>The application may or may not require the ability to create and
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destroy mutexes dynamically. For most embedded systems it is both less
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error-prone and more efficient to create objects like mutexes
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statically. Dynamic creation of mutexes can be implemented using a
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pre-allocated pool of mutex objects, involving some extra code to
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manipulate the pool and an additional configuration option to define
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the size of the pool. Alternatively it can be implemented using a
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general-purpose memory allocator, involving quite a lot of extra code
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and configuration options. However this general-purpose memory
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allocator may be present anyway to support the application itself or
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some other component. The ability to create and destroy mutexes
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dynamically is a configuration option, and there may not be a sensible
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default that is appropriate for all applications.</P
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></LI
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><LI
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><P
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>An important issue for mutex locks is the handling of priority
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inversion, where a high priority thread is prevented from running
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because it needs a lock owned by a lower priority thread. This is only
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an issue if there is a scheduler with multiple priorities: some
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systems may need multi-threading and hence synchronization primitives,
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but a single priority level may suffice. If priority inversion is a
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theoretical possibility then the application developer may still want
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to ignore it because the application has been designed such that the
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problem cannot arise in practice. Alternatively the developer may want
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some sort of exception raised if priority inversion does occur,
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because it should not happen but there may still be bugs in the code.
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If priority inversion can occur legally then there are three main ways
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of handling it: priority ceilings, priority inheritance, and ignoring
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the problem. Priority ceilings require little code but extra effort on
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the part of the application developer. Priority inheritance requires
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more code but is automatic. Ignoring priority inversion may or may not
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be acceptable, depending on the application and exactly when priority
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inversion can occur. Some of these choices involve additional
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configuration options, for example there are different ways of raising
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an exception, and priority inheritance may or may not be applied
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recursively.</P
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></LI
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><LI
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><P
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>As a further complication some mutexes may be hidden inside a
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component rather than being an explicit part of the application. For
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example, if the C library is configured to provide a
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<TT
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CLASS="FUNCTION"
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>malloc</TT
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> call then there may be an associated mutex
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to make the function automatically thread-safe, with no need for
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external locking. In such cases the memory allocation component of the
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C library can impose a constraint on the kernel, requiring that
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mutexes be provided. If the user attempts to disable mutexes anyway
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then the configuration tools will report a conflict.</P
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></LI
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><LI
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><P
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>The mutex code should contain some general debugging code such as
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assertions and tracing. Usually such debug support will be enabled or
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disabled at a coarse level such as the entire system or everything
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inside the kernel, but sometimes it will be desirable to enable the
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support more selectively. One reason would be memory requirements: the
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target may not have enough memory to hold the system if all debugging
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is enabled. Another reason is if most of the system is working but
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there are a few problems still to resolved; enabling debugging in the
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entire system might change the system's timing behavior too much, but
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enabling some debug options selectively can still be useful. There
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should be configuration options to allow specific types of debugging
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to be enabled at a fine-grain, but with default settings inherited
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from an enclosing component or from global settings.</P
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></LI
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><LI
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><P
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>The mutex code may contain specialized code to interact
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with a debugging tool running on the host. It should be
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possible to enable or disable this debugging code, and there may
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be additional configuration options controlling the detailed
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behavior.</P
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></LI
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></OL
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><P
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>Altogether there may be something like ten to twenty configuration
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options that are specific to the mutex code. There may be a similar
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number of additional options related to assertions and other debug
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facilities. All of the options should have sensible default values,
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possibly fixed, possibly calculated depending on what is happening
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elsewhere in the configuration. For example the default setting for
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an assertion option should generally inherit from a kernel-wide
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assertion control option, which in turn inherits from a global option.
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This allows users to enable or disable assertions globally or at
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a more fine-grained level, as desired.</P
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><P
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>Different components may be configurable to different degrees, ranging
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from no options at all to the fine-grained configurability of the
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above mutex example (or possibly even further). It is up to component
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writers to decide what options should be provided and how best to
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serve the needs of application developers who want to use that
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component.</P
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