Subversion Repositories openrisc_2011-10-31

&eCos; was designed from the very beginning as a configurable

component architecture. The core &eCos; system consists of a number of

different components such as the kernel, the C library, an

infrastructure package. Each of these provides a large number of

configuration options, allowing application developers to build a

system that matches the requirements of their particular application.

To manage the potential complexity of multiple components and lots of

configuration options, &eCos; comes with a component framework: a

collection of tools specifically designed to support configuring

multiple components. Furthermore this component framework is

extensible, allowing additional components to be added to the system

at any time.

The &eCos; component architecture involves a number of key concepts.

The phrase component framework is used to describe

the collection of tools that allow users to configure a system and

administer a component repository. This includes the 

class="software">ecosconfig command line tool, the

graphical configuration tool, and the package administration tool.

Both the command line and graphical tools are based on a single

underlying library, the &CDL; library.

The option is the basic unit of configurability. Typically each option

corresponds to a single choice that a user can make. For example there

is an option to control whether or not assertions are enabled, and the

kernel provides an option corresponding to the number of scheduling

priority levels in the system. Options can control very small amounts

of code such as whether or not the C library's

strtok gets inlined. They can also control quite

large amounts of code, for example whether or not the

printf supports floating point conversions.

Many options are straightforward, and the user only gets to choose

whether the option is enabled or disabled. Some options are more

complicated, for example the number of scheduling priority levels is a

number that should be within a certain range. Options should always

start off with a sensible default setting, so that it is not necessary

for users to make hundreds of decisions before any work can start on

developing the application. Once the application is running the

various configuration options can be used to tune the system for the

specific needs of the application.

The component framework allows for options that are not directly

user-modifiable. Consider the case of processor endianness: some

processors are always big-endian or always little-endian, while with

other processors there is a choice. Depending on the user's choice of

target hardware, endianness may or may not be user-modifiable.

A component is a unit of functionality such as a particular kernel

scheduler or a device driver for a specific device. A component is

also a configuration option in that users may want to enable

or disable all the functionality in a component. For example, if a

particular device on the target hardware is not going to be used by

the application, directly or indirectly, then there is no point in

having a device driver for it. Furthermore disabling the device driver

should reduce the memory requirements for both code and data.

Components may contain further configuration options. In the case of a

device driver, there may be options to control the exact behavior of

that driver. These will of course be irrelevant if the driver as a

whole is disabled. More generally options and components live in a

hierarchy, where any component can contain options specific to that

component and further sub-components. It is possible to view the

entire &eCos; kernel as one big component, containing sub-components

for scheduling, exception handling, synchronization primitives, and so

on. The synchronization primitives component can contain further

sub-components for mutexes, semaphores, condition variables, event

flags, and so on. The mutex component can contain configuration

options for issues like priority inversion support.

A package is a special type of component. Specifically, a package is

the unit of distribution of components. It is possible to create a

distribution file for a package containing all of the source code,

header files, documentation, and other relevant files. This

distribution file can then be installed using the appropriate tool.

Afterwards it is possible to uninstall that package, or to install a

later version. The core &eCos; distribution comes with a number of

packages such as the kernel and the infrastructure. Other packages

such as network stacks can come from various different sources and can

be installed alongside the core distribution.

Packages can be enabled or disabled, but the user experience is a

little bit different. Generally it makes no sense for the tools to

load the details of every single package that has been installed. For

example, if the target hardware uses an ARM processor then there is no

point in loading the HAL packages for other architectures and

displaying choices to the user which are not relevant. Therefore

enabling a package means loading its configuration data into the

appropriate tool, and disabling a package is an unload operation. In

addition, packages are not just enabled or disabled: it is also

possible to select the particular version of a package that should be

used.

A configuration is a collection of user choices. The various

tools that make up the component framework deal with entire

configurations. Users can create a new configuration, output a

savefile (by default ecos.ecc), manipulate a

configuration, and use a configuration to generate a build tree prior

to building &eCos; and any other packages that have been selected.

A configuration includes details such as which packages have been

selected, in addition to finer-grained information such as which

options in those packages have been enabled or disabled by the user.

The target is the specific piece of hardware on which the application

is expected to run. This may be an off-the-shelf evaluation board, a

piece of custom hardware intended for a specific application, or it

could be something like a simulator. One of the steps when creating a

new configuration is need to specify the target. The component

framework will map this on to a set of packages that are used to

populate the configuration, typically HAL and device driver packages,

and in addition it may cause certain options to be changed from their

default settings to something more appropriate for the

specified target.

A template is a partial configuration, aimed at providing users with

an appropriate starting point. &eCos; is shipped with a small number

of templates, which correspond closely to common ways of using the

system. There is a minimal template which provides very little

functionality, just enough to bootstrap the hardware and then jump

directly to application code. The default template adds additional

functionality, for example it causes the kernel and C library packages

to be loaded as well. The uitron template adds further functionality

in the form of a &uITRON; compatibility layer. Creating a new

configuration typically involves specifying a template as well as a

target, resulting in a configuration that can be built and linked with

the application code and that will run on the actual hardware. It is

then possible to fine-tune configuration options to produce something

that better matches the specific requirements of the application.

The component framework needs a certain amount of information about

each option. For example it needs to know what the legal values are,

what the default should be, where to find the on-line documentation if

the user needs to consult that in order to make a decision, and so on.

These are all properties of the option. Every option (including

components and packages) consists of a name and a set of properties.

Choices must have consequences. For an &eCos; configuration the main

end product is a library that can be linked with application code, so

the consequences of a user choice must affect the build process. This

happens in two main ways. First, options can affect which files get

built and end up in the library. Second, details of the current option

settings get written into various configuration header files using C

preprocessor #define directives, and package source

code can #include these configuration headers and

adapt accordingly. This allows options to affect a package at a very

fine grain, at the level of individual lines in a source file if

desired. There may be other consequences as well, for example there

are options to control the compiler flags that get used during the

build process.

Configuration choices are not independent. The C library can provide

thread-safe implementations of functions like

rand, but only if the kernel provides support for

per-thread data. This is a constraint: the C library option has a

requirement on the kernel. A typical configuration involves a

considerable number of constraints, of varying complexity: many

constraints are straightforward, option A requires

option B, or option C precludes

option D. Other constraints can be more

complicated, for example option E may require the

presence of a kernel scheduler but does not care whether it is the

bitmap scheduler, the mlqueue scheduler, or something else.

Another type of constraint involves the values that can be used for

certain options. For example there is a kernel option related to the

number of scheduling levels, and there is a legal values constraint on

this option: specifying zero or a negative number for the number of

scheduling levels makes no sense.

As the user manipulates options it is possible to end up with an

invalid configuration, where one or more constraints are not

satisfied. For example if kernel per-thread data is disabled but the C

library's thread-safety options are left enabled then there are

unsatisfied constraints, also known as conflicts. Such conflicts will

be reported by the configuration tools. The presence of conflicts does

not prevent users from attempting to build &eCos;, but the

consequences are undefined: there may be compile-time failures, there

may be link-time failures, the application may completely fail to run,

or the application may run most of the time but once in a while there

will be a strange failure… Typically users will want to resolve

all conflicts before continuing.

To make things easier for the user, the configuration tools contain an

inference engine. This can examine a conflict in a particular

configuration and try to figure out some way of resolving the

conflict. Depending on the particular tool being used, the inference

engine may get invoked automatically at certain times or the user may

need to invoke it explicitly. Also depending on the tool, the

inference engine may apply any solutions it finds automatically or it

may request user confirmation.

The configuration tools require information about the various options

provided by each package, their consequences and constraints, and

other properties such as the location of on-line documentation. This

information has to be provided in the form of &CDL; scripts. CDL

is short for Component Definition Language, and is specifically

designed as a way of describing configuration options.

A typical package contains the following:

Some number of source files which will end up in a library. The

application code will be linked with this library to produce an

executable. Some source files may serve other purposes, for example to

provide a linker script.

Exported header files which define the interface provided by the

package.

On-line documentation, for example reference pages for each exported

function.

Some number of test cases, shipped in source format, allowing users to

check that the package is working as expected on their particular

hardware and in their specific configuration.

One or more &CDL; scripts describing the package to the configuration

system.

Not all packages need to contain all of these. For example some

packages such as device drivers may not provide a new interface,

instead they just provide another implementation of an existing

interface. However all packages must contain a &CDL; script that

describes the package to the configuration tools.

All &eCos; installations include a component repository. This is a

directory structure where all the packages get installed. The

component framework comes with an administration tool that allows new

packages or new versions of a package to be installed, old packages to

be removed, and so on. The component repository includes a simple

database, maintained by the administration tool, which contains

details of the various packages.

Generally application developers do not need to modify anything inside

the component repository, except by means of the administration tool.

Instead their work involves separate build and install trees. This

allows the component repository to be treated as a read-only resource

that can be shared by multiple projects and multiple users. Component

writers modifying one of the packages do need to manipulate files in

the component repository.

The &eCos; component framework places a great deal of emphasis on

configurability. The fundamental goal is to allow large parts of

embedded applications to be constructed from re-usable software

components, which does not a priori require that those components be

highly configurable. However embedded application development often

involves some serious constraints.

Many embedded applications have to work with very little memory, to

keep down manufacturing costs. The final application image that will

get blown into EPROM's or used to manufacture ROMs should contain only

the code that is absolutely necessary for the application to work, and

nothing else. If a few tens of kilobytes are added unnecessarily to a

typical desktop application then this is regrettable, but is quite

likely to go unnoticed. If an embedded application does not fit on the

target hardware then the problem is much more serious. The component

framework must allow users to configure the components so that any

unnecessary functionality gets removed.

Many embedded applications need deterministic behavior so that they

can meet real-time requirements. Such deterministic behavior can

often be provided, but at a cost in terms of code size, slower

algorithms, and so on. Other applications have no such real-time

requirements, or only for a small part of the overall system, and the

bulk of the system should not suffer any penalties. Again the

component framework must allow the users control over the timing

behavior of components.

Embedded systems tend to be difficult to debug. Even when it is

possible to get information out of the target hardware by means other

than flashing an LED, the more interesting debugging problems are

likely to be timing-related and hence very hard to reproduce and track

down. The re-usable components can provide debugging assistance in

various ways. They can provide functionality that can be exploited by

source level debuggers such as gdb, for example per-thread debugging

information. They can also contain various assertions so that problems

can be detected early on, tracing mechanisms to figure out what

happened before the assertion failure, and so on. Of course all of

these involve overheads, especially code size, and affect the timing.

Allowing users to control which debugging features are enabled for any

given application build is very desirable.

However, although it is desirable for re-usable components to provide

appropriate configuration options this is not required. It is possible

to produce a package which does not provide a single configuration

option — although the user still gets to choose

whether or not to use the package. In such cases it is still necessary

to provide a minimal CDL script, but its main purpose would be to

integrate the package with the component framework's build system.

The purpose of configurability is to control the behavior of

components. A scheduler component may or may not support time slicing;

it may or may not support multiple priorities; it may or may not

perform error checking on arguments passed to the scheduler routines.

In the context of a desktop application a button widget may contain

some text or it may contain a picture; the text may be displayed in a

variety of fonts; the foreground and background color may vary. When

an application uses a component there must be some way of specifying

the desired behavior. The component writer has no way of knowing in

advance exactly how a particular component will end up being used.

One way to control the behavior is at run time. The application

creates an instance of a button object, and then instructs this object

to display either text or a picture. No special effort by the

application developer is required, since a button can always support

all desired behavior. There is of course a major disadvantage in

terms of the size of the final application image: the code that gets

linked with the application has to provide support for all possible

behavior, even if the application does not require it.

Another approach is to control the behavior at link-time, typically

by using inheritance in an object-oriented language. The button

library provides an abstract base class Button

and derived classes TextButton and

PictureButton. If an application only uses text

buttons then it will only create objects of type

TextButton, and the code for the

PictureButton class does not get used. In

many cases this approach works rather well and reduces the final image

size, but there are limitations. The main one is that you can only

have so many derived classes before the system gets unmanageable: a

derived class

TextButtonUsingABorderWidthOfOnePlusAWhiteBackgroundAndBlackForegroundAndATwelvePointTimesFontAndNoErrorCheckingOrAssertions

is not particularly sensible as far as most application developers are

concerned.

The &eCos; component framework allows the behavior of components to

be controlled at an even earlier time: when the component source code

gets compiled and turned into a library. The button component could

provide options, for example an option that only text buttons need to

be supported. The component gets built and becomes part of a library

intended specifically for the application, and the library will

contain only the code that is required by this application and nothing

else. A different application with different requirements would need

its own version of the library, configured separately.

In theory compile-time configurability should give the best possible

results in terms of code size, because it allows code to be controlled

at the individual statement level rather than at the function or

object level. Consider an example more closely related to embedded

systems, a package to support multi-threading. A standard routine

within such a package allows applications to kill threads

asynchronously: the POSIX routine for this is

pthread_cancel; the equivalent routine in &uITRON;

is ter_tsk. These routines themselves tend to

involve a significant amount of code, but that is not the real

problem: other parts of the system require extra code and data for the

kill routine to be able to function correctly. For example if a thread

is blocked while waiting on a mutex and is killed off by another

thread then the kill operation may have to do two things: remove the

thread from the mutex's queue of waiting threads; and undo the

effects, if any, of priority inheritance. The implementation requires

extra fields in the thread data structure so that the kill routine

knows about the thread's current state, and extra code in the mutex

routines to fill in and clear these extra fields correctly.

Most embedded applications do not require the ability to kill off a

thread asynchronously, and hence the kill routine will not get linked

into the final application image. Without compile-time configurability

this would still mean that the mutex code and similar parts of the

system contain code and data that serve no useful purpose in this

application. The &eCos; approach allows the user to select that the

thread kill functionality is not required, and all the components can

adapt to this at compile-time. For example the code in the mutex lock

routine contains statements to support the killing of threads, but

these statements will only get compiled in if that functionality is

required. The overall result is that the final application image

contains only the code and data that is really needed for the

application to work, and nothing else.

Of course there are complications. To return to the button example,

the application code might only use text buttons directly, but it

might also use some higher-level widget such as a file selector and

this file selector might require buttons with pictures. Therefore the

button code must still be compiled to support pictures as well as

text. The configuration tools must be aware of the dependencies

between components and ensure that the internal constraints are met,

as well as the external requirements of the application code. An area

of particular concern is conflicting requirements: a button component

might be written in such a way that it can only support either text

buttons or picture buttons, but not both in one application; this

would represent a weakness in the component itself rather than in the

component framework as a whole.

Compile-time configurability is not intended to replace the other

approaches but rather to complement them. There will be times when

run-time selection of behavior is desirable: for example an

application may need to be able to change the baud rate of a serial

line, and the system must then provide a way of doing this at

run-time. There will also be times when link-time selection is

desirable: for example a C library might provide two different random

number routines rand and

lrand48; these do not affect other code so there

is no good reason for the C library component not to provide both of

these, and allow the application code to use none, one, or both of

them as appropriate; any unused functions will just get eliminated at

link-time. Compile-time selection of behavior is another option, and

it can be the most powerful one of the three and the best suited to

embedded systems development.

Components can support configurability in varying degrees. It is not

necessary to have any configuration options at all, and the only user

choice is whether or not to load a particular package. Alternatively

it is possible to implement highly-configurable code. As an example

consider a typical facility that is provided by many real-time

kernels, mutex locks. The possible configuration options include:

If no part of the application and no other component requires mutexes

then there is no point in having the mutex code compiled into a

library at all. This saves having to compile the code. In addition

there will never be any need for the user to configure the detailed

behavior of mutexes. Therefore the presence of mutexes is a

configuration option in itself.

Even if the application does make use of mutexes directly or

indirectly, this does not mean that all mutex functions have to be

included. The minimum functionality consists of lock and unlock

functions. However there are variants of the locking primitive such as

try-lock and try-with-timeout which may or may not be needed.

Generally it will be harmless to compile the try-lock function even if

it is not actually required, because the function will get eliminated

at link-time. Some users might take the view that the try-lock

function should never get compiled in unless it is actually needed, to

reduce compile-time and disk usage. Other users might argue that there

are very few valid uses for a try-lock function and it should not be

compiled by default to discourage incorrect uses. The presence of a

try-lock function is a possible configuration option, although it may

be sensible to default it to true.

The try-with-timeout variant is more complicated because it adds a

dependency: the mutex code will now rely on some other component to

provide a timer facility. To make things worse the presence of this

timer might impact other components, for example it may now be

necessary to guard against timer interrupts, and thus have an

insidious effect on code size. The presence of a lock-with-timeout

function is clearly a sensible configuration option, but the default

value is less obvious. If the option is enabled by default then the

final application image may end up with code that is not actually

essential. If the option is disabled by default then users will have

to enable the option somehow in order to use the function, implying

more effort on the part of the user. One possible approach is to

calculate the default value based on whether or not a timer component

is present anyway.

The application may or may not require the ability to create and

destroy mutexes dynamically. For most embedded systems it is both less

error-prone and more efficient to create objects like mutexes

statically. Dynamic creation of mutexes can be implemented using a

pre-allocated pool of mutex objects, involving some extra code to

manipulate the pool and an additional configuration option to define

the size of the pool. Alternatively it can be implemented using a

general-purpose memory allocator, involving quite a lot of extra code

and configuration options. However this general-purpose memory

allocator may be present anyway to support the application itself or

some other component. The ability to create and destroy mutexes

dynamically is a configuration option, and there may not be a sensible

default that is appropriate for all applications.

An important issue for mutex locks is the handling of priority

inversion, where a high priority thread is prevented from running

because it needs a lock owned by a lower priority thread. This is only

an issue if there is a scheduler with multiple priorities: some

systems may need multi-threading and hence synchronization primitives,

but a single priority level may suffice. If priority inversion is a

theoretical possibility then the application developer may still want

to ignore it because the application has been designed such that the

problem cannot arise in practice. Alternatively the developer may want

some sort of exception raised if priority inversion does occur,

because it should not happen but there may still be bugs in the code.

If priority inversion can occur legally then there are three main ways

of handling it: priority ceilings, priority inheritance, and ignoring

the problem. Priority ceilings require little code but extra effort on

the part of the application developer. Priority inheritance requires

more code but is automatic. Ignoring priority inversion may or may not

be acceptable, depending on the application and exactly when priority

inversion can occur. Some of these choices involve additional

configuration options, for example there are different ways of raising

an exception, and priority inheritance may or may not be applied

recursively.

As a further complication some mutexes may be hidden inside a

component rather than being an explicit part of the application. For

example, if the C library is configured to provide a

malloc call then there may be an associated mutex

to make the function automatically thread-safe, with no need for

external locking. In such cases the memory allocation component of the

C library can impose a constraint on the kernel, requiring that

mutexes be provided. If the user attempts to disable mutexes anyway

then the configuration tools will report a conflict.

The mutex code should contain some general debugging code such as

assertions and tracing. Usually such debug support will be enabled or

disabled at a coarse level such as the entire system or everything

inside the kernel, but sometimes it will be desirable to enable the

support more selectively. One reason would be memory requirements: the

target may not have enough memory to hold the system if all debugging

is enabled. Another reason is if most of the system is working but

there are a few problems still to resolved; enabling debugging in the

entire system might change the system's timing behavior too much, but

enabling some debug options selectively can still be useful. There

should be configuration options to allow specific types of debugging

to be enabled at a fine-grain, but with default settings inherited

from an enclosing component or from global settings.

The mutex code may contain specialized code to interact

with a debugging tool running on the host. It should be

possible to enable or disable this debugging code, and there may

be additional configuration options controlling the detailed

behavior.

Altogether there may be something like ten to twenty configuration

options that are specific to the mutex code. There may be a similar

number of additional options related to assertions and other debug

facilities. All of the options should have sensible default values,

possibly fixed, possibly calculated depending on what is happening

elsewhere in the configuration. For example the default setting for

an assertion option should generally inherit from a kernel-wide

assertion control option, which in turn inherits from a global option.

This allows users to enable or disable assertions globally or at

a more fine-grained level, as desired.

Different components may be configurable to different degrees, ranging

from no options at all to the fine-grained configurability of the

above mutex example (or possibly even further). It is up to component

writers to decide what options should be provided and how best to

serve the needs of application developers who want to use that

component.

Large parts of &eCos; were developed concurrently with the development

of the configuration technology, or in some cases before design work

on that technology was complete. As a consequence the various &eCos;

packages often make only limited use of the available functionality.

This situation is expected to change over time. It does mean that many

of the descriptions in this guide will not correspond exactly to how

the &eCos; packages work right now, but rather to how they could work.

Some of the more extreme discrepancies such as the location of on-line

documentation in the component repository will be mentioned in the

appropriate places in the guide.

A consequence of this is that developers of new components can look at

existing &CDL; scripts for examples, and discover discrepancies

between what is recommended in this guide and what actually happens at

present. In such cases this guide should be treated as authoritative.

It is also worth noting that the current component framework is not

finished. Various parts of this guide will refer to possible changes

and enhancements in future versions. Examining the source code of the

configuration tools may reveal hints about other likely developments,

and there are many more possible enhancements which only exist at a

conceptual level right now.