Why Linux?

Intelligent dedicated systems and appliances used in interface, monitoring, communications, and control applications increasingly demand the services of a sophisticated, state-of-the-art operating system. Many such systems require advanced capabilities like: high resolution and user-friendly graphical user interfaces (GUIs); TCP/IP connectivity; substitution of reliable (and low… power) flash memory solid state disk for conventional disk drives; support for 32-bit ultra-high-speed CPUs; the use of large memory arrays; and seemingly infinite capacity storage devices including CD-ROMs and hard disks.

This is not the stuff of yesteryear's “standalone” code, “roll-your-own” kernels, or “plain old DOS”. No, those days are gone — forever!

Then too, consider the rapidly accelerating pace of hardware and chipset innovation — accompanied by extremely rapid obsolescence of the older devices. Combine these two, and you can see why it's become an enormous challenge for commercial RTOS vendors to keep up with the constant churning of hardware devices. Supporting the newest devices in a timely manner — even just to stay clear of the unrelenting steamroller of chipset obsolescence — takes a large and constant resource commitment. If it's a struggle for the commercial RTOS vendors to keep up, going it alone by writing standalone code or a roll-your-own kernel certainly makes no sense.

With the options narrowing, embedded system developers find themselves faced with a dilemma:

• On the one hand, today's highly sophisticated and empowered intelligent embedded systems — based on the newest chips and hardware capabilities — demand nothing less than the power, sophistication, and currency of support provided by a popular high-end operating system like Windows.

• On the other hand, embedded systems demand extremely high reliability (for non-stop, unattended operation) plus the ability to customize the OS to match an application's unique requirements.

Here's the quandary: general purpose desktop OSes (like Windows) aren't well suited to the unique needs of appliance-like embedded systems. However, commercial RTOSes, while designed to satisfy the reliability and configuration flexibility requirements of embedded applications, are increasingly less desirable due to their lack of standardization and their inability to keep pace with the rapid evolution of technology.

What's a developer to do?

Fortunately, a new and exciting alternative has emerged: open-source Linux. Linux offers powerful and sophisticated system management facilities, a rich cadre of device support, a superb reputation for reliability and robustness, and extensive documentation. Best of all (say system developers), Linux is available at no charge — and with completely free source code.

Is Linux, like Windows, too large and demanding of system resources to fit the constraints of embedded systems? Unlike Windows, Linux is inherently modular and can be easily scaled into compact configurations — barely larger than DOS — that can even fit on a single floppy. What's more, since Linux source code is freely available, it's possible to customize the OS according to unique embedded system requirements.

It's not surprising, then, that open-source Linux has created a new OS development and support paradigm wherein thousands of developers continually contribute to a constantly evolving Linux code base. In addition, dozens of Linux-oriented software companies have sprung up — eager to support the needs of developers building a wide range of applications, ranging from factory automation to intelligent appliances.

Which Linux?

Because Linux is openly and freely available in source form, there are many available variations and configurations. There are implementations specifically for “thin server” or “firewall” applications, small footprint versions, and real-time enhanced versions. There are also Linux ports for non-“x86” CPUs, including PowerPC, RISC, 68xxx, and microcontrollers.

So, how do you decide which distribution to use? That depends.

First, realize that all Linux distributions are variations on the same theme — that is, they are pretty much collections of the same basic components, including the Linux kernel, command shells (command processors), and many common utilities. The differences tend to center around which of the many hundreds of Linux utilities have been included, what extras are included (open-source or proprietary) on the “distribution CD” (e.g. Star Office is included in some cases), and how the installation process is managed.

Even though Linux is free, purchasing a “commercial” Linux distribution can have many advantages including development tools, useful utilities, and support. The many companies who are now building businesses around distributing and supporting Linux are busy investing in developing tools and services to differentiate their Linux offerings from the pack. Some of the special capabilities being developed include:

• Installation tools to automate and simplify the process of generating a Linux configuration that is tuned to a specific target's hardware setup can save weeks, or even months, of development effort.

• A variety of Windows-like GUIs that vary in size, appearance, features, capabilities to support a wide range of embedded requirements.

• Support for the specific needs of various embedded and real-time computing platforms and environments (e.g. special CompactPCI system features).

Yet despite this great diversity, all Linux implementations share a common core, including: kernel; drivers; several popular GUIs; utilities.

Small foot-print Linux

For many embedded systems, the main challenge in embedding Linux is to minimize system resource requirements in order to fit within constraints such as RAM, solid state disk (SSD), processor speed, and power consumption. Embedded operation may require booting from (and fitting within) a DiskOnChip or CompactFlash SSD; or booting and running without a display and keyboard (“headless” operation); or loading the application from a remote device via an Ethernet LAN connection.

There are many sources of ready-made small foot-print Linux. Included among these are a growing number of application-oriented Linux configurations and distributions that are tuned to specific applications. Some examples are routers, firewalls, internet/network appliances, network servers, gateways, etc.

You may also opt to create your own flavor of embedded Linux, starting from a standard distribution and leaving out modules you don't need. Even so, you should consider jump-starting your efforts by beginning with someone else's working configuration, since the source code of their version will be available for that purpose. Best of all, this sort of building on the efforts of others in the Linux community is not only completely legal — it's encouraged!

Real-time Linux

Many embedded systems require predictable and bounded responses to real-world events. Such “real-time” systems include factory automation, data acquisition and control systems, audio/video applications, and many other computerized products and devices. What's a “real-time system”? The commonly accepted definition of “real-time” performance is that real-world events must be responded to within a defined, predicable, and relatively short time interval.

Although Linux is not a real-time operating system (the Linux kernel does not provide the required event prioritization and preemption functions), there are currently several add-on options available that can bring real-time capabilities to Linux-based systems. The most common method is the dual-kernel approach. Using this approach, a general purpose (non-real-time) OS runs as a task under a real-time kernel. The general purpose OS provides functions such as disk read/write, LAN/communications, serial/parallel I/O, system initialization, memory management, etc., while the real-time kernel handles real-world event processing. You might think of this as a “have your cake and eat it too” strategy, because it can preserve the benefits of a popular general purpose OS, while adding the capabilities of a real-time OS. In the case of Linux, you can retain full compatibility with standard Linux, while adding real-time functions in a non-interfering manner.

Of course, you could also dive in and modify Linux to convert it into a real-time operating system, since its source is openly available. But if you do this, you will be faced with the severe disadvantage of having a real-time Linux that can't keep pace, either features-wise or drivers-wise, with mainstream Linux. In short, your customized Linux won't benefit from the continual Linux evolution that results from the pooled efforts of thousands of developers world-wide.

Who needs real-time, anyhow?

In any case, how many applications really require real-time enhancements to Linux? Bear in mind that “real-time” is a relative, not absolute, expression. As mentioned, a real-time system must handle real-world tasks within acceptable — and predictable — time windows. Although CPUs run at ever increasing speeds (approaching 500MHz), the world around them goes on at a constant speed. Therefore, real-time performance is becoming ever easier to achieve.

Back when the “traditional” RTOSes were first developed, embedded systems depended on 4- and 8-bit CPUs clocked at single-digit-megahertz speeds and running out of kilobytes of RAM. Now, with CPUs speeding along at up to 500 MHz, and with memories measured in the hundreds of megabytes, “real-time” performance is becoming less of a concern. The greater concern, these days, has become speed-to-market and sophistication of functionality. Where execution efficiency was the watchword of the CPU-bound past, protocols are the key to the Internet-centric future. (You might say, “It's the protocols, stupid!”)

Going soft

There's a term to describe the use of ordinary OSes in real-world applications with acceptable results: “soft real-time” In many systems, you can ensure that the real-world constraints of your application can be achieved without resorting to using a specialized RTOS. However, this only tends to be practical when required response times are in the milliseconds — not microseconds! Assuming that's the case, a minimally configured Linux, on a reasonably fast processor (486-133 or faster) without special real-time add-ons, may well suit your needs. If soft real-time sounds like what you need, you may want to check out a Linux add-on called Linux-SRT (SRT = soft real-time).

On the other hand, your system may indeed require microsecond-level response times. In that case, you can either dedicate an inexpensive microcontroller or DSP to handling the time-critical events, or you can use one of several available real-time Linux add-ons (e.g. RTLinux or RTAI).

Embedded and real-time Linux solution providers

Since Linux is free, how can anyone build a profitable business based on offering commercial Linux distributions? It's a lot like bottled water — basically, what you pay for is services: packaging, delivery, quality assurance, etc.

A word of caution: don't assume that every Linux related program you download from the web or obtain from a Linux CD can be freely reproduced and incorporated into the devices you develop. Some commercial Linux distributions that target embedded and real-time applications include proprietary third-party tools and utilities that require licensing and royalty payments if you incorporate them into multiple systems. In other words: read the fine print!

For now, however, licensed Linux system software is the exception rather than the rule. With the market placing such a high value on Linux and its associated software being open source and royalty-free, most Linux software companies serving the embedded and real-time Linux market have opted to build their businesses based on selling tools, offering engineering services, and providing technical support.

Gazing into the embedded Linux crystal ball . . .

Given the strong position of Microsoft Windows in the end user desktop/laptop market, it's not likely that the “average” desktop/laptop PC user will be running Linux any time soon. On the other hand, in embedded and real-time applications, where the OS is an underlying and hidden technology supporting appliance-like operation of a non-computer device, several key features of Linux are making it a growing preference among system developers:

• Source is available and free
  
• There are no runtime royalties
  
• Linux supports a vast array of devices
  
• Linux is truly a global standard
  
• Linux is sophisticated, efficient, robust, reliable, modular, and highly configurable

Time will tell, but it certainly looks like Linux has already altered the embedded and real-time operating system landscape in a fundamental and irreversible way. The result? Developers now have greater control over their embedded OS; manufacturers are spared the costs and headaches of software royalties; and end users get more value.

. . . and the Penguins of the South Pole are celebrating. ;)

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For further reading . . .

LinuxDevices.com has created a series of Embedded Linux Quick Reference Guides which represent a continually updated index to Embedded Linux software, hardware, tools, suppliers, resources, and applications. Use these handy guides as launching-pads whenever you set out on a quest for Embedded Linux information and solutions . . .

• The Embedded Linux Overview Quick Reference Guide — includes an introduction to Embedded Linux including a discussion of the features and benefits of using it, an index to the other sections of LinuxDevices.com's Embedded Linux Quick Reference Guide series, and a reading list of recommended articles and whitepapers.

• The Embedded Linux Distributions Quick Reference Guide — distributions and implementations of the Linux kernel and associated system software components that are tailored to fit the limited resources and other constraints of a wide range of intelligent devices and embedded systems.

• The Real-time Linux Software Quick Reference Guide — distributions and implementations of the Linux kernel, Linux add-ons, and other software that support the enhanced responsiveness required for process control, high speed communications, streaming media, and other real-time applications.

• The Embedded Linux GUI/Windowing Quick Reference Guide — windowing and graphics software solutions that support Linux-based intelligent devices and embedded systems, including graphical user interfaces (GUIs), window managers, and browsers.

• The Linux-friendly System-on-Chip Quick Reference Guide — a survey of highly integrated system-on-chip processors that can be used as the basis of Linux-based embedded systems and devices.

• The Linux-friendly Embedded Single Board Computers Quick Reference Guide — a tutorial and perspective on the rapidly diverging shapes, sizes, and architectures for embedded single-board computers (SBCs), and highlights the growing importance of Linux to the embedded SBC market. Popular embedded SBC form-factors are defined and described, and sources for product are identified.

• The Linux-based “Cool Devices” Quick Reference Guide — a catalog of end products in which Linux serves as the embedded operating system, including webpads, cell phones, set-top boxes, Internet appliances, entertainment appliances, and other intelligent devices.

• The Linux-PDA and PDA-Linux Quick Reference Guide — an overview of available and developing support for Linux on PDAs. Includes PDAs that use Linux as their internal operating system, Linux-based operating system packages that support multiple PDAs, and a list of relevant articles for further reading.

Please note that these guides are updated frequently, so be sure to check back periodically for the latest info. Additional guides will be added to this series on an ongoing basis.

 
This article was originally published on LinuxDevices.com and has been donated to the open source community by QuinStreet Inc. Please visit LinuxToday.com for up-to-date news and articles about Linux and open source.