1.2 Real Life and Embedded Linux Systems
What types of embedded systems are built with Linux? Why do people choose Linux? What issues are specific to the use of Linux in embedded systems? How many people actually use Linux in their embedded systems? How do they use it? All these questions and many more come to mind when pondering the use of Linux in an embedded system. Finding satisfactory answers to the fundamental questions is an important part of building the system. This isn't just a general statement. These answers will help you convince management, assist you in marketing your product, and most of all, enable you to evaluate whether your initial expectations have been met.
1.2.1 Types of Embedded Linux Systems
We could use the traditional segments of embedded systems such as aerospace, automotive systems, consumer electronics, telecom, and so on to outline the types of embedded Linux systems, but this would provide no additional information in regard to the systems being designated, because embedded Linux systems may be structured alike regardless of the market segment. Rather, let's classify embedded systems by criteria that will provide actual information about the structure of the system: size, time constraints, networkability, and degree of user interaction.
The size of an embedded linux system is determined by a number of different factors. First, there is physical size. Some systems can be fairly large, like the ones built out of clusters, while others are fairly small, like the Linux watch built by IBM. Most importantly, there are the size attributes of the various electronic components of the system, such as the speed of the CPU, the size of the RAM, and the size of the permanent storage.
In terms of size, I will use three broad categories of systems: small, medium, and large. Small systems are characterized by a low-powered CPU with a minimum of 2 MB of ROM and 4 MB of RAM. This isn't to say Linux won't run in smaller memory spaces, but it will take you some effort to do so. If you plan to run Linux in a smaller space than this, think about starting your work from one of the various distributions that put Linux on a single floppy. If you come from an embedded systems background, you may find that you could do much more using something other than Linux in such a small system. Remember to factor in the speed at which you could deploy Linux, though.
Medium-sized systems are characterized by a medium-powered CPU with around 32 MB or ROM and 64 MB of RAM. Most consumer-oriented devices built with Linux belong to this category. This includes various PDAs, MP3 players, entertainment systems, and network appliances. Some of these devices may include secondary storage in the form of solid-state drives, CompactFlash, or even conventional hard drives. These types of devices have sufficient horsepower and storage to handle a variety of small tasks or can serve a single purpose that requires a lot of resources.
Large systems are characterized by a powerful CPU or collection of CPUs combined with large amounts of RAM and permanent storage. Usually, these systems are used in environments that require large amounts of calculations to carry out certain tasks. Large telecom switches and flight simulators are prime examples of such systems. Typically, such systems are not bound by costs or resources. Their design requirements are primarily based on functionality while cost, size, and complexity remain secondary issues.
In case you were wondering, Linux doesn't run on any processor below 32 bits. This rules out quite a number of processors traditionally used in embedded systems. Actually, according to traditional embedded system standards, all systems running Linux would be classified as large systems. This is very true when compared to an 8051 with 4K of memory. Keep in mind, though, current trends: processors are getting faster, RAM is getting cheaper and larger, systems are as integrated as ever, and prices are going down. With growing processing demands and increasing system requirements, the types of systems Linux runs on are quickly becoming the standard. In some cases, however, it remains that an 8-bit microcontroller might be the best choice.
188.8.131.52 Time constraints
There are two types of time constraints for embedded systems: stringent and mild. Stringent time constraints require that the system react in a predefined time frame. Otherwise, catastrophic events happen. Take for instance a factory where workers have to handle materials being cut by large equipment. As a safety precaution, optical detectors are placed around the blades to detect the presence of the specially colored gloves used by the workers. When the system is alerted that a worker's hand is in danger, it must stop the blades immediately. It can't wait for some file to get swapped or for some task to relinquish the CPU. This system has stringent time requirements; it is a hard real-time system.
Streaming audio systems would also qualify as having stringent requirements, because any transient lagging is usually perceived as bothersome by the users. Yet, this later example would mostly qualify as a soft real-time system because the failure of the application to perform in a timely fashion all the time isn't catastrophic as it would be for a hard real-time system. In other words, although infrequent failures will be tolerated, the system should be designed to have stringent time requirements.
Mild time constraints vary a lot in requirements, but they generally apply to systems where timely responsiveness isn't necessarily critical. If an automated teller takes 10 more seconds to complete a transaction, it's generally not problematic. The same is true for a PDA that takes a certain number of seconds to start an application. The extra time may make the system seem slow, but it won't affect the end result.
Networkability defines whether a system can be connected to a network. Nowadays, we can expect everything to be accessible through the network, even the refrigerator. This, in turn, places special requirements on the systems being built. One factor pushing people to choose Linux as an embedded OS is its proven networking capabilities. Falling prices and standardization of networking components are accelerating this trend. Most Linux devices have one form or another of network capability. You can attach a wireless network card in the Linux distribution built for the Compaq iPAQ, for instance, simply by inserting the adapter in the PCMCIA jacket. Networking issues will be discussed in detail in Chapter 10.
184.108.40.206 User interaction
The degree of user interaction varies greatly from one system to another. Some systems, such as PDAs, are centered around user interaction, while others, such as industrial process control systems, might only have LEDs and buttons for interaction. Some other systems, have no user interface whatsoever. For example, some components of an autopilot system in a plane might take care of wing control but have no direct interaction with the human pilots.
The best way to get an idea of what an embedded Linux system might do is to look at examples of such systems. Trouble is, if you try to look for example embedded systems whose details are publicly available on the Internet, you will mostly find consumer devices. Very few examples of Linux in aerospace, industrial control, telecom, or automotive systems are publicly detailed. Yet, it isn't as if Linux wasn't used in those types of applications. Rather, in contrast to consumer devices, the builders of such devices see little advantage in advertising their designs. For all they know, they may be providing critical information to competitors who may decide to switch to Linux to catch up with them. Consumer device builders, on the other hand, leverage the "hype" factor into promoting their consumer products. And given the different market dynamics between consumer products and industrial products, they can afford to play to the crowd.
Surprisingly (or maybe not so surprising after all), some of the best examples of Linux in critical systems are provided in the pages of Linux Journal magazine. Digging back a few years, I was able to uncover a treasure of non-consumer-oriented embedded applications based on Linux. This, combined with the consumer devices detailed on the Internet and the statistics we shall see below, provide a fair image of Linux's capabilities and future as an embedded operating system. Table 1-1 contains a summary of the example embedded Linux systems discussed below. The first column is a brief description of the system. The second column details the type of the embedded system. The next four columns characterize the system based on the criteria outlined in the previous section.
220.127.116.11 Accelerator control
The accelerator control system was built at the European Synchrotron Radiation Facility and is described in issue 66 of Linux Journal. The accelerator equipment is built of many hardware and software components that control all the aspects of experimentation. While not all software was transferred to Linux, some interesting parts have been. This includes the serial line and stepper motor controllers. Many instances of these devices are employed to control various aspects of the system. Serial lines, for instances, control vacuum devices, power supplies, and programmable logic controllers (PLCs). Stepper motors, on the other hand, are used in positioning goniometers, slits, and translation stages. Serial lines are controlled via serial boards running on PC/104.
The PC/104 single board computer (SBC) controlling the serial boards has a Pentium 90 MHz with 20 MB of RAM and a 24 MB solid-state hard disk. A standard workstation distribution, SuSE 5.3, was trimmed down to fit in the limited permanent storage space. Some stepper motor controllers run on a similar configuration, while others run on VME boards that have 8 to 32 MB of memory and load the operating system from a Unix-type server using BOOTP/TFTP. These boards run a modified version of Richard Hirst's Linux for 680x0-based VME boards. All the equipment is network accessible and controllable through a TCP/IP network. Here, Linux, in the broad sense, was chosen because it is configurable, stable, free, and well supported, contains support for many standards, and its source code is accessible.
18.104.22.168 Computer-aided training system
The computer-aided training system (CATS) was built at CAE Electronics and is described in issue 64 of Linux Journal. Unlike full flight simulators, which include visual, sound, and motion simulation, CATS provides only a visual representation of the various aircraft panels. A CATS isn't a cheap version of a flight simulator. Instead, it complements a flight simulator by providing entry-level training. Conventional CAE CATS were built on IBM RS/6000 workstations running AIX. A port to Linux was prompted by the low cost of powerful x86 systems and the portability of Linux itself.
The CATS come in three different versions: one-, three-, and seven-screen systems. Development and testing was done on a workstation equipped with a Pentium II 350 MHz processor, 128 MB of RAM, and Evolution4 graphic cards from Color Graphics Systems, which provide for control of four displays each. Xi Graphics' AcceleratedX X server was used to control the Evolution4 and provide adequate multiheaded display. A single-screen version could still run easily on a Linux system equipped with the standard XFree86 X server.
Because of customer requirements, the system was provided on a bootable CD-ROM to avoid local installation. Hence, the complete CATS is run from the CD-ROM using a RAM filesystem. The end system has been found to be reliable, predictable, dependable, stable, and in excess of performance requirements. Work on prototype flight simulators running Linux began in April 2000. Having had very positive results, most full flight simulators currently shipped run Linux.
22.214.171.124 Ericsson "blip"
The Ericsson "blip" is a commercial product. Details of the product can be found on Ericsson's blip web site at http://www.ericsson.com/about/blipnet/ and on LinuxDevices.com. "blip" stands for "Bluetooth Local Infotainment Point" and enables Bluetooth devices to access local information. This product can be used either in public places to provide services or at home for accessing or synchronizing with local information.
The blip houses an Atmel AT91F40816 ARM7TDMI paced at 22.5 MHz with 2 MB of RAM, 1 MB of system flash, and 1 MB of user flash. The Atmel chip runs the uClinux distribution, with kernel 2.0.38 modified for MMU-less ARM, provided by Lineo along with uClibc, the miniature C library, and talks via a serial link to a standalone Bluetooth chip. Access to the device is provided by a proprietary Bluetooth stack, an Ethernet interface, and a serial port. Custom applications can be developed for the blip using an SDK provided by Ericsson and built using customized GNU software. Linux was chosen, because it provided an open and inexpensive development environment both for the host and the target, hence encouraging and stimulating the development of third-party software.
126.96.36.199 SCADA protocol converter
The System Control and Data Acquisition (SCADA) protocol converter is detailed in issue 77 of Linux Journal. Here, an existing Digital Control System (DCS) controlling a turbocompressor in an oil extraction plant had to be integrated into a SCADA system to facilitate management of the plant. Converting the complete DCS for better integration would have been expensive, hence the choice was made to build a conversion gateway that interfaced between the existing DCS and the SCADA system.
Linux was chosen because it is easy to tailor, it is well documented, it can run from RAM, and development can be done directly on the target system. An 8 MB DiskOnChip (DOC) from M-Systems provides a solid-state drive for the application. To avoid patching the kernel with the binary drivers provided by M-Systems, the DOC's format is left in its shipped configuration as a DOS filesystem. The kernel and root filesystem are compressed and placed in the DOC along with DOS. Upon bootup, the batch files invoke Loadlin to load Linux and the root filesystem. The system files are therefore read-only and the system is operated using a RAM root filesystem. The root filesystem was built using Red Hat 6.1 following the BootDisk HOWTO instructions. The system is an industrial PC with 32 MB of RAM.
188.8.131.52 Sharp Zaurus
The Sharp Zaurus is a commercial product sold by Sharp Electronics. Details on the Zaurus can be found on its web site at http://www.myzaurus.com/ and on LinuxDevices.com. The Zaurus is a Personal Digital Assistant (PDA) completely based on Linux. As such, it comes equipped with all the usual PDA applications, such as contacts, to do list, schedule, notes, calculator, email, etc.
The original Zaurus, the SL-5500, was built around an Intel StrongARM 206 MHz processor with 64 MB of RAM and 16 MB of flash. A newer version, the SL-5600, is built around an Intel XScale 400 MHz processor with 32 MB of RAM and 64 MB of flash. The system is based on Lineo's Embedix embedded Linux distribution and uses QT's Palmtop GUI. Independent development of the Zaurus software is encouraged by Sharp who maintains a developer web site at http://developer.sharpsec.com/.
184.108.40.206 Space vehicle control
The space vehicle control was built at the European Space Agency (ESA) and is detailed in issue 59 of Linux Journal. The Automatic Transfer Vehicle (ATV) is an unmanned space vehicle used in the refueling and reboosting of the International Space Station (ISS). The docking process between the ATV and the ISS requires the ATV to catch up to the ISS and dock with precision. This process is governed by complex mathematical equations. Given this complexity, monitoring systems are needed to ensure that all operations proceed as planned. This is the role of the Ground Operator Assistant System (GOAS) and the Remote ATV Control at ISS (RACSI).
The GOAS runs on the ground and provides monitoring and intervention capabilities. It used to run on a Sun UltraSPARC 5-based workstation with 64 MB of RAM and 300 MB of disk space. It was ported to a Pentium 233 MHz system with 48 MB of RAM running Linux.
The RACSI runs on the ISS and provides temporary mission interruption and collision avoidance. It runs on an IBM ThinkPad with 64 MB of RAM and uses 40 MB of the available disk space. The system runs the Slackware 3.0 distribution. Moo-Tiff libraries are used to provide Motif-like widgets.
Linux was chosen, because it provides the reliability, portability, performance, and affordability needed by space applications. Despite these benefits, the ESA finally decided to run the RACSI and GOAS on Solaris, using the same equipment, for operational reasons.
As these examples show, Linux can be put to use in many fields in many ways, using different hardware and software configurations. The fastest way to build an embedded system with Linux is often to look at similar projects that have used Linux in their systems. There are many more examples of embedded systems based on Linux that I have not discussed. A search through the various resources listed in Appendix B may yield fruitful leads. Keep in mind, though, that copying other projects may involve copying other people's mistakes. In that case, the best way to guard yourself from chasing down other people's problems is to ensure that you have an understanding of all the aspects of the system or, at least, have a pointer where you can find more information regarding the gray areas of your system.
1.2.3 Survey Findings
Since Linux started being used as an embedded operating system, many surveys have been published providing information regarding various aspects of Linux's use in this way. Though the complete results of many of the surveys are part of commercial reports, which are relatively expensive, there are a few interesting facts that have been publicized. Let's look at the findings of some of these surveys.
In 2000, Embedded Systems Programming (ESP) magazine conducted a survey on 547 subscribers. The survey found that, though none considered it in 1998 and 1999, 38% of readers were considering using Linux as the operating system for their next design. This is especially interesting, as Linux came in only second to VxWorks, WindRiver's flagship product. The survey also found that, though none were using it in 1998 and 1999, 12% of respondents were already using Linux in their embedded systems in 2000.
As part of reporting on embedded Linux, LinuxDevices.com set up a web-based survey in 2000 and 2001 that site visitors could fill to provide information regarding their use of Linux in embedded systems. Both years, a few hundred respondents participated in the survey. Though there were no control mechanisms to screen respondents, the results match those of other more formal surveys. Both surveys contained a lot of information. For the sake of simplicity, I will only mention the surveys' most important findings.
In 2000, the LinuxDevices.com survey found that most developers considering the use of Linux in embedded systems were planning to use an x86, ARM, or PPC target with a custom board. The survey shows that most developers plan to boot Linux from a DiskOnChip or from a native flash device, and that the main peripherals included in the system would be Ethernet and data acquisition cards. The most important reasons developers have for choosing Linux are the superiority of open source software over proprietary offerings, the fact that source availability facilitates understanding the operating system, and the elimination of the dependency on a single operating system vendor. Developers reported using Red Hat, Debian, and MontaVista as their main embedded Linux distributions.
In 2001, the LinuxDevices.com survey found that developers plan to use Linux in embedded systems mostly based on x86, ARM, and PPC systems with custom boards. As in the previous survey, most developers plan to boot their system from some form of flash storage. In contrast with the previous survey, this survey provides insight regarding the amount of RAM and persistent storage developers intend to use. The majority of developers seem to want to use Linux with system having more than 8 MB of RAM and 8 MB of persistent storage. In this survey, developers justify their choice of Linux based on source code availability, Linux's reliability and robustness, and its high modularity and configurability. Developers reported that Red Hat and Debian were their main embedded Linux distributions. Combined with the 2000 survey, the results of the 2001 LinuxDevices.com survey confirm a steady interest in Linux.
Another organization that has produced reports on Linux's use in embedded systems is the Venture Development Corporation (VDC). Though mainly aimed at companies selling products to embedded Linux developers, the VDC's reports published in 2001 and 2002 provide some interesting facts. First, the 2001 report states that the market for embedded Linux development tools products was worth $20 million in 2000 and would be worth $306 million by 2005. The 2001 report also finds that the leading vendors are Lineo, MontaVista, and Red Hat. The report finds that the key reasons developers have for selecting Linux are source code availability and the absence of royalties.
The 2002 VDC report included a web-based survey of 11,000 developers. This survey finds that the Linux distributions currently used by developers are Red Hat, Roll-Your-Own, and non-commercial distributions. Developers' key reasons for choosing Linux are source code availability, reduced licensing, reliability, and open source development community support. Interestingly, the report also lists the most important factors inhibiting Linux's use in embedded applications. The most important factor is real-time limitations, followed by doubts about availability and quality of support, and fragmentation concerns. In addition, the report states that respondents consult the open source community for support with technical issues regarding Linux, and that most are satisfied with the answers they get.
The Evans Data Corporation (EDC) has also conducted surveys in 2001 and 2002 regarding Linux's use in embedded systems. The 2001 survey conducted on 500 developers found that Linux is fourth in the list of operating systems currently used in embedded systems, and that Linux was expected to be the most used embedded operating system in the following year. In 2002, the survey conducted on 444 developers found that Linux was still fourth in the list of operating systems currently used in embedded systems, and that Linux is as likely to be used as Windows as the operating system of choice for future designs.
While these results are partial and though it is too early to predict Linux's full impact on the embedded world, it is clear that there is great interest in embedded Linux and that this interest is growing. Moreover, the results show that the interest for Linux isn't purely amateuristic. Rather, Linux is being considered for and used in professional applications and is being preferred to a lot of the traditional embedded OSes. Also, contrary to popular belief and widespread FUD (fear, uncertainty, and doubt) Linux isn't interesting only because it's free. The fact that its source code is available, is highly reliable, and can easily be tailored to the task are other important reasons, if not more important. Interestingly, the Debian distribution is one of the favorite embedded distributions, even though no vendor is pushing this distribution on the market.
1.2.4 Reasons for Choosing Linux
220.127.116.11 Quality and reliability of code
Quality and reliability are subjective measures of the level of confidence in the code. Although an exact definition of quality code would be hard to obtain, there are properties common programmers come to expect from such code:
The properties expected from reliable code are:
Most programmers agree that the Linux kernel and most projects used in a Linux system fit this description of quality and reliability of their codebase. The reason is the open source development model (see note below), which invites many parties to contribute to projects, identify existing problems, debate possible solutions, and fix problems effectively. You can expect to run Linux for years unattended without problems, and people have effectively done so. You can also select which system components you want to install and which you would like to avoid. With the kernel, too, you can select which features you would like during build configuration. As a testament to the quality of the code making up the various Linux components, you can follow the various mailing lists and see how quickly problems are pointed out by the individuals maintaining the various components of the software or how quickly features are added. Few other OSes provide this level of quality and reliability.
18.104.22.168 Availability of code
Code availability relates to the fact that Linux's source code and all build tools are available without any access restrictions. The most important Linux components, including the kernel itself, are distributed under the GNU General Public License (GPL). Access to these components' source code is therefore compulsory. Other components are distributed under similar licenses. Some of these licenses, such as the BSD license, for instance, permit redistribution of binaries without the original source code or the redistribution of binaries based on modified sources without requiring publication of the modifications. Nonetheless, the code for the majority of projects that contribute to the makeup of Linux is readily available without restrictions.
When source access problems arise, the open source and free software community seeks to replace the "faulty" software with an open source version providing similar capabilities. This contrasts with traditional embedded OSes, where the source code isn't available or must be purchased for very large sums of money. The advantages of having the code available are the possibility of fixing the code without exterior help and the capability of digging into the code to understand its operation. Fixes for security weaknesses and performance bottlenecks, for example, are often very quickly available once the problem has been publicized. With traditional embedded OSes you have to contact the vendor, alert them of the problem, and await a fix. Most of the time, people simply find workarounds instead of waiting for fixes. For sufficiently large projects, managers even resort to purchasing access to the code to alleviate outside dependencies.
22.214.171.124 Hardware support
Broad hardware support means that Linux supports different types of hardware platforms and devices. Although a number of vendors still do not provide Linux drivers, considerable progress has been made and more is expected. Because a large number of drivers are maintained by the Linux community itself, you can confidently use hardware components without fear that the vendor may one day discontinue that product line. Broad hardware support also means that Linux runs on dozens of different hardware architectures, at the time of this writing. Again, no other OS provides this level of portability. Given a CPU and a platform, you can reasonably expect that Linux runs on it or that someone else has gone through a similar porting process and can assist you in your efforts. You can also expect that the software you write on one architecture be easily ported to another architecture Linux runs on. There are even device drivers that run on different hardware architectures transparently.
126.96.36.199 Communication protocol and software standards
Linux also provides broad communication protocol and software standards support as we'll see throughout this book. This makes it easy to integrate Linux within existing frameworks and to port legacy software to Linux. You can easily integrate a Linux system within an existing Windows network and expect it to serve clients through Samba, while clients see little difference between it and an NT server. You can also use a Linux box to practice amateur radio by building this feature into the kernel. Likewise, Linux is a Unix clone, and you can easily port traditional Unix programs to it. In fact, most applications currently bundled with the various distributions were first built and run on commercial Unixes and were later ported to Linux. This includes all the software provided by the FSF. Most traditional embedded OSes are, in this regard, very limited and often provide support only for a limited subset of the protocols and software standards available.
188.8.131.52 Available tools
The variety of tools existing for Linux make it very versatile. If you think of an application you need, chances are others felt the need for this application before you. It is also likely that someone took the time to write the tool and made it available on the Internet. This is what Linus Torvalds did, after all. You can visit Freshmeat (http://www.freshmeat.net) and SourceForge (http://www.sourceforge.net) and browse around to see the variety of tools available.
184.108.40.206 Community support
Community support is perhaps one of the biggest strengths of Linux. This is where the spirit of the free software and open source community can most be felt. As with application needs, it is likely that someone has encountered the same problems as you in similar circumstances. Often, this person will gladly share his solution with you, provided you ask. The development and support mailing lists are the best place to find this community support, and the level of expertise found there often surpasses what can be found over expensive support phone calls with proprietary OS vendors. Usually, when you call a technical support line, you never get to talk to the engineers who built the software you are using. With Linux, an email to the appropriate mailing list will often get you straight to the person who wrote the software. Pointing out a bug and obtaining a fix or suggestions is thereafter a rapid process. As many programmers experience, seldom is a justified plea for help ignored, provided the sender takes the care to search through the archives to ensure that her question hasn't already been answered.
Licensing enables programmers to do with Linux what they could only dream of doing with proprietary software. In essence, you can use, modify, and redistribute the software with only the restriction of providing the same rights to your recepients. This, though, is a simplification of the various licenses used with Linux (GPL, LGPL, BSD, MPL, etc.) and does not imply that you lose control of the copyrights and patents embodied in the software you generate. These considerations will be discussed in Section 1.2.6. Nonetheless, the degree of liberty available is quite large.
220.127.116.11 Vendor independence
Vendor independence, as was demonstrated by the polls presented earlier, means that you do not need to rely on any sole vendor to get Linux or to use it. Furthermore, if you are displeased with a vendor, you can switch, because the licenses under which Linux is distributed provide you the same rights as the vendors. Some vendors, though, provide additional software in their distributions that isn't open source, and you might not be able to receive service for this type of software from other vendors. Such issues must be taken in account when choosing any distribution. Mostly, though, you can do with Linux as you would do with a car. Since the hood isn't welded shut, as with proprietary software, you can decide to get service from a mechanic other than the one provided by the dealership where you purchased it.
The cost of Linux is a result of open source licensing and is different from what can be found with traditional embedded OSes. There are three components of software cost in building a traditional embedded system: initial development setup, additional tools, and runtime royalties. The initial development setup cost comprises the purchase of development licenses from the OS vendor. Often, these licenses are purchased for a given number of "seats," one for each developer. In addition, you may find the tools provided with this basic development package to be insufficient and may want to purchase additional tools from the vendor. This is another cost. Finally, when you deploy your system, the vendor will ask for a per-unit royalty. This may be minimal or large, depending on the type of device you produce and the quantities produced. Some mobile phone manufacturers, for instance, choose to implement their own OSes to avoid paying any royalties. This makes sense for them, given the number of units sold and the associated profit margins.
With Linux, this cost model is turned on its head. All development tools and OS components are available free of charge, and the licenses under which they are distributed prevent the collection of any royalties on these core components. Most developers, though, may not want to go chasing down the various software tools and components and figure out which versions are compatible and which aren't. Most developers prefer to use a packaged distribution. This involves purchasing the distribution or may involve a simple download. In this scenario, vendors provide support for their distribution for a fee and offer services for porting their distributions to new architectures and developing new drivers for a fee. This is where their money is made. They may also charge for additional proprietary software packaged with their distribution. Compared to the traditional embedded software cost model, though, this is relatively inexpensive, depending on the distribution you choose.
1.2.5 Players of the Embedded Linux Scene
Unlike proprietary OSes, Linux is not controlled by a single authority who dictates its future, its philosophy, and its adoption of one technology or another. These issues and others are taken care of by a broad ensemble of players with different but complementary vocations and goals.
18.104.22.168 Free software and open source community
The free software and open source community is the basis of all Linux development and is the most important player in the embedded Linux arena. It is made up of all the developers who enhance, maintain, and support the various software components that make up a Linux system. There is no central authority within this group. Rather, there is a loosely tied group of independent individuals, each with his specialty. These folks can be found discussing technical issues on the mailing lists concerning them or at gatherings such as the Ottawa Linux Symposium. It would be hard to characterize these individuals as a homogeneous group, because they come from different backgrounds and have different affiliations. Mostly, though, they care a great deal about the technical quality of the software they produce. The quality and reliability of Linux, as discussed earlier, are a result of this level of care.
Your author is actually part of the free software community and has made a number of contributions. Besides maintaining a presence on some mailing lists and participating in the advancement of free software in various ways, I wrote and maintain the Linux Trace Toolkit, which is a set of components for the tracing of the Linux kernel. I have also contributed to other free software and open source projects, including RTAI and Adeos.
Throughout this book, I will describe quite a few components that are used in Linux systems. Each maintainer of or contributor to the components I will describe is a player in the free software and open source community.
Having recognized the potential of Linux in the embedded market, many companies have moved to embrace and promote Linux in this area. Industry players are important because they are the ones pushing Linux as an end-user product. Often, they are the first to receive feedback from those end users. Although postings on the various mailing lists can tell the developer how the software is being used, not all users participate in those mailing lists. Vendors must therefore strike an equilibrium between assisting their users and helping in the development of the various projects making up Linux without falling in the trap of wanting to divert development to their own ends. In this regard, many vendors have successfully positioned themselves in the embedded Linux market. Here are some of them.
There are also many small players who provide a variety of services around open source and free software. In fact, many open source and free software contributions are made by individuals who are either independent or work for small-size vendors. As such, the services provided by such small players are often on a par or sometimes surpass those provided by larger players. Here are some individuals and small companies who provide embedded Linux services and are active contributors to the open source and free software community: Alessandro Rubini, Bill Gatliff, CodePoet Consulting, DENX Software Engineering, Opersys, Pengutronix, System Design & Consulting Services, and Zee2.
There are currently three organizational bodies aimed at promoting and encouraging the adoption of Linux in embedded applications: the Embedded Linux Consortium (ELC), Emblix, the Japan Embedded Linux Consortium, and the TV Linux alliance. The ELC was founded by 23 companies as a nonprofit vendor-neutral association and now includes more than 100 member companies. Its current goals include the creation of an embedded Linux platform specification inspired by the Linux Standard Base and the Single Unix Specification. It remains unclear whether the ELC's specification will gain any acceptance from the very open source and free software developers that maintain the software the ELC is trying to standardize, given that the drafting of the standard is not open to the public, which is contrary to the open source and free software traditions. Emblix was founded by 24 Japanese companies with similar aims as the ELC but with particular emphasis on the Japanese market. The TV Linux alliance is a consortium that includes cable, satellite, and telecom technology suppliers and operators who would like to support Linux in set-top boxes and interactive TV applications.
These efforts are noteworthy, but there are other organizational bodies that influence Linux's advancement, in the broad sense, although they do not address embedded systems particularly.
First and foremost, the Free Software Foundation (FSF), launched in 1984 by Richard Stallman, is the maintainer of the GNU project from which most components of a Linux system are based. It is also the central authority on the GPL and LGPL, the licenses most software in a Linux system fall under. Since its foundation, the FSF has promoted the usage of free software in all aspects of computing. The FSF has taken note of the recent rise in the use of GNU and GPL software in embedded systems and is moving to ensure that user and developer rights are preserved.
The OpenGroup maintains the Single Unix Specification (SUS), which describes what should be found in a Unix system. There is also the Linux Standard Base (LSB) effort, which aims at developing and promoting "a set of standards that will increase compatibility among Linux distributions and enable software applications to run on any compliant Linux system," as stated on the LSB web site at http://www.linuxbase.org/. In addition, the Filesystem Hierarchy Standard (FHS) maintained by the Filesystem Hierarchy Standard Group specifies the content of a Linux root tree. The Free Standards Group (FSG) maintains the Linux Development Platform Specification (LDPS), which specifies the configuration of a development platform to enable applications developed on conforming platforms to run on most distributions available. Finally, there is the Real-Time Linux Foundation, which aims at promoting and standardizing real-time enhancements and programming in Linux.
Most developers connect to the embedded Linux world through various resource sites and publications. It is through these sites and publications that the Linux development community, industry, and organizations publicize their work and learn about the work of the other players. In essence, the resource sites and publications are the meeting place for all the people concerned with embedded Linux. A list of resources can be found in Appendix B, but there are two resources that stand out, LinuxDevices.com and Linux Journal.
LinuxDevices.com was founded on Halloween day 1999 by Rick Lehrbaum. It has since been acquired by ZDNet and, later still, been purchased by a company owned by Rick. To this day, Rick continues to maintain the site. LinuxDevices.com features news items, articles, polls, forums, and many other links pertaining to embedded Linux. Many key announcements regarding embedded Linux are made on this site. The site contains an archive of actively maintained articles regarding embedded Linux. Though its vocation is clearly commercial, I definitely recommend taking a peek at the site once in a while to keep yourself up to date with the latest in embedded Linux. Among other things, LinuxDevices.com was instrumental in launching the Embedded Linux Consortium.
As part of the growing interest in the use of Linux in embedded systems, the Embedded Linux Journal (ELJ) was launched by Specialized System Consultants, owners of Linux Journal (LJ), in January 2001 with the aim of serving the embedded Linux community, but was later discontinued. Though ELJ is no longer published as a separate magazine, LJ now contains an "embedded" section every month, which contains articles that otherwise would have been published in ELJ.
1.2.6 Copyright and Patent Issues
You may ask: what about using Linux in my design? Isn't Linux distributed under this weird license that may endanger the copyrights and patents of my company? What are all those licenses anyway? Is there more than one license to take care of? Are we allowed to distribute binary-only kernel modules? What about all these articles I read in the press, some even calling Linux's license a "virus"?
These questions and many more have probably crossed your mind. You probably even discussed some of these issues with some of your coworkers. The issues can be confusing and can come back to haunt you if they aren't dealt with properly. I don't say this to scare you. The issues are real, but there are known ways to use Linux without any fear of any sort of licensing contamination. With all the explanations provided below, it would be important to keep in mind that this isn't legal counsel and I am not a lawyer. If you have any doubts about your specific project, consult your attorney.
OK, now that I've given you ample warning, let us look at what is commonly accepted thought on Linux's licensing and how it applies to Linux systems in general, including embedded systems.
22.214.171.124 Textbook GPL
For most components making up a Linux system, there are two licenses involved, the GPL and the LGPL, introduced earlier. Both licenses are available from the FSF's web site at http://www.gnu.org/licenses/, and should be included with any package distributed under the terms of these licenses. The GPL is mainly used for applications, while the LGPL is mainly used for libraries. The kernel, the binary utilities, the gcc compiler, and the gdb debugger are all licensed under the GPL. The C library and the GTK widget toolkit, on the other hand, are licensed under the LGPL.
Some programs may be licensed under BSD, Mozilla, or another license, but the GPL and LGPL are the main licenses used. Regardless of the license being used, common sense should prevail. Make sure you know the licenses under which the components you use fall and understand their implications.
The GPL provides rights and imposes obligations very different from what may be found in typical software licenses. In essence, the GPL is meant to provide a higher degree of freedom to developers and users, enabling them to use, modify, and distribute software with few restrictions. It also makes provisions to ensure that these rights are not abrogated or hijacked in any fashion. To do so, the GPL stipulates the following:
As you can see, the GPL protects authors' copyrights while providing freedom of use. This is fairly well accepted. The application of the modification and distribution clauses, on the other hand, generates a fair amount of confusion. To clear this confusion, two issues must be focused on: running GPL software and modifying GPL software. Running the software is usually the reason why the original authors wrote it. The authors of gcc, for example, wrote it to compile software with. As such, the software compiled by an unmodified gcc is not covered by the GPL, since the person compiling the program is only running gcc. In fact, you can compile proprietary software with gcc, and people have been doing this for years, without any fear of GPL "contamination." Modifying the software, in contrast, creates a derived work that is based on the original software, and is therefore subject to the licensing of that original software. If you take the gcc compiler and modify it to compile a new programming language of your vintage, for example, your new compiler is a derived work and all modifications you make cannot be distributed outside your organization under the terms of any license other than the GPL.
Most anti-GPL speeches or writings play on the confusion between running and modifying GPL software, to give the audience an impression that any software in contact with GPL software is under threat of GPL "contamination." This is not the case.
There is a clear difference between running and modifying software. As a developer, you can safeguard yourself from any trouble by asking yourself whether you are simply running the software as it is supposed to be run or if you are modifying the software for your own ends. As a developer, you should be fairly capable of making out the difference.
Note that the copyright law makes no difference between static and dynamic linking. Even if your proprietary application is integrated to the GPL software during runtime through dynamic linking, that doesn't exclude it from falling under the GPL. A derived work combining GPL software and non-GPL software through any form of linking still cannot be distributed under any license other than the GPL. If you package gcc as a dynamic linking library and write your new compiler using this library, you will still be restricted from distributing your new compiler under any license other than the GPL.
Whereas the GPL doesn't allow you to include parts of the program in your own program unless your program is distributed under the terms of the GPL, the LGPL allows you to use unmodified portions of the LGPL program in your program without any problem. If you modify the LGPL program, though, you fall under the same restrictions as the GPL and cannot distribute your modifications outside your organization under any license other than the LGPL. Linking a proprietary application, statically or dynamically, with the C library, which is distributed under the LGPL, is perfectly acceptable. If you modify the C library, on the other hand, you are prohibited from distributing all modifications under any license other than the LGPL.
Much like the running versus modifying GPL software discussion above, there is a clear difference between linking against LGPL software and modifying LGPL software. You are free to distribute your software under any license when it is linked against an LGPL library. You are not allowed to distribute any modifications to an LGPL library under any license other than LGPL.
126.96.36.199 Pending issues
Up to now, I've discussed only textbook application of the GPL and LGPL. Some areas of application are, unfortunately, less clearly defined. What about applications that run using the Linux kernel? Aren't they being linked, in a way, to the kernel's own code? And what about binary kernel modules, which are even more deeply integrated to the kernel? Do they fall under the GPL? What about including GPL software in my embedded system?
I'll start with the last question. Including a GPL application in your embedded system is actually a textbook case of the GPL. Remember that you are allowed to redistribute binary copies of any GPL software as long as your recipients receive the original source code. Distributing GPL software in an embedded system is a form of binary distribution and is allowed, granted you respect the other provisions of the GPL regarding running and modifying GPL software.
Some proprietary software vendors have tried to cast doubts about the use of GPL software in embedded systems by claiming that the level of coupling found in embedded systems makes it hard to differentiate between applications and, hence, between what falls under GPL and what doesn't. This is untrue. As we shall see in Chapter 6 and Chapter 8, there are known ways to package embedded Linux systems that uphold modularity and the separation of software components.
To avoid any confusion regarding the use of user applications with the Linux kernel, Linus Torvalds has added a preamble to the GPL license found with the kernel's source code. This preamble has been reproduced verbatim in Appendix C and stipulates that user applications running on the kernel are not subject to the GPL. This means that you can run any sort of application on the Linux kernel without any fear of GPL "contamination." A great number of vendors provide user applications that run on Linux while remaining proprietary, including Oracle, IBM, and Adobe.
The area where things are completely unclear is binary-only kernel modules. Modules are software components that can be dynamically loaded and unloaded to add functionality to the kernel. While they are mainly used for device drivers, they can and have been used for other purposes. Many components of the kernel can actually be built as loadable modules to reduce the kernel image's size. When needed, the various modules can be loaded during runtime.
Although this was intended as a facilitating and customizing architecture, many vendors and projects have come to use modules to provide capabilities to the kernel while retaining control over the source code or distributing it under licenses different from the GPL. Some hardware manufacturers, for instance, provide closed-source binary-only module drivers to their users. This enables the use of the hardware with Linux without requiring the vendor to provide details regarding the operation of their device.
The problem is that once a module is loaded in the kernel, it effectively becomes part of its address space and is highly coupled to it because of the functions it invokes and the services it provides to the kernel. Because the kernel is itself under the GPL, many contend that modules cannot be distributed under any other license than the GPL because the resulting kernel is a derived work. Others contend that binary-only modules are allowed as long as they use the standard services exported to modules by the kernel. For modules already under the GPL, this issue is moot, but for non-GPL modules, this is a serious issue. Linus has said more than once that he allows binary-only modules as long as it can be shown that the functionality implemented is not Linux specific, as you can see in some of his postings included in Appendix C. Others, however, including Alan Cox, have come to question his ability to allow or disallow such modules, because not all the code in the kernel is copyrighted by him. Others, still, contend that because binary modules have been tolerated for so long, they are part of standard practice.
There is also the case of binary-only modules that use no kernel API whatsoever. The RTAI and RTLinux real-time tasks inserted in the kernel are prime examples. Although it could be argued that these modules are a class of their own and should be treated differently, they are still linked into kernel space and fall under the same rules as ordinary modules, whichever you think them to be.
At the time of this writing, there is no clear, definitive, accepted status for binary-only modules, though they are widely used and accepted as legitimate. Linus' latest public statements on the matter, made during a kernel mailing list debate on the Linux Security Module infrastructure (reproduced verbatim in Appendix C), seem to point to the fact that the use of binary-only modules is an increasingly risky decision. In fact, the use of binary-only modules is likely to remain a legally dubious practice for the foreseeable future. If you think you need to resort to binary-only proprietary kernel modules for your system, I suggest you follow Alan Cox's advice and seek legal counsel beforehand. Actually, I also suggest you reconsider and use GPL modules instead. This would avoid you many headaches.
188.8.131.52 RTLinux patent
Perhaps one of the most restrictive and controversial licenses you will encounter in deploying Linux in an embedded system is the license to the RTLinux patent held by Victor Yodaiken, the RTLinux project leader. The patent covers the addition of real-time support to general purpose operating systems as implemented by RTLinux.
Although many have questioned the patent's viability, given prior art, and Victor's handling of the issue, it remains that both the patent and the license are currently legally valid, at least in the United States, and have to be accounted for. The U.S. Patent Number for the RTLinux patent is 5,995,745, and you can obtain a copy of it through the appropriate channels. The patent license that governs the use of the patented method is available on the Web at http://www.fsmlabs.com/about/patent/.
The license lists a number of requirements for gratis use of the patented method. Notably, the license stipulates that there are two approved uses of the patented process. The first involves using software licensed under the terms of the GPL, and the second involves using an umodified version of the "Open RTLinux" as distributed by FSMLabs, Victor Yodaiken's company. The traditional way in which these requirements have been read by real-time Linux developers is that anyone distributing non-GPL real-time applications needs to purchase a license from FSMLabs. Not so says Eben Moglen, the FSF's chief legal counsel. In a letter that was sent to the RTAI community, the original of which is available at http://www.aero.polimi.it/~rtai/documentation/articles/moglen.html, Moglen makes the following statement: "No application in a running RTLinux or RTAI system does any of the things the patent claims. No applications program is therefore potentially infringing, and no applications program is covered, or needs to be covered, by the license."
Though Moglen's authoritative statement is clear on the matter, it remains that FSMLabs' continued refusal to provide explanations regarding the patent's reach has left a cloud of uncertainty regarding all real-time extensions using the patented process.
It follows from this that the only way to stay away from this mess is to avoid using the patented process altogether. In other words, another method than that covered by the patent must be used to obtain deterministic response times from Linux. Fortunately such a method exists.
Basing myself entirely on scientific articles on nanokernel research published more than one year earlier than the preliminary patent application, I wrote a white paper describing how to implement a Linux-based nanokernel to enable multiple OSes to share the same hardware. The white paper, entitled "Adaptive Domain Environment for Operating Systems," was published in February 2001 and is available from http://www.opersys.com/adeos/ along with other papers on other possible uses of this method. Given that your author started working on this book soon after the paper's publication, there was little development effort being put on the project, and the idea lay dormant for over a year.
The situation changed in late April 2002 when Philippe Gerum, a very talented free software developer, picked up the project and decided to push it forward. By early June, we were sufficiently satisfied with the project's status to make the first public release of the Adeos nanokernel. The release made on June 3, 2002, was endorsed by several free software organizations throughout the world, including Eurolinux (http://www.eurolinux.org/) and April (http://www.april.org/), as a patent-free method for allowing real-time kernels to coexist with general purpose kernels. Though, as with any other patent, such endorsements do not constitute any guarantee against patent infringement claims, the consensus within the open source and free software community is that the Adeos nanokernel and its applications are indeed patent free. For my part, I encourage you to make your own verifications, as you should do for any patent. Among other things, review the original white paper and, most importantly, the scientific articles mentioned therein.
Already, Adeos is being used by developers around the world for allowing different types of kernels to coexist. RTAI, for instance, which previously used the patented process to take control from Linux, and was therefore subject to the patent license, has already been ported to Adeos. Though at the time of this writing Adeos runs on single processor and SMP x86 systems only, ports to other architectures should be relatively straightforward, given the nanokernel's simplicity. If you are interested in contributing to Adeos, by porting it to other architectures for example, or if you would just like to use it or get more information, visit the project's web site at http://www.adeos.org/.
1.2.7 Using Distributions
Wouldn't it be simpler and faster to use a distribution instead of setting up your own development environment and building the whole target system from scratch? What's the best distribution? Unfortunately, there are no straightforward answers to these questions. There are, however, some aspects of distribution use that might help you find answers to these and similar questions.
184.108.40.206 To use or not to use
First and foremost, you should be aware that it isn't necessary to use any form of distribution to build an embedded Linux system. In fact, all the necessary software packages are readily available for download on the Internet. It is these same packages that distribution providers download and package for you to use. This approach provides you with the highest level of control and understanding over the packages you use and their interactions. Apart from this being the most thorough approach and the one used within this book, it is also the most time-consuming, as you have to take the time to find matching package versions and then set up each package one by one while ensuring that you meet package interaction requirements.
Hence, if you need a high degree of control over the content of your system, the "do it yourself" method may be best. If, however, like most people, you need the project ready yesterday or if you do not want to have to maintain your own packages, you should seriously consider using both a development and a target distribution. In that case, you will need to choose the development and target distributions most appropriate for you.
220.127.116.11 How to choose a distribution
There are a number of criteria to help in the choice of a distribution, some of which have already been mentioned in Section 1.2.3. Depending on your project, you may also have other criteria not discussed here. In any case, if you choose commercial distributions, make sure you have clear answers to your questions from the distribution vendor when you evaluate his product. As in any situation, if you ask broad questions, you will get broad answers. Use detailed questions and expect detailed answers. Unclear answers to precise questions are usually a sign that something is amiss. If, however, you choose an open source distribution, make sure you have as much information as possible about it. The difference between choosing an open source distribution and a commercial distribution is the way you obtain answers to your questions about the distribution. Whereas the commercial distribution vendor will provide you with answers to your questions about his product, you may have to look for the answers to those same questions about an open source distribution on your own.
An initial factor in the choice of a development or target distribution is the license or licenses involved. Some commercial distributions are partly open source and distribute value-added packages under conventional software licenses prohibiting copying and imposing royalties. Make sure the distribution clearly states the licenses governing the usage of the value-added software and their applicability. If unsure, ask. Don't leave licensing issues unclear.
Before evaluating a distribution, make yourself a shopping list of packages you would like to find in it. The distribution may have something better to offer, but at least you know if it fits your basic requirements. A development distribution should include items covered in Section 1.4.2, whereas a target distribution should automate and/or facilitate, to a certain degree, items covered in Section 1.4.1 and Section 1.4.4. Of course, no distribution can take away issues discussed in Section 1.4.3, since only the system developers know what type of programming is required for the system to fit its intended purpose.
One thing that distinguishes commercial distributions from open source distributions is the support provided by the vendor. Whereas the vendor supplying a commercial distribution almost always provides support for her own distribution, the open source community supplying an open source distribution does not necessarily provide the same level of support that would be expected from a commercial vendor. This, however, does not preclude some vendors from providing commercial support for open source distributions. Through serving different customers with different needs in the embedded field, the various vendors build a unique knowledge about the distributions they support and the problems clients might encounter during their use, and are therefore best placed to help you efficiently. Mainly, though, these vendors are the ones who keep up with the latest and greatest in Linux and are therefore the best source of information regarding possible bugs and interoperability problems that may show up.
Reputation can also come into play when choosing a distribution, but it has to be used wisely, as a lot of information circulating may be presented as fact while being mere interpretation. If you've heard something about one distribution or another, take the time to verify the validity of the information. In the case of a commercial distribution, contact the vendor. Chances are he knows where this information comes from and, most importantly, the rational explanation for it. This verification process, though, isn't specific to embedded Linux distributions. What is specific to embedded Linux distributions is the reputation commercial distributions build when their vendors contribute to the open source community. A vendor that contributes back by providing more open source software or by financing development shows that he is in contact with the open source community and has therefore a privileged position in understanding how the changes and developments of the various open source projects will affect his future products and ultimately his clients. In short, this is a critical link and a testament to the vendor's understanding of the dynamics involved in the development of the software he provides you. In the case of open source distributions, this criterion is already met, as the distribution itself is an open source contribution.
Another precious tool commercial distributions might have to offer is documentation. In this day and age where everything is ever-changing, up-to-date and accurate documentation is a rare commodity. The documentation for the majority of open source projects is often out of date, if available at all. Linus Torvalds' words ring true here. "Use the source, Luke," he says, meaning that if you need to understand the software you should read the source code. Yet not everyone can invest the amount of time necessary to achieve this level of mastery, hence the need for appropriate documentation. Because the open source developers prefer to invest time in writing more code than in writing documentation, it is up to the distribution vendors to provide appropriately packaged documentation with their distributions. When evaluating a distribution, make sure to know the type and extent of accompanying documentation. Although there is less documentation for open source distributions, in comparison with commercial distributions, some open source distributions are remarkably well documented.
Given the complexity of some aspects of development and target setup, the installation of a development and/or target distribution can be hard. In this regard, you may be looking for easy-to-install distributions. Although this is legitimate, keep in mind that once you've installed the distributions, you should not need to reinstall them afterward. Notice also that installation does not really apply for a target distribution, as it was defined earlier, because target distributions are used to facilitate the generation of target setups and don't have what is conventionally known as an "installation" process. The three things you should look for in the installation process of a distribution are clear explanations (whether textually during the installation, in a manual, or both), configurability, and automation. Configurability is a measure of how much control you have over the packages being installed, while automation is the ability to automate the process using files containing the selected configuration options.
With some CPU models and boards being broadly adopted for embedded systems development, commercial distribution vendors have come to provide prepackaged development and/or target distributions specifically tailored for those popular CPU models and boards. If you are intending to use a specific CPU model or board, you may want to look for a distribution that is already tested for your setup.
18.104.22.168 What to avoid doing with a distribution
There is one main course of action to avoid when using a distribution: using the distribution in a way that makes you dependent solely on this same distribution for all future development. Remember that one of the main reasons to use Linux is that you aren't subject to anyone's will and market decisions. If your development relies solely on proprietary tools and methods of the distribution you chose, you are in risk of being locked into continuous use of that same distribution for all future development. This does not mean, though, that you shouldn't use commercial distributions with value-added software the likes of which cannot be found on other distributions. It only means that you should have a backup plan to achieve the same results with different tools from different distributions, just in case.