The Use of Linux in an Embedded System
Several aspects of using Linux needed to be considered before actually committing to this program. Many companies have been battling with these issues as Linux has struggled to move into the mainstream. The most important to us was whether or not the development resources would be available to us. Having worked with the GNU utilities, namely gcc and gdb, on a previous project, I felt developing the main program would present no problem. The next issue was the support required to allow the database interface to be provided through the user's browser. Again, a little research showed not only was the support there, but also there were many choices. One question that was asked of me was how we would handle the constant flux of kernel development. It didn't take long to realize it was not necessary to continually upgrade these systems just to stay up to date. All we needed was a workable version that could be propagated through the whole product. Since the source code is available, if the kernel were to develop in a direction incompatible with our system, we would not be abandoned with an albatross.
It seems that programming for a serial line in any system is a challenge and nothing comes easy. This was particularly true for this project as I had minimal experience with the UNIX environment and had to handle up to eight serial ports. Because of previous experience, the design team had already selected the Digi International eight-port 422 card. My first task was to set it up to run under Linux. I felt like I was groping around in the dark and looked for any kind of confirmation that I was doing things right. I tired to use minicom with a loop-back plug just to see if it worked. If it had, there'd be little mention of it here, but it didn't, and I spent considerable time reviewing what I had done. Finally, I called the Digi International support people. Of course, they had me try all the usual, and I was willing to entertain them on the off chance that I had overlooked something. To make a long story short, after several cycles of e-mail, one of their second-line support people picked up on my plight. While working with him, I certainly learned much about the inner workings of Linux, and it was worth the time for that alone. Finally, we discovered the problem was in the version of the driver I had; in fact, it had not been written to support the 422 card. A simple change and it started working. This experience proved to me that Linux is indeed a supported system.
Now that all the hardware was in place, it was my job to make it work. I was rather puzzled as to how I would handle up to eight serial interfaces and so I did some reading. This included the appropriate HOWTOs and any books I could get. Fortunately one of them was Beginning Linux Programming by Neil Matthew and Richard Stones, Wrox press, 1996. In it, they develop an excellent example of the use of FIFOs to communicate between tasks. I took their example and expanded it, so that each serial interface had its own task and all communicated through FIFOs to the central controlling task. To see how the various tasks relate, see Figure 2. This proved to be an excellent choice for two reasons. The first is that the central controlling task could spawn a task for each serial link. If only one was needed for an application, only one task was spawned, etc. The second is that the serial task could perform all polling, error checking and retransmission without involving the controlling task in any way. This made the controlling task much simpler in that it dealt only with valid messages that needed action.
One of the problems I've had in the past with programming serial links is that, unless you can master the interrupt mechanism, the system spends a large amount of time spinning its wheels waiting for something to happen. In the Linux system, I was able to use the select call to allow each task to go into the idle state until something needed to be done. This happens while the controlling task is waiting for a message to arrive from one of the readers. The timeout feature of the select call was also used to trigger a background task. If no messages arrived from any reader within ten seconds, a subtask would perform background housekeeping. It also happens in the serial task. In this case, the select is set to wake up on arrival of a message from either a reader or the controlling task. Again, the timeout feature was used, but this time it indicated that one of the door-access modules had failed to respond, which is indicative of a hardware problem.
With these basics in place, the development of the rest of the program progressed very rapidly. I want to point out that the multi-tasking power of Linux and the capabilities of gdb came through during this phase. Since the serial task was spawned by the controlling task, no screen was attached, and thus, there was no way to print the usual diagnostic messages during the development phase. One of the first things I did was to learn how to use syslogd to report error conditions within these tasks. Secondly, I used the capability of gdb to connect to an already running task and debug it.