ADEOS - avoiding real-time Linux patents [LWN.net]

The current systems for performing hard real-time tasks with Linux (RTLinux and RTAI) take the same approach: use a real-time kernel at the lowest level of the system, and run the general purpose kernel (i.e. Linux) as the lowest-priority task. This technique has been patented by Victor Yodaiken, and the patent has been licensed to the community under terms similar to those used by Red Hat: as long as your work is licensed under the GPL, you can use the patented technology without paying royalties.

This patent and its license have been the subject of endless controversy in the free software community. In particular, developers and users of RTAI have always felt a little nervous, especially when the RTAI core was licensed under the LGPL. The recent relicensing of the RTAI core was undertaken to be sure of compliance with the patent license, but not everybody has been satisfied. In particular, people who wish to run or distribute proprietary systems using RTAI have been unsure of their status.

An obvious solution, one might think, would be to not use the patented technology; until recently, however, alternatives have been somewhat scarce. That situation may have changed, however, as the result of the announcement of the upcoming first release of ADEOS.

ADEOS is, essentially, a small "nanokernel" which takes charge of low-level hardware resources (interrupts in particular). Any number of higher level operating systems can run in parallel on top of ADEOS; they run independently and know little of each other. ADEOS implements an abstraction called an "interrupt pipeline," which is essentially an ordered list of systems which are interested in a particular interrupt. Real-time systems put themselves at the head of the list, and are thus able to respond quickly to interrupts. General purpose systems can handle any interrupts which the real-time systems allow to pass down the pipeline.

The idea of running multiple operating systems over a small, low-level kernel is not particularly new; IBM has been doing it for decades. It is interesting, however, in that the ADEOS developers claim that it is suffiently different from the RTLinux approach that it is not covered by the patent. The reality of the situation, however, may not be determined until a fair number of lawyers have been involved.

The RTAI project plans to move over to ADEOS and thus, with luck, free itself of patent worries. Whether RTAI will be able to rid itself of the persistent claims that it is a derivative of RTLinux could be another story. RTLinux supporters will point out strong similarities between a number of source files in the two projects. If RTAI really used RTLinux code at the beginning, and released that code under a different license (even if that license was the LGPL), the project has, in theory, lost its rights to use that code. Nobody has seriously pursued a GPL infringement claim against RTAI, but, as long as the allegations persist, such an action is a possibility.

Meanwhile, ADEOS embodies a different approach to dealing with software patents: find a way to work around them. From the press release:

Often a piece of Free Software, into which many people have invested years of work, can be turned into non-free software by a patent for one simple but essential calculation rule. Our example shows that the developers need not always give up. Sometimes, by trying very hard, an alternative calculation rule that does the work and is not completely useless can be found. Developers should not shy [from] the effort that this takes, because even if the patent owner offers a license on friendly terms, usually the project will be restricted in some way or other and the intentions of the developers to create real Free Software will be betrayed.

This approach has been taken with other patents: using JPEG and PNG files to get around the GIF patent, and Ogg Vorbis as an alternative to MP3, for example. As software patents proliferate in the U.S., however (and possibly spread to Europe), it will get always harder to dodge them all. And tiresome as well. Software patents remain a threat to free software.

(See also: a more detailed, technical description of how ADEOS works.)

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