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A dedicated licence for open-source hardware: CERN OHL approved by OSI

How do you license open-source hardware? Existing software licences will not do, says CERN

The OSI (Open Source Intitiative) has approved version 2 of CERN's Open Hardware License (OHL), meaning it conforms to its Open Source Definition and respects the ideals and ethos of the movement.

Geneva-based CERN (Conseil Européen pour la Recherche Nucléaire) says it has an open-source culture. "Our main mandate at CERN is to conduct basic research. But there is a lesser-known part of our mandate, which is to make things that we do available for the public… very often these are engineering things that we develop," Javier Serrano, head of the hardware and timing section, Beam Controls Group, told The Register.

The desire to share hardware designs publicly led to the creation of the OHL, for which version 2.0 was released last year. But why not use existing licences like GPL, MIT or Apache 2.0?

"There were no adequate open-source licences that we could rely on to share our hardware designs," said Serrano.

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In hardware, working with commercial companies is a necessity, because "you need somebody who's going to manufacture it, assemble it to test and sell it to you," he said. "Companies don't like the legal risk, so there was a need for a licence to bring clarity as to what the conditions would be."

CERN ended up with three variants. There is a strong reciprocal licence (CERN-OHL-S), which is for designs that remain free along with all their derivatives, a copyleft principle similar to GPL.

There is a weak reciprocal licence where the design can be used as a component in other designs without the whole becoming open source, but if the design of the component is modified, that must be shared back (CERN-OHL-W). And there is a permissive licence, CERN-OHL-P, which lets users mix the design freely with proprietary designs provided it is acknowledged, similar to Apache 2.0 in the software world.

"Permissive licensing is much easier than reciprocal licensing," Serrano told us. "To take a modern example, Apache 2.0 is used by the lowRISC people in Cambridge. The problems arise mainly in reciprocal licensing... the realities of the chip design world and the FPGA design world are such that interpreting GPL would lead to an impossibility to license your design."

One issue with hardware is that "it is very difficult to make fully free designs that do not rely on any primitive or any block provided by the vendors under a proprietary licence," said Serrano. The GPL has the concept of system libraries, but "that does not match what you have in the chip design world."

Another problem is with standard components. "If somebody designs a printed circuit board, it has resistors in it. Of course we should not require people to publish the recipe for making resistors out of carbon and metal because anybody can buy resistors."

Open-source lawyer Andrew Katz, partner and head of tech at Moorcrofts, helped to draft the OHL and came up with the idea of available components.

"Anything which is available, basically you don't need to publish, and we reuse that for these tricky problems of building blocks in ASIC design," said Serrano. "In IC design we use the same concept, these blocks qualify as available components and therefore excluded from the obligation to release the source code."

There are verbs that people use in software like linking. What does that mean in hardware? When I connect my mouse to the USB port of a PC, am I linking?

Software licences have the wrong terms for hardware. "There are verbs that people use in software like linking. What does that mean in hardware? When I connect my mouse to the USB port of a PC, am I linking? These things are better served by proper language that people in hardware design can relate to," said Serrano.

CERN has little idea how many are using its OHL licences, but they are available as a drop-down choice for GitHub repositories, encouraging take-up. Hardware has a broad scope; it might not mean electronic designs. "There were examples of face masks for COVID-19 that were using the CERN OHL," said Serrano, "and many examples of ventilators."

Why bother with the OSI? "It is a question of principle. Does somebody have the authority to tell me if this is open source or not? The closest you can get is OSI," Serrano told us. "It is also pragmatic. There are many institutions that rely on OSI to define a set of licences that are acceptable for them to work with." Thanks to the approval process, CERN's open hardware licences are now in this list.

There is a curious aspect to this, which is that the OSI is about software licensing. CERN's OHL can be used for software, but that is not its primary purpose. Pamela Chestek, a lawyer on the OSI board and chair of its licence committee, told us that "the OSI really only deals with software licences. This licence is intended for a hardware and software combination… what we look at is, in the software only use case, does it fit within the open source definition? It was a really well crafted licence and the answer I came to was yes, it does."

Does OSI approval then give comfort to those using it for licensing hardware designs? "My understanding was that the reason CERN sought the approval of the Open Source Initiative was use cases of blended products which are both hardware and software," Chestek said.

"If your question is, 'Can you consume this as an open hardware licence?', I don't know the answer to that because I don't do hardware licences... anybody who uses this licence will know that for the software on there they have all of the assurances that are guaranteed by the Open Source Definition."

Chestek's comments suggest that the open source hardware community has more work to do in establishing standards in open hardware licensing, having said which, CERN's OHL is significant progress.

"Being able to cover multiple use cases including hardware, software and data under one licence – rather than trying to apply multiple licences to the various components – will be very useful as people develop their projects," said Amanda Brock of OpenUK, an open-source advocacy body. ®

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