Why some drivers are not merged early

Arjan van de Ven's kernel oops report always makes for interesting reading; it is a quick summary of what is making the most kernels crash over the past week. It thus points to where some of the most urgent bugs are to be found. Sometimes, though, this report can raise larger issues as well. Consider the June 16 report, which notes that quite a few kernel crashes were the result of a not-quite-ready wireless update shipped by Fedora. Ingo Molnar was quick to jump on this report with a process-related complaint:

He then took the discussion further with this observation:

This comment drew some unhappy responses from the networking developers, who feel that they have been unfairly targeted for criticism. Wireless drivers have been merged at the first real opportunity, they say, and trying to put them in earlier would have only made things worse. In fact, your editor will submit that mistakes were made with wireless drivers, but those mistakes have little to do with delaying their inclusion into the mainline. What went wrong with wireless is this:

• Early wireless developers did not really try to solve the wireless networking problem; they just wanted to get their adaptor to work. Wireless maintainer John Linville once told your editor that, for years, these adaptors were treated as if they were Ethernet adaptors, which they certainly are not. When these developers did get around to dealing with issues specific to wireless networking, they created their own wireless stacks contained within their drivers. So no general wireless framework was created.

  It's only in 2004 that Jeff Garzik started a project to create a generic wireless stack for Linux - and he started with a stack (HostAP) which, sometime later on, was seen as not being the best choice. So the work on HostAP - late to begin in the first place - was eventually abandoned.

• The networking stack which was eventually developed - mac80211 - began its life as a proprietary code base created with no community review or oversight at all. Predictably, it had all kinds of problems which required well over a year of work to resolve. Until mac80211 was in reasonable shape, there was no real way to get drivers ready for inclusion.

The result of all this (and the occasional legal hassle as well) is that wireless networking on Linux lagged for years, and is only now reaching something close to a stable state. So it is not surprising that there has been a lot of code churn in this area, or that things occasionally break. But it is hard to see how trying to merge wireless drivers sooner would have helped the situation significantly.

The non-merging of the Nouveau driver - the reverse-engineered driver for NVIDIA adapters - also has a simple explanation: the developers have not yet asked for this merge to happen. Nouveau is not considered to be at a point where it works yet, and, importantly, there are still user-space API issues which must be worked out. Breaking user-space code is severely frowned upon, so merging of code is nearly impossible if its user-space interfaces are still in flux.

James Bottomley put forward another reason why a driver may stay out of the mainline even though the author would like to see it merged:

In other words, their control over access to the mainline tree is the one club subsystem maintainers have at hand when they feel the need to push a developer to make changes to a driver. It may well be that simply merging drivers regardless of technical objections (something which a number of developers are pushing for) will reduce the incentive for developers to get their code into top shape - and it's not always clear that others will step in and do the work for them.

On the other hand, the idea that in-tree code tends to be less buggy than out-of-tree code is relatively uncontroversial. So, for many drivers at least, a "merge first and fix it up later" policy may well lead to the best results in the shortest period of time. One thing that is clear is that this discussion will not be going away anytime soon; chances are good that this year's kernel summit (happening in September) will end up revisiting the issue.

Index entries for this article
Kernel	Development model/Driver merging
Kernel	Device drivers

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to post comments

Couldn't disinclusion be used as a stick? i.e. 'Fix these critical issues 
or your driver gets tossed'

Seems like we might be able to learn a lesson from the delay in getting a decent wireless
stack.

Something (like this) likely to affect everybody is not that hard to identify. Every
distribution should recognize that a coordinated effort, probably with centralized resources,
possibly with special task funding would be a benefit to themselves. Could not someone (at
each distro) be charged with the job of looking for these needs/opportunities?

Regards,
..jim