Zack's Kernel News

Git Merge "Simplification" Advice

Bjorn Helgaas submitted some PCI patches in the form of a merge request from another Git tree. This is a standard part of the development process for larger distributed projects like the Linux kernel, and this one included work from dozens of contributors. The idea is that a bunch of people work on a given sub-project in relative isolation so their changes don't break everyone else's work on the main Linux tree. Then, with the merge request, the contributors ask Linus Torvalds to resolve any conflicts that their changes might have produced with other changes going into the kernel at the same time. No biggie, nothing to see here. Tens of thousands of contributors can get their hands dirty at the same time, without throwing dirt onto any of their fellow contributors' hands while they're at it.

In this case, Linus noticed some wonky twirling going on behind the scenes, and it posed a problem for him. Specifically, Bjorn and his fellow PCI travelers had already done some merging from multiple separate trees (used for different sub-sub-projects within their sub-project), followed by a patch reversion, so that all merges going into Linus's official repository would seem to come from the same tree. It's not psychotic; they were just trying to keep things simple.

So first of all, Linus objected to the patch reversion itself. Patch reversions remove a patch that was previously accepted into a tree, but a reversion is itself a patch that also needs to be accepted via the same process as other patches – including having a meaningful commit message, which the PCI patch reversion did not. However, it's a relatively common occurrence for patch reversions to have no meaningful commit message – developers don't tend to see the point of it because all the patch reversion does is take something out that had recently been put in.

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