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SubjectRe: merging other repos into linux-2.6


On Thu, 30 Oct 2008, Greg KH wrote:
>
> But as for the 'bisectability' at one point in the merge, you will be
> adding a stand-alone driver into the kernel itself. So for anyone
> traversing down that path, all you would be building would be the driver
> itself, the whole rest of the kernel is "gone".

Right. This was exactly what happened initially in the btrfs thing. And it
was horrid.

It was horrid because it was totally unexpected for users, and causes huge
churn and confusion when trying to check out a totally different directory
layout (and git won't remove the old *.o files, so trust me, it _will_ be
confusing).

But it was horrid also because if you do it the straightforward but stupid
way (the way that git definitely does support) by using a "subdirectory
merge" to merge the individual driver into a new location, it also makes
the history almost totally useless. Suddenly trivial things like

git log drivers/xyz/file.c

don't work well at all any more, because in the old history it will all be
under the pathname "file.c" _without_ the drivers/xyz/ part. And the
merge diff will be just one huge mess..

So yes, git can do it that way, but the end result is so horrible as to be
worse than not importing the history at all!

What I got Chris Mason to do was to run

git filter-branch --index odd-script-goes-here

with that odd script looking something like:

> #!/bin/sh
> exists=$(git ls-files fs/btrfs/)
> [ -z "$exists" ] &&
> git ls-files --stage |
> awk -F '\t' '{ print "0 0000000000000000000000000000000000000000\t" $2 "\n" $1 "\tfs/btrfs/" $2 }' |
> git update-index --index-info

which basically does a directory rename in the branch (obviously, in this
case into fs/btrfs, which is not what _you_ want). That way, at least the
directory structure of the tree you merge has the same layout, and you
don't get _that_ particular directory jumping back and forth.

Chris also merged in the history at the 2.6.26 tree (I think), so that
while his original history had had just a stand-alone btrfs build, the end
result was actually *totally* bisectable. Again, you should ask him about
any other scripts he ran.

Linus


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