[U-Boot] ARM Workflow: rebase on ARM repositories

[Stefano Babic]

Hi all ARM Custodians,

I was thinking about the way we are usual to manage our trees. This
comes because I know about issues from users who set their development
on ARM-repositories.

One of them uses u-boot-imx for his development, and of course after I
rebased my tree he got into trouble, due to using a commit that does not
exist anymore.
Nevertheless there are boards, where the official documentation explain
how to set patches on bases of u-boot-arm. For example,

	http://www.ti.com/tool/tmdsevm3530

Detlev discovers that the official documentation refers directly to
commit cf6ec699a6dc21a538b039a0392cd38132072090 in u-boot-arm. After a
rebase this commit does not exist anymore.

Of course, we can really say that setting a development on a ARM
repository instead of main repository is not the best ;-). But we know
that sometimes setting on a partial repository is the best because some
patches that are strictly required are already merged. And I do not know
if we can say that our trees are "private" or "development" only: they
are published, and available for everybody.

Albert has described the way we are currently using in
http://www.denx.de/wiki/view/U-Boot/CustodianGitTrees. I think you konow
very well and it is the way we follow, and we usually rebase our tree
after u-boot-arm is merged by Wolgang in mainline. I want to discuss
here if we really need it and if this is the correct way to do.

In Linux, every maintainer makes a "git pull" from Linus' tree. Nobody
rebases, and I had never had the problem that my tree diverges when I
update my kernel's trees from a mainatiner tree. However, this happens
continuosly for the users of u-boot-imx. The way we are following  can
be seen in the linux-next trees, not in the main trees.

The worst thing I think is that we lose the history of our tree, and the
behavior can be different. Rebased patches can be different, and testing
done until the rebase can be worthless and should be (theoretically)
done again. Testers can say they have successfully test a patch, but it
was on a pre-rebased tree. A tested-by in a rebased tree can be worthless.

My big question is if we should not to come back using "git pull" to
downstream mainline from Wolfgang's tree, instead of continuos rebase. I
know that we switched to rebase to avoid a lot of "git merge commits",
but maybe this is not so bad as rebasing.

What is your opinion ?

Regards,
Stefano

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