[U-Boot] Custodian branch base commits

[Stephen Warren <swarren@wwwdotorg.org>]

The instructions at:

http://www.denx.de/wiki/U-Boot/CustodianGitTrees

... indicate that downstream branches should be rebased onto
${upstream}/master immediately prior to sending a pull request. I'm not
sure this is correct any more, given that upstream master branches now
pull in downstream branches via "git merge" rather than "git rebase".
Should that page be changed?

The question then becomes: What commit should downstream branches use as
their baseline. I assert that they should be free to choose any commit
in their upstream master branch (or merge in any upstream commit), or
any further upstream branch all the way up to u-boot/master.

As background, I want to send a commit to the Tegra tree that enables
use of CONFIG_CMD_FS_GENERIC, and additionally, re-write bootcmd in
Tegra's default environment to use the new shell commands that config
option enables.

However, the commits that implement CONFIG_CMD_FS_GENERIC are in
u-boot/master but not yet in u-boot-arm/master. So, Tegra's master
branch can't simply be based on current u-boot-arm/master, since if it
was, applying my patch would break functionality until it was merged
with u-boot/master and so picked up the actual implementation.

Hopefully, u-boot-arm/master will merge in (or get reset to)
u-boot/master fairly shortly. However, should Tegra's repo have to wait
for this?

Instead, I propose that 2nd-level custodians should be able to do
something like:

# (in u-boot-tegra.git, assuming everything is merged already)
git checkout master
git reset --hard u-boot/master
git am patch_that_depends_on_uboot_master
# send pull request to u-boot-arm/master

# (in u-boot-tegra.git, assuming some patches already applied locally)
git checkout master
git merge u-boot/master
git am patch_that_depends_on_uboot_master
# send pull request to u-boot-arm/master

This should work fine provided that u-boot/master and u-boot-arm/master
guarantee never to rebase. If they don't make that guarantee, then I
don't think we can be using merges at all anyway, so that shouldn't be
an issue.

What do people think?