[PATCH v3 5/8] doc: devicetree: Updates for devicetree-rebasing subtree

[Tom Rini]

On Thu, Jan 04, 2024 at 03:45:10PM +0100, Michal Simek wrote:
> Hi Tom,
> 
> On 1/4/24 14:50, Tom Rini wrote:
> > On Thu, Jan 04, 2024 at 06:52:32PM +0530, Sumit Garg wrote:
> > > Hi Michal,
> > > 
> > > On Thu, 4 Jan 2024 at 15:39, Michal Simek <michal.simek at amd.com> wrote:
> > > > 
> > > > 
> > > > 
> > > > On 1/3/24 17:32, Tom Rini wrote:
> > > > > On Wed, Jan 03, 2024 at 09:19:40AM -0700, Rob Herring wrote:
> > > > > > On Thu, Dec 28, 2023 at 4:59 AM Sumit Garg <sumit.garg at linaro.org> wrote:
> > > > > [snip]
> > > > > > > +In order to maintain devicetree files sync, U-Boot maintains a Git subtree for
> > > > > > > +devicetee-rebasing repo as `devicetee-rebasing/` sub-directory. It is regularly
> > > > > > > +kept updated with every new kernel major release via subtree pull as follows::
> > > > > > 
> > > > > > I would suggest dropping "-rebasing" in the u-boot tree. (I wish we
> > > > > > had in the original repo). I don't think it's relevant.
> > > > > > 
> > > > > > We're not likely to regenerate the tree, but any clue what 'git
> > > > > > subtree pull' would do in this case? It could happen if we switched to
> > > > > > git-filter-repo.
> > > > > > 
> > > > > > > +
> > > > > > > +    git subtree pull --prefix devicetree-rebasing \
> > > > > > > +        git://git.kernel.org/pub/scm/linux/kernel/git/devicetree/devicetree-rebasing.git \
> > > > > > > +        <release-tag> --squash
> > > > > > 
> > > > > > I'd put this in a script to run. Documentation tends to be not quite
> > > > > > correct. A script could also be smarter and figure out <release-tag>.
> > > > > 
> > > > > Since you had mentioned elsewhere moving the kernel scripts/dtc/ to
> > > > > something using subtree, I do hope to learn some lessons from what you
> > > > > do there. The first thing is that given the size/nature of the commits,
> > > > > I had figured I'd be the one doing this as a subtree pull+squash then
> > > > > push, rather than posting to the ML since it'll be huge and
> > > > > un-reviewable, but with a note sent off to the ML that people should be
> > > > > aware the next sync has been done and retest as needed. But Sumit, were
> > > > > you planning to do some of this instead?  The second thing is that if we
> > > > > move the subtree part in to dts/ instead (where we will still have the
> > > > > Makefile/Kconfig we have today and then our Makefiles within the
> > > > > directories, this might get more complex and so really require a script
> > > > > to keep it from getting error prone?
> > > > 
> > > > I played with this series and didn't really have time to dig into subtree
> > > > mechanics but one thing I see is that when squash and merge happens you can't do
> > > > simple rebase anymore which is time to time very useful.
> > > > I see some different rebase strategies and --rebase-merges option but if this is
> > > > adopted it should be properly described how to rebase u-boot tree which contains
> > > > some subtrees.
> > > 
> > > Did you observe any specific difference with respect to subtree merge
> > > commits (Tom will likely do those in the beginning of next release
> > > cycle) when compared with normal merge commits when Tom pulls in code
> > > from different custodians like one example below? The same rebasing
> > > rules should apply to both types of merge commits.
> > > 
> > > commit dffa6d0210f57793f1e4e1e209d91ca5642e4d05 (origin/next)
> > > Merge: 2b28c3b871c e266d273114
> > > Author: Tom Rini <trini at konsulko.com>
> > > Date:   Mon Jan 1 12:38:15 2024 -0500
> > > 
> > >      Merge tag 'dm-next-1124' of
> > > https://source.denx.de/u-boot/custodians/u-boot-dm into next
> > > 
> > >      support propagating supernode properties with bootph schema
> > >      align bloblist with v0.9 of Firmware Handoff spec
> > 
> > There's two cases. The first and uncommon case is that if you want to do
> > something like:
> > - Branch next
> > - Add this subtree, do a few subtree updates on it (like my experiment
> >    of starting with v6.1-dts and merging up to v6.6-dts)
> > - Wait a while, have changes happen in next
> > - Rebase on to current next
> > 
> > It gets painful and odd, but I don't know that this will ever really
> > happen.
> > 
> > The second, common case is:
> > - Assume "next" already has this subtree + some merge commits in it.
> > - Make a new local branch.
> > - Make change in your new local branch.
> > - This hypothetical "next" branch moves forward with changes
> > - Rebase your local branch on to this "next" branch
> > 
> > In this case things work as expected. Even if  "next" in that case gets
> > a new subtree merge that you need to rebase in.
> > 
> > A third case that I'm not sure if really will happen or is a case where
> > the custodian should rework their tree instead (to drop the subtree
> > merge) is:
> > - Assume "next" already has this subtree + some merge commits in it.
> > - Make a new local branch.
> > - Make a subtree merge+squash in your local branch.
> > - This hypothetical "next" branch moves forward with changes
> > - Rebase your local branch on to this "next" branch.
> > 
> > This will then make you try and resolve some conflicts from that subtree
> > merge.  But is this a valid use case? Yes, I can see wanting to get a
> > start on testing a dts merge before it happens, but is this a tree which
> > needs to be rebased, or just re-created as needed, if the parent branch
> > moves forward?
> > 
> > Can you explain a bit more your case Michal since it sounded to me like
> > you had a problem that happened rather than a worry about what might
> > happen?
> 
> I remember couple of bisects which I was doing and it works fine on the tree
> till the point you have 2 issues to deal with. It means pretty much you have
> config change/fix which has to be applied to see second issue.
> 
> The normal way what I have done was simply find major version apply that fix
> and then take the latest tree and rebase latest on the top of it. There were
> normally not so many conflicts to resolved (or easy to resolve/ignore). Then
> because the first fix is present all the time bisect is able to help me to
> find second issue.
> 
> If there is subtree update the whole rebase will horribly fail.
> 
> I see that Rob and you in DTC case just simply do c&p and create regular
> commit with all changes inside. From my perspective this is more
> straightforward to deal with which will handle all cases and it is also
> proven over that years.

Ah, bisecting.  Maybe the (annoying) answer is to turn that merge commit
in to a regular commit in the bisect branch?  This is a good point, and
I need to go and construct a bigger example tree and check around at
what happens.

-- 
Tom
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