Re: Process for cherry-picking patches from other projects

[George Dunlap <George.Dunlap@xxxxxxxxxx>]

> On May 13, 2022, at 3:52 PM, Juergen Gross <JGross@xxxxxxxx> wrote:
> 
> On 13.05.22 16:33, George Dunlap wrote:
>> Starting a new thread to make it clear that we’re discussing a wider policy 
>> here.
>> This question is aimed at Jan and Andy in particular, as I think they’ve 
>> probably done the most of this; so I’m looking to them to find out what our 
>> “standard practice” is.
>> There have recently been some patches that Bertrand has submitted which pull 
>> in code from Linux ("[PATCH 1/3] xen/arm: Sync sysregs and cpuinfo with 
>> Linux 5.18-rc3”), which has caused a discussion between him, Julien, and 
>> Stefano about the proper way to do such patches.
>> The “Origin:” tag section of xen.git/docs/process/sending-patches.pandoc 
>> suggests that there are some standards, but doesn’t spell them out.
>> The questions seem to be:
>> 1) When doing this kind of update, is it permissible to send a single patch 
>> which “batches” several upstream commits together, or should each patch be 
>> backported individually?
>> 2) If “batches” are permissible, when? When would individual patches be 
>> preferred?
>> 3) For “batch updates”, what tags are necessary? Do we need to note the 
>> changesets of all the commits, and if so, do we need multiple “Origin” tags? 
>> Do we need to include anything from the original commits — commit messages? 
>> Signed-off-by’s?
>> And a related question:
>> 4) When importing an entire file from an upstream like Linux, what tags do 
>> we need?
>> My recollection is that we often to a “accumulated patch” to update, say, 
>> the Kconfig tooling; so it seems like the answer to this is sometimes “yes”.
>> It seems to me that in a case where you’re importing a handful of patches — 
>> say 5-10 — that importing them one-by-one might be preferred; but in this 
>> case, since the submission was already made as a batch, I’d accept having it 
>> as a batch.
>> I think if I were writing this patch, I’d make a separate “Origin” tag for 
>> each commit.
>> I wouldn’t include the upstream commit messages or S-o-b’s; I would write my 
>> own commit message summarizing why I’m importing the commits, then have the 
>> ‘origin’ tags, then my own S-o-b to indicate that I am attesting that it 
>> comes from an open-source project (and for whatever copyright can be 
>> asserted on the commit message and the patch as a collection).
>> And for #4, I would do something similar: I would write my own commit 
>> message describing what the file is for and why we’re importing it; have the 
>> Origin tag point to the commit at the point I took the file; and my own 
>> S-o-b.
> 
> IMO we should add another tag for that purpose, e.g.:
> 
> File-origin: <repository> <tag> <path> [# <local-path>]
> 
> Specifying the repository the file(s) are coming from, the tag (e.g. a
> tagged version, or the top git commit), and the path of the original
> file(s) in that repository (<path> could either be a common directory
> of multiple files, or a single file; multiple "File-origin:" tags should
> be possible). In case the file is being renamed locally, its new name
> can be added as <local-path>.
> 
> This variant should be used to add _new_ files to Xen. In case of
> updating a file which has seem lots of commits since its last update or
> introduction, it might be easier to just use the "File-origin:" tag,
> probably with a note below the "---" marker that listing more than <x>
> patches (x > 10?) or splitting into more than <x> patches would be
> just useless work (common sense should apply here, especially regarding
> the readability of the patch and the related review effort).

You don’t mention what to do about SoB’s — I assume you agree with my 
assessment above, that a single  SoB from the submitter of the patch to Xen, 
asserting that they’re satisfied that all of the code has been asserted by 
other people as having a suitable license, is sufficient.

In which case, barring a contradiction from Andy or Jan as to our standard 
practice, I think that we don’t need to collect SoBs from the original commits; 
a single SoB by Bertrand (or whomever) that it all comes from Linux and that 
suitable SoBs can be tracked down should it become necessary, will be 
sufficient.

That should be enough to get the specific patch currently under discussion from 
the ARM maintainers un-stuck.

 -George

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