Process for cherry-picking patches from other projects

[George Dunlap <George.Dunlap@xxxxxxxxxx>]

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.

Andy and Jan, what do you guys normally do?

Thanks,
 -George

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