linux-kernel - Re: lpc18xx: undefined Kconfig option ARCH

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Date:	Tue, 12 May 2015 14:03:33 +0200
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Valentin Rothberg <valentinrothberg@...il.com>
Cc:	Joachim Eastwood <manabian@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	Alexandre Courbot <gnurou@...il.com>,
	Paul Bolle <pebolle@...cali.nl>,
	hengelein Stefan <stefan.hengelein@....de>,
	Andreas Ruprecht <andreas.ruprecht@....de>
Subject: Re: lpc18xx: undefined Kconfig option ARCH_LPC18XX

On Tue, May 12, 2015 at 12:48 PM, Valentin Rothberg
<valentinrothberg@...il.com> wrote:
> On Tue, May 12, 2015 at 12:40 PM, Linus Walleij
> <linus.walleij@...aro.org> wrote:
>> On Wed, May 6, 2015 at 10:02 AM, Valentin Rothberg
>> <valentinrothberg@...il.com> wrote:
>>
>>> Hi Joachim,
>>>
>>> there are two of your commits [1, 2] in today's linux-next (i.e.,
>>> next-20150506).  Both of them add Kconfig options that depend on
>>> ARCH_LPC18XX, which is not defined in Kconfig, see:
>>>
>>> +       depends on OF && (ARCH_LPC18XX || COMPILE_TEST)
>>>
>>> Is there a patch queued somewhere to add ARCH_LPC18XX ?
>>
>> Yes, and in this case it solves more problems to merge these
>> patches out-of-order than not to.
>
> Could you explain those problems?  I have some research interest in
> such cases and want to understand and identify reasons for such
> things.

What we want as maintainers is to keep GIT trees in isolation
without too much cross-dependencies. Often done by assuming
optimistically that patches are orthogonal.

Non-trivial dependencies are for example if one patch introduce
a header file that another patch is using, the result will
not compile, so we need to resolve to the exit strategies of:

- (A) Applying the same patch to two trees, which will be
  handled by git as the patches are textually identical.
- (B) Creating an immutable branch pulled into both trees, which
  is better since it means the same hash is in both trees
- (C) apply to one tree, wait for the merge window, and postpone
  to the next kernel cycle, which delays development

Whenever we can orthogonalize patches it makes our job
easier.

Kconfig symbols added in one tree and used in another
does not create compilation wreckage and will resolve nicely
in linux-next and during the merge window, however the
trees still have a formal dependency which would argue that
you should go back to option (B) above in the ideal case
and create an immutable branch.

However doing that for something that you, as a human
maintainer, know will fix itself later, is surplus process for
the sake of process and "never making mistakes" which
cost more than it yields in maintainer time, thus making it
a time-cost economically rational decision not to go to all
the trouble of setting up an immutable branch but instead
rely on some uncertainty and laziness to save time.

The answer to whether a certain maintainer will dare to do so
or not is per individual preference. The crucial point is that
"time savings" trumps "nothing can ever go wrong".

Yours,
Linus Walleij
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