linux-kernel - Re: diet-kconfig: a script to trim unneeded kconfigs

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Date:	Fri, 19 Sep 2008 00:14:23 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	Valdis.Kletnieks@...edu
Cc:	Mauro Carvalho Chehab <mchehab@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: diet-kconfig: a script to trim unneeded kconfigs

At Thu, 18 Sep 2008 17:09:27 -0400,
Valdis.Kletnieks@...edu wrote:
> 
> On Thu, 18 Sep 2008 22:17:22 +0200, Takashi Iwai said:
> 
> > There are different perspectives for this.  My goal to reduce the
> > build time with the existing kernel config for a specific hardware.
> > That is, it makes eaiser to create another set of kernel with test
> > patches or bisect.
> 
> One has to be careful here - I've more than once hit kernel bugs that I ended
> up bisecting, which were dependent on the exact .config used.

That's true.  But, you should at least test the trimmed kernel once
before bisecting to reproduce the bug.
And, my script doesn't do anything so intrusive.  It trims just unused
CONFIG_*=m.  Since they are not loaded anyway, reducing them must be
harmless (in theory).

>  The most recent
> was a deadlock during early boot if CONFIG_HID_COMPAT=y, which would have
> been turned off by any sane kconfig trimmer.  On the flip side, running a
> trimmer wouldn't minimize my .config very much, because I've already set most
> stuff I *never* use to 'n'.

Heh, you are obviously not a "typical" user ;)

My scenario was for users of distro kernel who want to help debugging
with some given patches or bisecting.

> And more than once, I've had a bisect go horribly awry and need to be re-started
> because the bisect crossed back and forth over a commit that added a config
> option, so doing a 'make oldconfig' at each iteration would re-prompt, and
> I wasn't consistent in the reply I gave.  Usually doesn't matter, except when
> the broken code is in the support for the feature...

Bisect isn't perfect, indeed.  But, in many cases, it can catch
the harmful commit more easily, especially if it's in the area
where you are unfamiliar.

> I *do* keep around a *very* minimal "only what I need to get to single-user"
> config if build time is an issue...
> 
> Might be different if you're starting off with a distro config that's basically
> an 'allmodconfig' - lotta fat to trim off *that*....

Exactly.

thanks,

Takashi
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