linux-kernel - Re: [RFC PATCH 0/5] Add CONFIG symbol as module attribute

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Date:   Tue, 23 Aug 2016 23:32:24 +0200
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     Michal Marek <mmarek@...e.com>
Cc:     "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Cristina Moraru <cristina.moraru09@...il.com>,
        "vegard.nossum@...il.com" <vegard.nossum@...il.com>,
        Valentin Rothberg <valentinrothberg@...il.com>,
        Hannes Reinecke <hare@...e.de>,
        Sam Ravnborg <sam@...nborg.org>, linux-kernel@...r.kernel.org,
        teg@...m.no, kay@...y.org, rusty@...tcorp.com.au,
        akpm@...ux-foundation.org, backports@...r.kernel.org,
        Guenter Roeck <linux@...ck-us.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "rafael.j.wysocki" <rafael.j.wysocki@...el.com>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Takashi Iwai <tiwai@...e.de>, Christoph Hellwig <hch@....de>,
        Mauro Carvalho Chehab <mchehab@....samsung.com>,
        Johannes Berg <johannes@...solutions.net>,
        Hauke Mehrtens <hauke@...ke-m.de>,
        Paul Bolle <pebolle@...cali.nl>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Alexey Khoroshilov <khoroshilov@...ras.ru>,
        Sathya Prakash Veerichetty <sathya.prakash@...adcom.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Laurence Oberman <loberman@...hat.com>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Tejun Heo <tj@...nel.org>,
        Jej B <James.Bottomley@...senpartnership.com>,
        Theodore Ts'o <tytso@....edu>, danijons@...dent.chalmers.se,
        Andrzej Wasowski <wasowski@....dk>
Subject: Re: [RFC PATCH 0/5] Add CONFIG symbol as module attribute

On Fri, Aug 19, 2016 at 11:07:36AM +0200, Michal Marek wrote:
> On 2016-08-18 19:55, Luis R. Rodriguez wrote:
> > On Wed, Aug 17, 2016 at 09:26:58PM +0200, Cristina Moraru wrote:
> > 
> >> This patchset implements dynamic pegging of kconfig symbol
> >> into driver modinfo section
> > 
> > First a little bit of motivation here helps, so let me try to
> > help fill in some gaps. This may help explain what you have
> > been working on a bit more.
> > 
> > First, for those that were not Cc'd but curious about this work
> > so far, you can read the patches here:
> > 
> > Original cover letter:
> > 
> > https://lkml.kernel.org/r/1471462023-119645-1-git-send-email-cristina.moraru09@gmail.com
> > 
> > https://marc.info/?l=linux-kernel&m=147146213519750&w=2 - patch 1
> > https://marc.info/?l=linux-kernel&m=147146209019744&w=2 - patch 2
> > https://marc.info/?l=linux-kernel&m=147146211819747&w=2 - patch 3
> > https://marc.info/?l=linux-kernel&m=147146209119745&w=2 - patch 4
> > https://marc.info/?l=linux-kernel&m=147146209519746&w=2 - patch 5
> > 
> > There are a few situations in which you may want to extract a
> > subset of Kconfig CONFIG_* symbols for a system. At least when
> > only considering modules:
> > 
> >  a) When optimizing build requirements for a kernel for a system.
> >     That is you boot into a distro kernel and then want to build
> >     a slim kernel only with sensible kernel configuration options.
> > 
> >  b) When you are on a distribution kernel but the distribution
> >     kernel provided lacks hardware support for your device, you
> >     may either want to upgrade the full kernel in which case you
> >     want to do a) or -- you may want to just a backports release
> >     which provides just the modules you need, you'd use it on top
> >     of the distribution kernel.
> 
> c) Having the mapping in sysfs would allow to simplify
> streamline_config.pl avoid parsing Makefiles in perl.

Indeed, this was actually a side possible goal here :) only it seems
we totally forgot about it while addressing the work.

> Only if the patch did not depend on streamline_config.pl :)

Indeed, lets get rid of that dependency.

> One idea would be to generate
> the Module.ksymb in a similar way we generate the modules.builtin file:
> Generate an alternate include/config/*.conf with all CONFIG_FOO=m
> replaced with
> 
> CONFIG_FOO=m-CONFIG_FOO
> 
> and in the Makefile, iterate over $(filter m-CONFIG_%, $(.VARIABLES)) to
> create the mapping. This would also properly cover cases where we build
> the $(obj-m) list from another list. It would certainly create other
> corner cases, but it's worth trying IMO.

Neat, would that cover lib-m then as well or other targets that would generate 
modules ?

> Another thing is that we do not necessarily need to record this
> information in .modinfo, but we can generate a list in
> /lib/modules/`uname -r`/ for consumption. It could also include drivers
> that are builtin in the current configuration.

Well so one future use would be for instance a 'make udevconfig' which
in theory should be able then to scrape all possible related needed
device CONFIG_'s and then give you an optimal kernel configuration.
It'd be nice to be able to extract this from the kernel and modules
without needing build sources in /lib/modules/`uname -r`/.

Thoughts?

  Luis