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

[Luis R. Rodriguez]

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@xxxxxxxxx
> > 
> > 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