Date:   Wed, 24 Aug 2016 13:05:04 +0200
From:   Michal Marek <mmarek@...e.com>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     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 2016-08-23 23:32, Luis R. Rodriguez wrote:
> 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
                                       should be obj-m-CONFIG_%

>> 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 ?

This would have to be handled as well by the patch. Obviously it's not
going to be as simple as described in a single paragraph of text :).


>> 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`/.

I meant that the file can be installed by modules_install. Like you have
/lib/modules/`uname -r`/modules.builtin with recent kernels and recent
distro packages. But really, it's a minor issue how the information is
presented to userspace.

Michal

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