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Date:   Wed, 3 Jul 2019 20:57:58 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Luis Chamberlain <mcgrof@kernel.org>
Cc:     Christoph Hellwig <hch@lst.de>,
        Brendan Higgins <brendanhiggins@google.com>,
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Subject: Re: [RFC PATCH 0/5] Add CONFIG symbol as module attribute
Message-ID: <20190703185758.GB14336@kroah.com>
References: <20160825074313.GC18622@lst.de>
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On Wed, Jul 03, 2019 at 04:50:20PM +0000, Luis Chamberlain wrote:
> On Wed, Jul 03, 2019 at 09:40:48AM +0200, Greg Kroah-Hartman wrote:
> > On Tue, Jul 02, 2019 at 08:51:06PM +0000, Luis Chamberlain wrote:
> > > On Sat, Jun 29, 2019 at 10:42:57AM +0200, Greg Kroah-Hartman wrote:
> > > > On Fri, Jun 28, 2019 at 11:40:22AM -0700, Luis Chamberlain wrote:
> > > > > On Wed, Jun 26, 2019 at 9:51 PM Christoph Hellwig <hch@lst.de> wrote:
> > > > > >
> > > > > > On Wed, Jun 26, 2019 at 03:21:08PM -0700, Luis Chamberlain wrote:
> > > > > > > On Tue, Feb 5, 2019 at 2:07 PM Luis Chamberlain <mcgrof@kernel.org> wrote:
> > > > > > > > In lieu of no Luke Skywalker, if you will, for a large kconfig revamp
> > > > > > > > on this, I'm inclined to believe *at least* having some kconfig_symb
> > > > > > > > exposed for some modules is better than nothing. Christoph are you
> > > > > > > > totally opposed to this effort until we get a non-reverse engineered
> > > > > > > > effort in place? It just seems like an extraordinary amount of work
> > > > > > > > and I'm not quite sure who's volunteering to do it.
> > > > > > > >
> > > > > > > > Other stakeholders may benefit from at least having some config -->
> > > > > > > > module mapping for now. Not just backports or building slimmer
> > > > > > > > kernels.
> > > > > > >
> > > > > > > Christoph, *poke*
> > > > > >
> > > > > > Yes, I'm still totally opposed to a half-backed hack like this.
> > > > > 
> > > > > The solution puts forward a mechanism to add a kconfig_symb where we
> > > > > are 100% certain we have a direct module --> config mapping.
> > > > > 
> > > > > This is *currently* determined when the streamline_config.pl finds
> > > > > that an object has only *one* associated config symbol associated. As
> > > > > Cristina noted, of 62 modules on a running system 58 of them ended up
> > > > > getting the kconfig_symb assigned, that is 93.5% of all modules on the
> > > > > system being tested. For the other modules, if they did want this
> > > > > association, we could allow a way for modules to define their own
> > > > > KBUILD_KCONF variable so that this could be considered as well, or
> > > > > they can look at their own kconfig stuff to try to fit the model that
> > > > > does work. To be clear, the heuristics *can* be updated if there is
> > > > > confidence in alternative methods for resolution. But since it is
> > > > > reflective of our current situation, I cannot consider it a hack.
> > > > > 
> > > > > This implementation is a reflection of our reality in the kernel, and
> > > > > as has been discussed in this thread, if we want to correct the gaps
> > > > > we need to do a lot of work. And *no one* is working towards these
> > > > > goals.
> > > > > 
> > > > > That said, even if you go forward with an intrusive solution like the
> > > > > one you proposed we could still use the same kconfig_symb...
> > > > > 
> > > > > So no, I don't see this as a hack. It's a reflection as to our current
> > > > > reality. And I cannot see how the kconfig_symb can lie or be
> > > > > incorrect. So in fact I think that pushing this forward also makes the
> > > > > problem statement clearer for the future of what semantics needs to be
> > > > > addressed, and helps us even annotate the problematic areas of the
> > > > > kernel.
> > > > > 
> > > > > What negative aspects do you see with this being merged in practice?
> > > > 
> > > > I'm trying to see what the actual problem that you are wanting to solve
> > > > here with this.  What exactly is it?
> > > 
> > > The problem is that there is no current maping of a module to respective
> > > kconfig symbol.
> > 
> > That's because it is not just "one" symbol per module.
> 
> This is true. But it is not the case for all modules.  In fact it seems
> its true that most modules do have *one* main symbol.

You mean "one unique symbol from all other modules", right?

That is much different than just "one" symbol, given that almost
every driver depends on something else being enabled as well (bus type,
platform type, arch, etc.)

And I would argue, that finding that "one" symbol is easy, just parse
the Makefiles.  But I would also state that this "one" symbol doesn't
really help you much as those are the "simple" things.  It's how to
turn on all of the required symbols to get to that "one" symbol that is
the hard part.

And conversely, if you disable that "one" symbol, does that also mean
you can disable the symbols it depended on?  If so, how far back?

And what about functionality?  If my usb-storage device is "enabled" in
the build, yet all filesystems are not, or the needed dm module is not,
it is useless.  Hardware requires usually more than one real "symbol" in
order to work properly, as you know.

And of course, what does this really matter to anyone?  If you build
"all modules" and you only load the modules you actually use for your
hardware (based on auto-loading), then your system uses the same amount
of memory as if you disabled all of the modules you did not need.

Yes, it's faster to build, but is that what you are trying to optimize
for here?

Anyway, if this is just an acidemic thing, have fun, but I would not be
adding anything else to the module image that is not really going to be
useful to anyone.

good luck!

greg k-h