[PATCH] kbuild: Enable building defconfigs from Kconfig files

[Linus Torvalds]

On Tue, Jul 13, 2010 at 10:47 PM, Grant Likely
<grant.likely at secretlab.ca> wrote:
>
> Okay, well I advocate for the -D /dev/null approach then.  I think
> that validating our defaults, and looking for the subtle interactions
> are exactly what we want to be doing when it comes to defconfigs.  The
> fact that a defconfig does *not* want the default value is exactly
> what the defconfigs should be capturing.

Sure. That does have the downside that it absolutely _requires_
changes to the Kconfig language. Because currently "select" can only
set a config variable, it cannot clear it or downgrade it from the
defaults - so if you start from anything but a noconfig, you can't
unset anything.

But that's not a big downside, since honestly either approach will
want it anyway (even -n wants it for setting integer or string options
- never mind the whole issue of making it easier/cleaner to write the
files in the first place).

>>          set NODES_SHIFT 10
>
> I'm partial to extending select statements myself because it fits
> nicely into the existing grammer; but I can see value in having a set
> statement too.

If it turns out easier to just implement the "set value" as a "select
with a value", that's fine by me too. If the syntax for setting a
particular value ends up being

   select NODES_SHIFT 10

that still works (with or without a "=" there). It looks a bit odd to
me, but that's probably just because it's new. It doesn't look _wrong_
by any stretch.

> It would eliminate the temporary config options that
> both my and Stephen's patch would add.

I don't think there is anything wrong with having the extra config
option just to drive the "select" syntax. In fact, in many cases I
think it would be "documentation", because I think the extra config
option should generally be named by the platform that you're actually
selecting for - even if that name may not be used for anything else.

So having the basic syntax for a Kconfig.xyz file be something like

         config SIDEWINDER_PLATFORM
              def_bool y
              select ...

where that initial config/def_bool is strictly not needed for the
actual end result, but is used to get to the select statements, and to
include that "SIDEWINDER_PLATFORM=y" in the resulting .config as a
kind of top-level comment all looks fine.

                 Linus