broken nouveau dependency on power supply

[Benjamin Herrenschmidt]

On Mon, 2012-04-02 at 11:06 +1000, Benjamin Herrenschmidt wrote:
> Hi folks !
> 
> With CONFIG_POWER_SUPPLY=m & nouveau built-in we get a build failure:
> 
> drivers/built-in.o: In function `.nouveau_pm_trigger':
> (.text+0xa56e8): undefined reference to `.power_supply_is_system_supplied'
> 
> nouveau probably needs to depends on CONFIG_POWER_SUPPLY to force a module
> build with the latter is =m

Ok, not that trivial...

The problem is more like POWER_SUPPLY should be a bool, not a tristate.

If you think about it: you don't want things like nouveau to depend on a
random subsystem like that, people will never get it. In fact,
POWER_SUPPLY provides empty inline stubs when not enabled, so that's
really designed to not have depends...

However that -cannot- work if POWER_SUPPLY is modular and the drivers
who use it are not. The only fixes here that make sense I can think of
that don't also involve Kconfig horrors are:

 - Ugly: in power_supply.h, use the extern variant if

	defined(CONFIG_POWER_SUPPLY) ||
	 (defined(CONFIG_POWER_SUPPLY_MODULE) && defined(MODULE))

IE. use the stub if power supply is a module and what is being built is
built-in. Of course that's not only ugly, it somewhat sucks from a user
perspective as the subsystem now exists but can't be used by some
drivers...

 - Better: Just make the bloody thing a bool :-) The power supply
framework itself is small enough, just make it a boolean option and
avoid the problem entirely. The actual power supply sub drivers can
remain modular of course.

Cheers,
Ben.