Re: [patch 2.6.25-rc5 1/2] gpiolib: dynamic gpio number allocation

[David Brownell <david-b@pacbell.net>]

On Thursday 13 March 2008, Andrew Morton wrote:

> > > Shouldn't ARCH_NR_GPIOS be CONFIG_NR_GPIOS?
> > 
> > No more than NR_IRQS is settable via Kconfig.  And for
> > rather similar reasons.  :)
> 
> What are those reasons?

Keeping a lid on the amount of space wasted by unused
table entries is one factor; it's an implementation
tradeoff.  In normal usage the number of IRQs (or GPIOs)
is defined by the board (or system) design, and there's
no real point to allowing more.

That said, NR_IRQS is kind of inflexible.  It's not easy
to provide board-specific overrides for cases like having
a few FPGAs or other external IRQ (or GPIO!) controllers
which chain IRQs and plug into those tables...

Both could use a way to extend a platform-defined minimum
to support a configurable number of external controllers.
Lacking that, both have somewhat ad-hoc solutions to make
sure board variants can be set up with the same kernel.
It basically boils down to making sure there are some extra
entries at end-of-table, and policies to allocate them.

- Dave