Re: [patch/rfc 1/4] GPIO implementation framework

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

> > >  struct gpio_desc {
> > >       struct gpio_chip *chip;
> > >       unsigned is_out:1;
> > > +     unsigned requested:1;
> > > +#ifdef CONFIG_DEBUG_FS
> > > +     const char *requested_str;
> > > +#endif
> >
> > Note that this means (on typical 32-bit embedded hardware)
> > twelve bytes per GPIO, which if you assume 256 GPIOs means
> > an extra 3 KB static memory compared to the patch I sent.

Actually, 2K is a more accurate number -- ignore DEBUG_FS.


> Note this reduces the memory in gpio_chip, so it consumes almost same
> memory as the patch you sent.

No; the amount of space shaved from a typical (32-bit banks)
gpio_chip is *exactly* the cost of one gpio_desc:  two words.
In one case, two bitmaps.  In the other, a pointer, two bits,
and internal struct padding.

So unless each bank has only a single GPIO, this approach
does cost more memory.  Both for the extra memory associated
with each gpio_chip that's used, and for unused gpio_desc.
 
That's not necessarily a bad thing, though it's always worth
avoiding bloat.

- Dave
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