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

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Message-Id: <200711132319.48821.david-b@pacbell.net>
Date:	Tue, 13 Nov 2007 23:19:47 -0800
From:	David Brownell <david-b@...bell.net>
To:	"eric miao" <eric.y.miao@...il.com>
Cc:	"Linux Kernel list" <linux-kernel@...r.kernel.org>,
	"Felipe Balbi" <felipebalbi@...rs.sourceforge.net>,
	"Bill Gatliff" <bgat@...lgatliff.com>,
	"Haavard Skinnemoen" <hskinnemoen@...el.com>,
	"Andrew Victor" <andrew@...people.com>,
	"Tony Lindgren" <tony@...mide.com>,
	"Jean Delvare" <khali@...ux-fr.org>,
	"Kevin Hilman" <khilman@...sta.com>,
	"Paul Mundt" <lethal@...ux-sh.org>,
	"Ben Dooks" <ben@...nity.fluff.org>
Subject: Re: [patch/rfc 1/4] GPIO implementation framework


> > >  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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