linux-kernel - Re: [PATCH 0/3] capebus moving omap

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Message-ID: <20121102110038.GA19493@arwen.pp.htv.fi>
Date:	Fri, 2 Nov 2012 13:00:38 +0200
From:	Felipe Balbi <balbi@...com>
To:	Russ Dill <Russ.Dill@...com>
CC:	<balbi@...com>, Pantelis Antoniou <panto@...oniou-consulting.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"Cousson, Benoit" <b-cousson@...com>,
	Tony Lindgren <tony@...mide.com>,
	<linux-kernel@...r.kernel.org>,
	Koen Kooi <koen@...inion.thruhere.net>,
	Matt Porter <mporter@...com>, <linux-omap@...r.kernel.org>,
	Kevin Hilman <khilman@...com>, Paul Walmsley <paul@...an.com>
Subject: Re: [PATCH 0/3] capebus moving omap_devices to mach-omap2

Hi,

On Fri, Nov 02, 2012 at 02:42:51AM -0700, Russ Dill wrote:
> >> browse through various detect functions, yes, some of them key off an
> >> ID, but a lot of them just check various registers to see if certain
> >> bits are zero, or certain bits are one. A lot of I²C devices I've
> >> dealt with have no good way of probing them, especially GPIO chips
> >> (you'll notice none of the I²C GPIO expanders have detect functions)
> >
> > it doesn't mean it can't be done.
> 
> Really? Please, do tell how you would write a detect function for a
> PCA9534. It has 4 registers, an input port registers, an output port
> register, a polarity inversion register, and a configuration register.

read them and match to their reset values, perhaps ?

> And don't forget, since we are probing, every detect routine for every
> I²C driver will have to run with every I²C address on every bus,
> possibly with both address formats.

not *every* I2C address. What you say is wrong, a ->detect() method will
only run for those addresses which the device can actually assume.

> >> On top of all this the detect routine does not tell you if the alert
> >> pin is connected to some IRQ, or in the case of a GPIO expander, what
> >> those GPIOs are connected to, etc, etc.
> >
> > so what ? All you want to do with detect is figure out if the far end is
> > who you think it is, not what it's doing.
> 
> If we already knew who was there, we wouldn't need a detect routine.

of course not :-) But the whole discussion has been about not knowing
which capes (and thus which devices) are attached to the bone.

-- 
balbi

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