Date:	Fri, 2 Nov 2012 10:57:18 +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 Thu, Nov 01, 2012 at 04:49:23PM -0700, Russ Dill wrote:
> On Thu, Nov 1, 2012 at 3:05 PM, Felipe Balbi <balbi@...com> wrote:
> > HI,
> >
> > On Thu, Nov 01, 2012 at 03:59:50PM +0200, Pantelis Antoniou wrote:
> >> Hi Alan,
> >>
> >> On Nov 1, 2012, at 3:51 PM, Alan Cox wrote:
> >>
> >> >> What they want, and what every user wants, is I plug this board in, and
> >> >> the driver make sure everything is loaded and ready. No, the end users
> >> >> don't want to see any of the implementation details of how the bitfile
> >> >> is transported; the driver can handle it.
> >> >
> >> > That doesn't necessarily make it a bus merely some kind of hotplug
> >> > enumeration of devices. That should all work properly both for devices
> >> > and busses with spi and i²c as the final bits needed for it got fixed
> >> > some time ago.
> >> >
> >> > In an ideal world you don't want to be writing custom drivers for stuff.
> >> > If your cape routes an i²c serial device to the existing system i²c
> >> > busses then you want to just create an instance of any existing driver on
> >> > the existing i²c bus not create a whole new layer of goop.
> >> >
> >> > It does need to do the plumbing and resource management for the plumbing
> >> > but thats not the same as being a bus.
> >> >
> >> > Alan
> >>
> >>
> >> Fair enough. But there's no such thing a 'hotplug enumeration
> >> construct' in Linux yet, and a bus is the closest thing to it. It does
> >> take advantage of the nice way device code matches drivers and devices
> >> though.
> >>
> >> I'm afraid that having the I2C/SPI drivers doing the detection won't
> >> work.  The capes can have arbitrary I2C/SPI devices (and even more
> >> weird components).  There is no way to assure for example that the I2C
> >> device responding to address 'foo' in cape A is the same I2C device
> >> responding to the same address in cape B.
> >
> > your ->detect() method should take care of that.
> 
> There isn't some magical serial number in I²C devices that a
> ->detect() method can read and the implementation of I²C is somewhat
> flexible. One devices read may be another devices write. A detect

look at what other drivers do. You can read a revision register, you can
write a command and see if the device responds as expected, it doesn't
matter.

> method that only performs reads could easily toggle a gpio that resets
> the board, rewrite and eeprom, or set the printer on fire. If you

how ? It's just a read.

> 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.

> 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.

-- 
balbi

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