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

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Message-ID: <20121102112104.4657fb7b@pyramind.ukuu.org.uk>
Date:	Fri, 2 Nov 2012 11:21:04 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Russ Dill <Russ.Dill@...com>
Cc:	balbi@...com, Pantelis Antoniou <panto@...oniou-consulting.com>,
	"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

> >> 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.

A bus is the wrong construct. You need something to add devices onto the
busses. You can do that. The Intel SFI layer on phones for example
enumerates devices through a firmware table set and creates them on the
right actual physical bus not on their own fake one.

It's not hotplug in the sense that the phone happens be a fixed
configuration but it does support hotplug behaviour because the order of
the drivers and enumeration is undefined (and both orders work).

> >>
> >> 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

It doesn't matter.

What you are basically talking about is

	cape layer
		- wtf is this
		- how do I plumb it

		- create device nodes with correct name for
		  binding, address etc on the right bus

	i2c layer
		- ooh a new i2c device
		- probe as indicated by device name
		- attach correct driver

Architecturally its possible you want to make some caps MFDs if they have
their own bus heirarchies etc but generally I doubt it.

Take a look at arch/x86/platform/mrst/mrst.c. It's a specific example of
a platform which parses tables and attaches devices to the right physical
bus in a manner they can be reliably probed even when the device has no
sane autodetect.

Alan
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