Enumerable I2C busses

[M. Warner Losh]

In message: <4929C6D8.7090305 at freebsd.org>
            Nathan Whitehorn <nwhitehorn at freebsd.org> writes:
: Rafał Jaworowski wrote:
: > 
: > On 2008-11-23, at 19:18, Dag-Erling Smørgrav wrote:
: > 
: >> Nathan Whitehorn <nwhitehorn at freebsd.org> writes:
: >>> The current I2C bus mechanism does not support the bus adding its own
: >>> children [...]
: >>
: >> That's because the I2C protocol does not support device enumeration or
: >> identification.  You have to know in advance what kind of devices are
: >> attached and at what address.  Even worse, it is not uncommon for
: >> similar but not entirely compatible devices to use the same I2C address
: >> (for instance, every I2C-capable RTC chip uses the same address, even
: >> though they have different feature sets)
: > 
: > Well, hard-coded addresses and conflicting assignments between vendors 
: > do not technically prevent from scanning the bus; actually, our current 
: > iicbus code can do bus scaning when compiled with a diag define. The 
: > problem however is some slave devices are not well-behaved, and they 
: > don't like to be read/written to other than in very specific scenario: 
: > if polled during bus scan strange effects occur e.g. they disappear from 
: > the bus, or do not react to consecutive requests etc.
: 
: All of this is true, but perhaps my question was badly worded. What I am 
: trying to figure out is how to shove information from an out-of-band 
: source (Open Firmware, in this case) into newbus without disrupting 
: existing code. In that way, my question is not I2C specific -- we run 
: into the same issue with the Open Firmware nexus node and pseudo-devices 
: like cryptosoft that attach themselves.

You are looking at the problem incorrectly.  In newbus, a case like
this the i2c bus should be a subclass (say i2c_of) that is derived
from the normal i2c bus stuff, but replaces the hints insertion of
devices with OF enumeration of devices.  The OF higher levels will
already know to attach this kind of i2c bus to certain i2c
controllers, or always on certain platforms.

: What I want to do is to have the I2C bus add the children that the 
: firmware says it has. What the firmware cannot tell in advance, however, 
: is which FreeBSD driver is responsible for those devices, and so the I2C 
: bus driver can't know that without a translation table that I would 
: prefer not to hack in to the bus driver.

This is the bigger problem.  Today, we are stuck with a lame table
that will translate the OF provided properties into FreeBSD driver
names.

: It seems reasonable to allow devices to use a real probe routine to look 
: at the firmware's name and compatible properties, like we allow on other 
: Open Firmware busses. The trouble is that existing drivers don't do 
: this, because they expect to be attached with hints, so they will attach 
: to all devices. I'm trying to figure out how to avoid this.
: 
: My basic question comes down to whether there is a good way in newbus to 
: handle busses that may be wholly or partially enumerated by firmware or 
: some other method, and may also have devices that can only attach 
: themselves if told to by hints.

Yes.  This is a bit of a problem.  The problem is that the existing
hints mechanism combines device tree and driver tree into one, and in
such a scenario, we wind up with the problem that you have.

One could make the probe routines return BUS_PROBE_GENERIC, and that
would help somewhat.  One could also have the probe routine check to
see if a specific driver is assigned to the device or not.  That would
also help, but does mean changing all the i2c bus attached drivers in
the tree.

Warner