Re: Enumerable I2C busses

On Wednesday 21 January 2009 9:47:50 am Nathan Whitehorn wrote:
John Baldwin wrote:
On Tuesday 06 January 2009 2:24:19 pm Nathan Whitehorn wrote:
M. Warner Losh wrote:
In message: <492AC8DE.6050902@xxxxxxxxxxx>
Nathan Whitehorn <nwhitehorn@xxxxxxxxxxx> writes:
: M. Warner Losh wrote:
: > In message: <4929C6D8.7090305@xxxxxxxxxxx>
: > Nathan Whitehorn <nwhitehorn@xxxxxxxxxxx> writes:
: > : Rafał Jaworowski wrote:
: > : >
: > : > On 2008-11-23, at 19:18, Dag-Erling Smørgrav wrote:
: > : >
: > : >> Nathan Whitehorn <nwhitehorn@xxxxxxxxxxx> 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.
:
: Yes, this is exactly what I wanted to do, like how ofw_pci works.
:
: > : 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.
:
: At the moment, I don't believe Apple uses any of the current very
small
: number of I2C device drivers in tree. So I may skip the table for the
: time being, assuming the hack below is OK. In future, this may change,
: since G5 systems require software thermal control. But that will be
the
: subject of another mail to this list...
:
: > : 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.
:
: I think changing existing I2C drivers may be unavoidable. Would there
be
: any objection to changing the MI iicbus drivers to return
: BUS_PROBE_NOWILDCARD in their probe routines? It seems to have been
: introduced (by you) to solve more or less exactly this problem. By my
: count, the relevant files are:
: dev/iicbus/ds133x.c
: dev/iicbus/icee.c
: dev/iicbus/ad7418.c
: dev/iicbus/iicsmb.c
: dev/iicbus/ds1672.c
: dev/iicbus/if_ic.c
: dev/iicbus/iic.c
:
: I would also like to change iicbus_probe to return -1000 like
: dev/pci/pci.c to allow it to be overridden by a subclass. Please let
me
: know if this is a terrible idea or if I have forgotten any I2C device
: drivers.

Short term, this is the right fix. There was an objection, I think by
Marcel, to this approach. However, his objections were part of a
larger set of objections and I think that we're working to solve
those.

Warner

This is now in the tree. Now for part 2, which I had not considered
previously: connecting the I2C bus layer to the I2C host adapters.

Right now, we have the following:
kiic/other i2c adapters
---iicbus
---ofw_iicbus

Since kiic provides an Open Firmware node, ofw_iicbus gets priority,
attaches, and everything after that is wonderful. The issue is how best
to attach the iicbus modules to kiic. Current I2C controllers contain a
line like this:
DRIVER_MODULE(iicbus, me, iicbus_driver, iicbus_devclass, 0, 0);

This explicitly specifies that you want the standard driver, so we need
additional glue to allow the ofw_iicbus driver to attach. One solution
is that each relevant host adapter can add an additional DRIVER_MODULE
line with the ofw_iicbus driver and class, which would have to exported
in a header somewhere. This is pretty ugly. Another solution is for the
ofw_iicbus module to grow a list of the names of interesting adapters.
This is worse.

The third option is to do what we do for pci, where all PCI adapters are
named 'pcib'. So we could make new I2C host adapters named 'iichb' or
something, and then move the DRIVER_MODULE logic for new code into the
bus modules, as is done for PCI. This decreases the amount of
information in the device names, but seems a bit cleaner. Thoughts?
-Nathan

If ofw_iicbus is simply an OF-aware version of iicbus (i.e. same
functionality) similar to the OF-aware PCI bus, then I would go the PCI
route
and just call it iicbus but give it a higher probe priority.


Which it is. What I meant was the bridge devices to which iicbus
attaches. For pci, they all end up with the same name (pcib) so that the
pci layer knows to attach to them. For I2C, they are called
iicbb/pcf/at91_twi/etc. and each bridge device explicitly attaches the
standard iicbus to itself, instead of letting it and any firmware-aware
versions probe.

I'm a bit torn on that one, especially since you have weird cases like one of
the via parts that has both smbus and iicbus children.

The other option would be to fix the attaching to subclasses thing (that
should make all pci drivers attach to cardbus0 devices since cardbus inherits
from pci, for example) and then you could have what would basically be an
abstract base class "iicbridge" with no devmethods that all bridge drivers
inherit from, and iicbus would attach to that.

--
John Baldwin
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