Enumerable I2C busses
M. Warner Losh
imp at bsdimp.com
Mon Nov 24 09:57:09 PST 2008
In message: <492AC8DE.6050902 at freebsd.org>
Nathan Whitehorn <nwhitehorn at freebsd.org> writes:
: M. Warner Losh wrote:
: > 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.
: 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:
: 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
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
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