Re: CVS commit: src/share/man/man4

[Paul Goyette <paul%whooppee.com@localhost>]

On Fri, 12 Sep 2008, Simon Burge wrote:

> > > > Document new I2C_SCAN option.
> > Added the following warning:
> >
> >         Scan each i2c bus to determine which addresses respond.  Note
> >         that using this option can access some devices in such a manner
> >         as to leave them in an unstable or unuseable state, and might
> >         prevent those devices from being properly matched and/or
> >         attached.
>
> Can you expand this a bit more?  Leaving some devices in an "unstable or
> unuseable state" is rather an understatement - enabling this option will
> stop my Dell 640m laptop from rebooting.  I suspect there's some sort of
> power controller on the I2C bus that we blindly try to probe and then
> leave in some state that renders it useless.
>
> From what I understand, there's no generic way to probe an I2C bus or
> SMbus, and therefore any unknown device(s) that are probed with the
> current code might be accessed in a way that could do anything that we
> can't predict.
>
> I'd prefer to see some large warning along the lines of "If you enable
> this, you may experience funny problems, up to and including not being
> able to reboot your machine (or even worse).  Don't enable this unless
> you really know what you're doing.".

How about if I reword it like this:

        Scan each i2c bus to determine which addresses respond.

        WARNING! Using this option can access some devices in such a
        manner as to leave them in an unstable or unuseable state, and
        can prevent those devices from being properly matched and/or
        attached.  It can also lock up the entire i2c bus and even
        prevent a machine from completing the boot process.  Don't
        use this option unless you know what you're doing and can
        accept all sorts of unforseen consequences.

It would really be nice if system/motherboard vendors could tell us
what they've put on the bus so we can actually behave intelligently.
Even better, it would've been nice if the i2c folks had thought things
through a bit more and insisted on some minimal device identification 
protocol.  (Of course, that would've made things more expensive, I 
guess.)


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