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

[Simon Burge <simonb%NetBSD.org@localhost>]

Paul Goyette wrote:

> On Thu, 11 Sep 2008, Jared D. McNeill wrote:
> 
> >> Document new I2C_SCAN option.
> >
> > Can you please add a note in the man page about the potential dangers of 
> > turning this on?
> 
> 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.".

Cheers,
Simon.