Re: PMBus support in Linux

[Guenter Roeck <guenter.roeck-IzeFyvvaP7pWk0Htik3J/w@public.gmane.org>]

On Sat, May 29, 2010 at 11:47:41AM -0400, Wolfram Sang wrote:
> 
> > Yes. PMBus uses i2c as transport, so it can use the existing i2c/smbus infrastructure. 
> > Data reported is voltage, temperature, current, power, and fan data as available
> > from the individual chip. Chips support a chip dependent number of channels.
> > Values reported are typically in the form of X = Y * 2^N, ie there is a mantissa
> > and an exponent.
> > 
> > So I would say it maps pretty well; I don't really see a substantial difference
> > to other HW monitoring chips in that respect. Key difference may be that PMBus devices 
> > typically also have a control component, but I don't have plans to implement that,
> > at least not for now.
> 
> I just had a glimpse, but I came to a similar conclusion. As I further
> understood, PMBus devices support a standard range of commands with the
> possibility of manufacturer extenstions.
> 
> So, the approach I see (unless I miss something) would be writing an
> hwmon-I2C-driver named pmbus-devices.c or so which covers the generic
> functionality and provides some hooks for manufacturer extensions, if those are
> necessary? (Altough I'd hope a number of devices would be covered by the
> generic driver) Makes sense?
> 
That would be one option. It would require either a register() API call, or a table
of supported chips, or a combination of both, to identify how many channels
(or pages, in PMBus terminology) per chip are supported and to identify supported
objects/registers.

The other option would be to write separate drivers (one for each chip) and not provide
a common infrastructure. Not sure if that is a good idea, though; I personally prefer
the first option.

Thanks,
Guenter