Re: [lm-sensors] [RFC PATCH] Allow the configuration register to be

[Guenter Roeck <guenter.roeck@ericsson.com>]

On Tue, Sep 14, 2010 at 10:57:19AM -0400, Jean Delvare wrote:
> On Tue, 14 Sep 2010 05:01:28 -0700, Guenter Roeck wrote:
> > Hi Jean,
> > 
> > On Tue, Sep 14, 2010 at 04:08:33AM -0400, Jean Delvare wrote:
> > > On Mon, 13 Sep 2010 11:09:06 -0700, Guenter Roeck wrote:
> > > > On Mon, Sep 13, 2010 at 01:41:21PM -0400, Jean Delvare wrote:
> > > > > Not too sure about that. I had the same reaction at first, but do we
> > > > > actually know of devices where the resolution isn't global?
> > > >
> > > > Yes, in the lm90 driver. Only max6657/58/59 and max6646 have extended local
> > > > temperatures, all other chips have only 8 bit resolution for the local temperature.
> > > > Remote temperatures are all extended, ie have higher resoution.
> > > 
> > > But resolution can't be changed on these chips, can it? I think the
> > 
> > Correct.
> > 
> > > main purpose of these files is to let the user change the resolution
> > > for chips which support this (usually this is an update speed /
> > > resolution trade-off). Or do you believe that having read-only
> > > attributes exposing fixed resolutions would be valuable?
> > > 
> > Good point. Probably not really. More like nice to have.
> > 
> > > I'm a little worried because resolution as a bit number isn't too
> > > informative and doesn't quite work as a device-independent value. If
> > > you really want the information to be exposed to user-space for all
> > > devices, then we'd rather use actual sensor units (mV, m°C, whatever)
> > > for resolution, and possibly add other attributes for the range. But
> > 
> > I assumed it would be in units. Not sure how to express resolution 
> > as bit number in the first place.
> 
> This is how datasheets usually refer to the future. The configuration
> bits let the user select between 9-, 10, 11 and 12-bit resolution ADC
> (with increasing integration time.)
> 
> As most temperature sensors we deal with agree that 9-bit resolution
> means LSB weight of 0.5°C, it could almost be used as a standard. But
> of course there could be a device doing things differently someday,
> which is why we probably don't want to use the ADC bit notation.
> 
Agreed.

> > > this means adding a whole lot of attribute files for some devices (if
> > > we do it on a per-channel basis), so we first have to determine whether
> > > it is really worth it. I don't think it is, but you can try and
> > > convince me.
> >
> > Thinking about it, I'd rather keep it in mind in case someone else
> > sees the need to it and convinces both of us...
> > 
> > Another question if both attributes (rate and resolution) would make sense 
> > as module parameters instead. Are those attributes expected to vary across
> > multiple instances of the same driver ?
> 
> I see no reason to use module parameters for these. It does make sense
> to use different settings on different chips in the same family (e.g.
> one is on the motherboard and the other on the graphics adapter), plus
> sysfs attribute values are easier to change at run-time (these
> attributes could even be added to libsensors.)
> 
Makes sense.

Guenter

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