linux-kernel - Re: [lm-sensors] Adding critical/fault limits to hwmon sysfs API

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Message-ID: <20100623162911.2028f817@hyperion.delvare>
Date:	Wed, 23 Jun 2010 16:29:11 +0200
From:	Jean Delvare <khali@...ux-fr.org>
To:	Guenter Roeck <guenter.roeck@...csson.com>
Cc:	"lm-sensors@...sensors.org" <lm-sensors@...sensors.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [lm-sensors] Adding critical/fault limits to hwmon sysfs API

On Wed, 23 Jun 2010 06:31:47 -0700, Guenter Roeck wrote:
> On Wed, Jun 23, 2010 at 08:43:46AM -0400, Jean Delvare wrote:
> > Hi Guenter,
> > 
> > On Sun, 20 Jun 2010 09:37:59 -0700, Guenter Roeck wrote:
> > > the current hwmon sysfs API does not specify critical or fault limits for voltage
> > > and current readings.
> > > 
> > > Many recent power controller/monitoring chips have support for such limits in addition
> > > to alarm limits. Typical action, when a the critical or fault limit is reached,
> > > may be a board reset or power shutdown, or to report the fault condition.
> > > 
> > > Examples for chips supporting critical/fault limits are SMM665 and variants as well
> > > as many PMBus devices, such as MAX8688, MAX16064, LTC2978, and others.
> > > 
> > > I think it would make sense to add critical/fault limits to the hwmon sysfs API,
> > > to be able to report those limits if supported by a chip.
> > > 
> > > Any thoughts on this ?
> > 
> > I agree it would be good to have standard names (and libsensors
> > support) if these features are popular. It might be a little difficult
> > to come up with the right attribute names though.
> > 
> > For temperatures, we have temp[1-*]_crit, for the critical limit on the
> > high end. We don't have a name for the critical limit on the low end,
> > because no chip ever implemented that. The name we chose doesn't offer
> > much possibilities for a nice name while staying consistent. Maybe
> > "lcrit" would be acceptable for the low end critical limit, and we keep
> > "crit" for the high end critical limit?
> > 
> How about {curr|in|temp}[1-*]_[min_]crit ?
> 
> In other words, keep _crit for the upper limit and introduce min_crit for the lower limit.
> This would be a bit better aligned with the existing _min while maintaining _crit for the 
> upper limit.

I expected a counter-proposal of this kind. The problem I see is that
the new limit we are adding is unrelated to _min. However, the other
_min_* file we have (_min_alarm) expresses something which is relative
to _min. Same as _max_hyst and _crit_hyst, which are relative to _max
and _critn respectively. So I have the feeling that _min_crit sends the
wrong signal to the reader. Especially if we keep _crit for the high
bound, the asymmetry raises questions.

This is my rationale for suggesting _crit and _lcrit. Now, I won't
argue forever if others disagree, these is really only a naming
convention and everything will be fine as long as the drivers and
libsensors agree.

-- 
Jean Delvare
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