Date:	Wed, 25 Oct 2006 16:42:35 +0200
From:	"Shem Multinymous" <multinymous@...il.com>
To:	"David Woodhouse" <dwmw2@...radead.org>
Cc:	"Richard Hughes" <hughsient@...il.com>,
	"Dan Williams" <dcbw@...hat.com>, linux-kernel@...r.kernel.org,
	devel@...top.org, sfr@...b.auug.org.au, len.brown@...el.com,
	greg@...ah.com, benh@...nel.crashing.org,
	"David Zeuthen" <davidz@...hat.com>,
	"linux-thinkpad mailing list" <linux-thinkpad@...ux-thinkpad.org>
Subject: Re: [PATCH v2] Re: Battery class driver.

Hi,

On 10/25/06, David Woodhouse <dwmw2@...radead.org> wrote:
> If you can summarise the bits I've missed in the meantime that would be
> wonderfully useful

OK. Looking at the current git snapshot:

current_now is missing.

time_remainig should be split into:
  time_to_empty_now
  time_to_empty_avg
  time_to_full
or even
  time_to_empty_now
  time_to_empty_avg
  time_to_full_now
  time_to_full_avg

s/charge_count/cycle_count/, that's the standard name and used by the SBS spec.

Why the reversed order, for example, in design_charge vs. charge_last?
Following hwmon style, I think it should be
s/design_charge/charge_design/
s/manufacture_date/date_manufactured/
s/first_use/date_first_used/
s/design_voltage/voltage_design/

s/charge_last/charge_last_full/ seems less ambiguous.

s/^charge$/charge_left/ follows SBS and seems better.

And, for the reasons I explained earlier, I strongly suggest not using
the term "charge" except when referring to the action of charging.
Hence:
s/charge_rate/rate/;  s/charge/capacity/

It would be nice to have power_{now,avg}, always in mW regardless of
the capacity units.

I take it you don't want to deal with battery control actions for now.


> > > one of the things I plan is to remove 'charge_units' and provide both
> > > 'design_charge' and 'design_energy' (also {energy,charge}_last,
> > > _*_thresh etc.) to cover the mWh vs. mAh cases.
> >
> > You can't do this conversion, since the voltage is not constant.
> > Typically the voltage drops when the charge goes down, so you'll be
> > grossly overestimating the available energy it. And the effect varies
> > with battery chemistry and condition.
>
> Absolutely. I don't want to do the conversion -- I want to present the
> raw data. I was just a question of whether I provide 'capacity' and
> 'units' properties, or whether I provide 'capacity_mWh' and
> 'capacity_mAh' properties (only one of which, presumably, would be
> available for any given battery). Likewise for the rates, thresholds,
> etc.

I think using one set of files and units string makes more sense, for
several reasons:
Reduces the number of attributes and kernel code duplication.
Can handle weird power sources that use other units.
Simpler userspace code. One can do
$ cd /sys/foo; echo `cat capacity_left` out of `cat capacity_last`
`cat capaity_units` left.
instead of checking multiple sets of files for valid values.

The great majority of apps don't care about the physical values, but
just need something that they can parse as a relative quantity and
something to show the user. The generic units scheme provides both. We
have current_*, voltage etc. for those that do care, but there's no
need to duplicate the whole set of _thresholds, _last_full, _design
etc.

  Shem
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