linux-kernel - Re: Implement devicetree support for AB8500 Btemp

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Date:	Thu, 13 Sep 2012 19:04:27 -0700
From:	Anton Vorontsov <anton.vorontsov@...aro.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Rajanikanth HV <rajanikanth.hv@...ricsson.com>,
	Rajanikanth HV <rajanikanth.hv@...aro.org>,
	Lee Jones <lee.jones@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus WALLEIJ <linus.walleij@...ricsson.com>,
	STEricsson_nomadik_linux <STEricsson_nomadik_linux@...t.st.com>,
	"linaro-dev@...ts.linaro.org" <linaro-dev@...ts.linaro.org>,
	Patch Tracking <patches@...aro.org>,
	Mathieu Poirier <mathieu.poirier@...aro.org>
Subject: Re: Implement devicetree support for AB8500 Btemp

(Thanks for Cc'ing me.)

On Thu, Sep 13, 2012 at 02:37:38PM +0000, Arnd Bergmann wrote:
[...]
> > > If this is true, I don't understand what makes the 'supplied-to'
> > > properties you list in the device tree binding board specific. Are
> > > they not always done the same way? If so, you could just leave them
> > > out.
> > Precisely 'supplied-to' is not board specific, it was maintained as
> > platform_data which i migrated to dt-node. It is meant to establish
> > dependency across bm drivers based on power_supply property and
> > runtime battery attributes.
> > Basically, 'supplied-to' provides a way of exporting change in
> > power_supply_property and runtime batter characteristics so that other
> > bm devs shall make use or refer the updated values.
> > Ref: external_power_changed(...) call back api.
> > Note: all the bm drivers handles subset of power_supply property and
> >       battery attributes,
> >       ref: include/linux/power_supply.h and get_property(...) call back
> >       api across bm drivers.
> 
> Ok, so you want to just remove the property from the device tree,
> or do you want to establish a different method to specify these
> connections?

Power supply subsystem's supplied_to describes not just how driver
should notify other devices, supplied_to is more generic stuff, in terms
that it describes power supply hierarchy. It's like a directed graph,
e.g.:

      <AC power> supplied_to <main battery> and <backup battery>
     <USB power> supplied_to <main battery> and <backup battery>
  <main battery> supplied_to <system>
<backup battery> supplied_to <system>
  <cmos battery> supplied_to <southbridge pci device>
  <mice battery> supplied_to <mice wireless hid>

How things interact in linux are just implementations details.
So, device tree is surely a perfect place to describe these things.

Although, in current bindings I see this:

+       ab8500-fg {
+               /* Other enery management module */
+               supplied_to = "ab8500_chargalg", "ab8500_usb";
+               num_supplicants = <2>;
+       };

Instead of addressing supplicants by name, it's better to address
via phandles. And, of course, num_supplicants is not needed, it can
be derived.

[...]
> > > possible batteries and require a property such as
> > > 
> > >  st-ericsson,battery-type: A string identifier for the type of battery,
> > > 			   which impacts how an operating system interpret
> > > 			   the sensor readings. Possible values include:
> > > 	* "none"	-- no battery connected
> > > 	* "li-ion-9100" -- Type 9100 Li-ION battery
> > > 	* <add any others that apply here>
> > Can do this, not precisely as "st-ericsson,battery-type", it will be as
> > battery-type = [unknown|NiMH|LION|...|]], reason being
> > allowable battery type is based on technology, as you can see the
> > possible types as:
> > POWER_SUPPLY_TECHNOLOGY_UNKNOWN = 0,
> >         POWER_SUPPLY_TECHNOLOGY_NiMH,
> >         POWER_SUPPLY_TECHNOLOGY_LION,
> >         POWER_SUPPLY_TECHNOLOGY_LIPO,
> >         POWER_SUPPLY_TECHNOLOGY_LiFe,
> >         POWER_SUPPLY_TECHNOLOGY_NiCd,
> >         POWER_SUPPLY_TECHNOLOGY_LiMn
> > Ref: include/linux/power_supply.h
> > Note: doing this will impact my of_probe(...), may slightly bloat the
> > code.
> 
> Ok.
> 
> If you want to make the battery type a generic property, it's probably
> best to start a separate binding document for this in
> Documentation/devicetree/bindings/power-supply/common.txt
> and document a string for each of these.

Fully agree. We need to document generic DT bindings for power supplies.

Thanks,
Anton.
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