linux-kernel - Re: [PATCH] rtc: ds1307: add trickle charger device tree binding

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Message-ID: <20140828164825.GA12153@roeck-us.net>
Date:	Thu, 28 Aug 2014 09:48:25 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Mark Rutland <mark.rutland@....com>
Cc:	Matti Vaittinen <matti.vaittinen@....com>,
	"a.zummo@...ertech.it" <a.zummo@...ertech.it>,
	"jason@...edaemon.net" <jason@...edaemon.net>,
	"jic23@...nel.org" <jic23@...nel.org>,
	"arno@...isbad.org" <arno@...isbad.org>,
	"jgunthorpe@...idianresearch.com" <jgunthorpe@...idianresearch.com>,
	"san@...etechnology.dk" <san@...etechnology.dk>,
	"hs@...x.de" <hs@...x.de>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"rtc-linux@...glegroups.com" <rtc-linux@...glegroups.com>,
	Sverdlin Alexander <alexander.sverdlin@....com>
Subject: Re: [PATCH] rtc: ds1307: add trickle charger device tree binding

On Thu, Aug 28, 2014 at 05:10:25PM +0100, Mark Rutland wrote:
> On Thu, Aug 28, 2014 at 04:51:57PM +0100, Guenter Roeck wrote:
> > On Thu, Aug 28, 2014 at 01:59:15PM +0100, Mark Rutland wrote:
> > > On Thu, Aug 28, 2014 at 01:42:44PM +0100, Matti Vaittinen wrote:
> > > > Patch adding support for specifying trickle charger setup from device
> > > > tree. Patch is based on linux-next tree.
> > > > 
> > > > 
> > > > Some DS13XX devices have "trickle chargers". Introduce a device tree binding
> > > > for specifying the setup and register values.
> > > > 
> > > > Signed-off-by: Matti Vaittinen <matti.vaittinen@....com>
> > > > ---
> > > >  .../devicetree/bindings/i2c/trivial-devices.txt    |  1 -
> > > >  .../devicetree/bindings/rtc/dallas,ds1339.txt      | 19 ++++++++++
> > > >  drivers/rtc/rtc-ds1307.c                           | 44 ++++++++++++++++++++--
> > > >  3 files changed, 59 insertions(+), 5 deletions(-)
> > > >  create mode 100644 Documentation/devicetree/bindings/rtc/dallas,ds1339.txt
> > > > 
> > > > diff --git a/Documentation/devicetree/bindings/i2c/trivial-devices.txt b/Documentation/devicetree/bindings/i2c/trivial-devices.txt
> > > > index 6af570e..e9206a4 100644
> > > > --- a/Documentation/devicetree/bindings/i2c/trivial-devices.txt
> > > > +++ b/Documentation/devicetree/bindings/i2c/trivial-devices.txt
> > > > @@ -35,7 +35,6 @@ catalyst,24c32		i2c serial eeprom
> > > >  cirrus,cs42l51		Cirrus Logic CS42L51 audio codec
> > > >  dallas,ds1307		64 x 8, Serial, I2C Real-Time Clock
> > > >  dallas,ds1338		I2C RTC with 56-Byte NV RAM
> > > > -dallas,ds1339		I2C Serial Real-Time Clock
> > > >  dallas,ds1340		I2C RTC with Trickle Charger
> > > >  dallas,ds1374		I2C, 32-Bit Binary Counter Watchdog RTC with Trickle Charger and Reset Input/Output
> > > >  dallas,ds1631		High-Precision Digital Thermometer
> > > > diff --git a/Documentation/devicetree/bindings/rtc/dallas,ds1339.txt b/Documentation/devicetree/bindings/rtc/dallas,ds1339.txt
> > > > new file mode 100644
> > > > index 0000000..9faf40e
> > > > --- /dev/null
> > > > +++ b/Documentation/devicetree/bindings/rtc/dallas,ds1339.txt
> > > > @@ -0,0 +1,19 @@
> > > > +* Dallas DS1339		I2C Serial Real-Time Clock
> > > > +
> > > > +Required properties:
> > > > +- compatible: Should contain "dallas,ds1339".
> > > > +- reg: I2C address for chip
> > > > +
> > > > +Optional properties:
> > > > +- trickle_setup : Used Trickle Charger configuration,
> > > > +        corresponding to 5 lowest bits in trickle charger register.
> > > 
> > The value is provided via platform data, so it is platform specific
> > and presumably needs to be configurable. I did, however, not find
> > a single in-kernel driver which is actually setting it.
> > So this is a good question.
> 
> I'm uncomfortable adding a field we don't understand to DT.
> 
> Is there any publicly-available documentation for the device?
> 

Lots ;-)

http://datasheets.maximintegrated.com/en/ds/DS1307.pdf
http://datasheets.maximintegrated.com/en/ds/DS1337.pdf
http://datasheets.maximintegrated.com/en/ds/DS1338.pdf
http://datasheets.maximintegrated.com/en/ds/DS1339.pdf
http://datasheets.maximintegrated.com/en/ds/DS1340.pdf
http://datasheets.maximintegrated.com/en/ds/DS1388.pdf
http://datasheets.maximintegrated.com/en/ds/DS3231.pdf

Code suggests that DS1339, DS1339, and DS1340 have the register.

Looking into the datasheets, the configuration consists of two parts:
- diode connected or not
- trickle charger resistor value (250, 2000, or 4000 ohm)

Given that, it seems to me that those values should be configured
explicitly instead of using a binary value, and that the driver
should perform the conversion from dt entry to register value.

Note that the relevant bits are the lower 4 bits, not the
lower 5 bits, so the above text is wrong.

Guenter
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