Date:	Fri, 29 Aug 2014 11:40:02 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Matti Vaittinen <matti.vaittinen@....com>
Cc:	ext Guenter Roeck <linux@...ck-us.net>,
	Jason Cooper <jason@...edaemon.net>,
	"a.zummo@...ertech.it" <a.zummo@...ertech.it>,
	"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 Fri, Aug 29, 2014 at 08:34:25AM +0100, Matti Vaittinen wrote:
> On Thu, Aug 28, 2014 at 10:40:34AM -0700, ext Guenter Roeck wrote:
> > On Thu, Aug 28, 2014 at 01:28:42PM -0400, Jason Cooper wrote:
> > > On Thu, Aug 28, 2014 at 09:48:25AM -0700, Guenter Roeck wrote:
> > > > 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.
> > > > > > > > 
> > > > > > > > +++ 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.
> > > 
> > > iiuc, there is no way for the kernel to determine what is being trickle
> > > charged, and thus no way to determine how it should set this register.
> > > 
> > > It may be a bit of overkill, but I think a DT macro would be the most
> > > maintainable solution here:
> > > 
> > > #define DS1339_TRCKL_DIODE   0x08
> > > #define DS1339_TRCKL_NODIODE 0x04
> > > #define DS1339_TRCKL_R250    0x01
> > > #define DS1339_TRCKL_R2000   0x02
> > > #define DS1339_TRCKL_R4000   0x03
> > > 
> > > trickle_setup = <DS1339_TRCKL_DIODE | DS1339_TRCKL_R250>;
> > > 
> > > And the driver would take care of oring it with the enable pattern.
> > > 
> > 
> > Yes, that sounds reasonable as well.
> > 
> I thought of this too. However the ds1307 seems to be designed to work
> with bunch of chips. What then when next chip supported by this driver
> introduces 75, 1000 and 5000 ohm resistors? Or something else. (Or add
> some other configuration). I do not see this approach to be
> maintainable in long run.

If a new chip comes out with new features, the driver will need an
update anyhow. We have no guarantee that the register placement will be
the same, let alone the layout.

If and when said new chip comes out we allocate a new compatible string.
If it's compatible iwith (i.e. is a superset of) an existing device's
programming model, we add that existing string as a fallback in the
compatible list so old kernels can drive the subset of features they
understand.

> I see strong possibility of polluting dt with endless amount of
> defines. Furthermore I believe the benefits of these defines would be
> negligible compared to effort maintaining defines and documentation of
> them causes. Surely the one who needs to add dt node for this chip in
> his board's dt has the manual for the chip he has on board. Especially
> so if he knows the trickle charger there and wants to configure it.
> The plain resistor type still gives zero information without knowing
> the other details.

I would rather that the driver had some idea of what it were doing
rather than being a glorified copy routine.

I would suggest we have two properties that describe the resistor's
rating and whether or not there is a diode:

trickle-resistor-ohms = <250>
diode-connected;

That's easy for a human to write and/or validate, we can easily extend
it in future, requires no proliferation of macros, and describes the
hardware rather than telling software what to do.

The driver becomes a little more complicated, but gains sanity checking,
which is a good thing.

I'm still worried that we have no idea what the device is intended to
charge. Surely that's important?

Thanks,
Mark.
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