linux-kernel - Re: [PATCH v5] rtc: support DS1302 RTC on ICP DAS LP-8x4x

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Date:	Sun, 20 Dec 2015 15:14:16 +0300
From:	Sergei Ianovich <ynvich@...il.com>
To:	Rob Herring <robh@...nel.org>
Cc:	linux-kernel@...r.kernel.org,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Alessandro Zummo <a.zummo@...ertech.it>,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
	<devicetree@...r.kernel.org>,
	"open list:REAL TIME CLOCK (RTC) SUBSYSTEM" 
	<rtc-linux@...glegroups.com>
Subject: Re: [PATCH v5] rtc: support DS1302 RTC on ICP DAS LP-8x4x

On Sat, 2015-12-19 at 21:38 -0600, Rob Herring wrote:
> On Tue, Dec 15, 2015 at 08:45:23PM +0300, Sergei Ianovich wrote:
> 
> Nothing in this is specific to ICP, so the subject should be updated.
> 
> > Signed-off-by: Sergei Ianovich <ynvich@...il.com>
> > CC: Alexandre Belloni <alexandre.belloni@...e-electrons.com>
> > ---
> >    v4..v5
> >    * drop THIS_MODULE from struct platform driver
> >    * use "dallas" for vendor name per vendor-prefixes.txt
> > 
> >    v3..v4
> >    * move DTS bindings to a different patch
> > 
> >    v2..v3
> >    * use usleep_range instead of custom nsleep
> >    * number change (07/16 -> 09/21)
> > 
> >    v0..v2
> >    * use device tree
> >    * use devm helpers where possible
> > 
> >  .../devicetree/bindings/rtc/rtc-ds1302.txt         |  14 +++
> >  drivers/rtc/Kconfig                                |   2 +-
> >  drivers/rtc/rtc-ds1302.c                           | 100
> > ++++++++++++++++++++-
> >  3 files changed, 113 insertions(+), 3 deletions(-)
> >  create mode 100644 Documentation/devicetree/bindings/rtc/rtc-
> > ds1302.txt
> > 
> > diff --git a/Documentation/devicetree/bindings/rtc/rtc-ds1302.txt
> > b/Documentation/devicetree/bindings/rtc/rtc-ds1302.txt
> > new file mode 100644
> > index 0000000..810613b
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/rtc/rtc-ds1302.txt
> > @@ -0,0 +1,14 @@
> > +* Dallas Semiconductor DS-1302 RTC
> > +
> > +Simple device which could be used to store date/time between
> > reboots.
> > +
> > +Required properties:
> > +- compatible : Should be "dallas,rtc-ds1302"
> > +- reg : Should be address and size of IO memory region
> 
> This device is a SPI (or SPI like?) interface. So you have some sort
> of 
> of FPGA logic in between the cpu and ds1302. The DT should have a node
> for the controller and then the ds1302 as a child of it. A full blown 
> SPI driver may be overkill here, but that's a separate discussion from
> the DT binding.

Below is the quote from the actual DT of LP-8x4x:
>                 fpga@5 {
>                         compatible = "simple-bus";
>                         #address-cells = <1>;
>                         #size-cells = <1>;
>                         ranges = <0 5 0x3000000 0x10000>;
>                         interrupt-parent = <&fpgairq>;
> 
>                         rtc@...c {
>                                 compatible = "dallas,rtc-ds1302";
>                                 reg = <0x901c 0x1>;
>                                 status = "okay";
>                         };

You are right about the topology. ds1302 is a half-duplex SPI device.
Does this mean I should rewrite the driver to handle the chip as a slave
SPI device, and then provide a master SPI functionality at the FPGA?
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