linux-kernel - Re: [PATCH 5/5] rtc: at91sam9: add DT bindings documentation

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Date:	Wed, 10 Sep 2014 15:16:25 +0200
From:	Johan Hovold <johan@...nel.org>
To:	Boris BREZILLON <boris.brezillon@...e-electrons.com>
Cc:	Johan Hovold <johan@...nel.org>,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	Andrew Victor <linux@...im.org.za>,
	Alessandro Zummo <a.zummo@...ertech.it>,
	rtc-linux@...glegroups.com, Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>, devicetree@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/5] rtc: at91sam9: add DT bindings documentation

On Wed, Sep 10, 2014 at 02:43:15PM +0200, Boris BREZILLON wrote:
> Hi Johan,
> 
> On Wed, 10 Sep 2014 14:14:24 +0200
> Johan Hovold <johan@...nel.org> wrote:
> 
> > On Wed, Sep 03, 2014 at 10:45:34AM +0200, Boris BREZILLON wrote:
> > > Signed-off-by: Boris BREZILLON <boris.brezillon@...e-electrons.com>
> > > ---
> > >  .../devicetree/bindings/rtc/atmel,at91sam9-rtc.txt   | 20 ++++++++++++++++++++
> > >  1 file changed, 20 insertions(+)
> > >  create mode 100644 Documentation/devicetree/bindings/rtc/atmel,at91sam9-rtc.txt
> > > 
> > > diff --git a/Documentation/devicetree/bindings/rtc/atmel,at91sam9-rtc.txt b/Documentation/devicetree/bindings/rtc/atmel,at91sam9-rtc.txt
> > > new file mode 100644
> > > index 0000000..9ca455f
> > > --- /dev/null
> > > +++ b/Documentation/devicetree/bindings/rtc/atmel,at91sam9-rtc.txt
> > > @@ -0,0 +1,20 @@
> > > +Atmel AT91SAM9260 Real Time Timer
> > > +
> > > +Required properties:
> > > +- compatible: should be: "atmel,at91sam9260-rtt"
> > > +- reg: should contain 2 memory regions
> > > +  * the first one encodes the memory region of the RTT controller
> > > +  * the second one encodes the GPBR (General Purpose Backup Resgisters)
> > > +    memory region used to store the current time
> > > +- interrupts: rtc alarm/event interrupt
> > > +- clocks: should contain one clock pointing the the slow clk
> > > +
> > > +Example:
> > > +
> > > +rtc@...ffe00 {
> > > +	compatible = "atmel,at91sam9260-rtt";
> > > +	reg = <0xfffffd20 0x10
> > > +	       0xfffffd50 0x4>;
> > > +	interrupts = <1 4 7>;
> > > +	clocks = <&clk32k>;
> > > +};
> > 
> > This does not describe the hardware, but rather a specific software
> > configuration.
> > 
> > The RTT is first of all not an RTC (although it can be used as one in a
> > specific software configuration). And the second register resource above
> > is not an RTT register, but a general-purpose backup register could be
> > used for other purposes (which register to use is currently configurable
> > for legacy booting using CONFIG_RTC_DRV_AT91SAM9_GPBR).
> > 
> > This was discussed in the thread where I posted an RFC for this last
> > year (which you linked to in your original submission thread), but no
> > conclusion was reached:
> > 
> > 	http://www.spinics.net/lists/arm-kernel/msg236292.html
> 
> Yes, I read this thread.

I'm sure you did. I just tried to summarise the main points of it above.

> Please, lets just find a solution, even if it's not a perfect one,
> because the situation is unacceptable.
> We're missing this features since the move to DT because we were not
> able to agree on a DT binding...

Agreed. My suggestion in the thread above was along the lines of generic
use-neutral rtt and gmbr nodes, and then an additional attribute to the
rtt node (which can be set in a specific board dts, when enabling the
rtt) providing a gmbr handle (and register number) for the rtc-at91sam9
driver to use.

This in itself does not resolve which rtt-driver would get bound if
there is ever another one (and the gmbr attribute is present), though.

> I know DT bindings are supposed to represent HW parts and not what
> they're used for or how they're configured, but do you see any other
> real usage of the RTT block ?

It's at least not hard to imagine other uses for the battery-backed up
gmbr registers.

I'll look into how that could be implemented.

> BTW, I don't care which binding/implementation is chosen but we need to
> sort this out!

Ok, let's do that. :)

Johan
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