Date:	Fri, 15 Aug 2014 11:07:31 -0500
From:	atull <atull@...nsource.altera.com>
To:	Steffen Trumtrar <s.trumtrar@...gutronix.de>
CC:	<tthayer@...nsource.altera.com>, <robherring2@...il.com>,
	<pawel.moll@....com>, <mark.rutland@....com>,
	<ijc+devicetree@...lion.org.uk>, <galak@...eaurora.org>,
	<rob@...dley.net>, <linux@....linux.org.uk>,
	<delicious.quinoa@...il.com>, <dinguyen@...nsource.altera.com>,
	<dougthompson@...ssion.com>, <grant.likely@...aro.org>,
	<bp@...en8.de>, <sameo@...ux.intel.com>, <lee.jones@...aro.org>,
	<devicetree@...r.kernel.org>, <linux-doc@...r.kernel.org>,
	<linux-edac@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <tthayer.linux@...il.com>
Subject: Re: [PATCHv10 2/2] arm: dts: Add Altera SDRAM EDAC bindings &
 devicetree entries.

On Fri, 15 Aug 2014, Steffen Trumtrar wrote:

> 
> Hi!

Hello

Thanks for the feedback...

> 
> tthayer@...nsource.altera.com writes:
> 
> > From: Thor Thayer <tthayer@...nsource.altera.com>
> >
> > Add the Altera SDRAM EDAC bindings and device tree changes to the Altera SoC project.
> >
> > Signed-off-by: Thor Thayer <tthayer@...nsource.altera.com>
> > ---
> > v2: Changes to SoC SDRAM EDAC code.
> >
> > v3: Implement code suggestions for SDRAM EDAC code.
> >
> > v4: Remove syscon from SDRAM controller bindings.
> >
> > v5: No Change, bump version for consistency.
> >
> > v6: Only map the ctrlcfg register as syscon.
> >
> > v7: No change. Bump for consistency.
> >
> > v8: No change. Bump for consistency.
> >
> > v9: Changes to support a MFD SDRAM controller with nested EDAC.
> >
> > v10: Revert to using syscon based on feedback.
> > ---
> >  .../bindings/arm/altera/socfpga-sdram-edac.txt     |   15 +++++++++++++++
> >  arch/arm/boot/dts/socfpga.dtsi                     |   11 +++++++++++
> >  2 files changed, 26 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/arm/altera/socfpga-sdram-edac.txt
> >
> > diff --git a/Documentation/devicetree/bindings/arm/altera/socfpga-sdram-edac.txt b/Documentation/devicetree/bindings/arm/altera/socfpga-sdram-edac.txt
> > new file mode 100644
> > index 0000000..d0ce01d
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/arm/altera/socfpga-sdram-edac.txt
> > @@ -0,0 +1,15 @@
> > +Altera SOCFPGA SDRAM Error Detection & Correction [EDAC]
> > +The EDAC accesses a range of registers in the SDRAM controller.
> > +
> > +Required properties:
> > +- compatible : should contain "altr,sdram-edac";
> > +- altr,sdr-syscon : phandle of the sdr module
> > +- interrupts : Should contain the SDRAM ECC IRQ in the
> > +	appropriate format for the IRQ controller.
> > +
> > +Example:
> > +	sdramedac {
> > +		compatible = "altr,sdram-edac";
> > +		altr,sdr-syscon = <&sdr>;
> > +		interrupts = <0 39 4>;
> > +	};
> > diff --git a/arch/arm/boot/dts/socfpga.dtsi b/arch/arm/boot/dts/socfpga.dtsi
> > index 4676f25..45b361e 100644
> > --- a/arch/arm/boot/dts/socfpga.dtsi
> > +++ b/arch/arm/boot/dts/socfpga.dtsi
> > @@ -603,6 +603,17 @@
> >  			};
> >  		};
> >  
> > +		sdr: sdr@...25000 {
> > +			compatible = "syscon";
> > +			reg = <0xffc25000 0x1000>;
> > +		};
> > +
> > +		sdramedac {
> > +			compatible = "altr,sdram-edac";
> > +			altr,sdr-syscon = <&sdr>;
> > +			interrupts = <0 39 4>;
> > +		};
> > +
> >  		L2: l2-cache@...ef000 {
> >  			compatible = "arm,pl310-cache";
> >  			reg = <0xfffef000 0x1000>;
> 
> Sorry, but I personally still don't like this approach.

This is a normal use of syscon.  Syscon is great for this case 
where multiple drivers need access to an ip block's register
range but there is otherwise no need to write a MFD from scratch
to allow that.  I think that's the purpose of syscon in the first
place.

> 
> If we do it like this however, shouldn't the different regions of the
> SDR (EDAC, FPGAPORTRST, ...) at least be child nodes of the syscon?
> That seems to match the syscon binding and describes the actual hardware
> better IMHO.

I like that; it would be clean and clear, but I don't think syscon is 
written such that that would actually work.  I haven't seen anybody else
do it that way.

> Oh, and "reg = <...>" for the sdram-edac, maybe ?

How would that work?  Syscon is already mapping the whole register range
for the sdr block.  Are you proposing a device tree binding that this
Linux driver would ignore, but some other driver in some other OS might
find useful in the future?  I'd rather not put duplicate information
in two places, too easy for it to get out of sync.

> How would I know where the start address of the EDAC is, if I would use
> this DT on another OS where I don't have the same driver?

The start address of the EDAC is an offset into the range mapped by 
syscon here.  The driver knows what the register offsets are.

If we are talking about this device tree being used by some other OS
that is not aware of syscon, then that OS won't know what's going on and
some modification of the DT will be needed.  I have been advised that
u-boot will need a significantly different DT, though I don't know
what the issues are there.

> 
> Regards,
> Steffen
> 
> -- 
> Pengutronix e.K.                           | Steffen Trumtrar            |
> Industrial Linux Solutions                 | http://www.pengutronix.de/  |
> Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0    |
> Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |
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