linux-kernel - Re: [PATCH v2 1/2] doc: bindings: Add bindings documentation for mtd nvmem

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Date:   Wed, 15 Mar 2017 20:41:41 +0100
From:   Alban <albeu@...e.fr>
To:     Rob Herring <robh@...nel.org>
Cc:     Aban Bedel <albeu@...e.fr>, linux-kernel@...r.kernel.org,
        Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        Maxime Ripard <maxime.ripard@...e-electrons.com>,
        Mark Rutland <mark.rutland@....com>,
        David Woodhouse <dwmw2@...radead.org>,
        Brian Norris <computersforpeace@...il.com>,
        Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Marek Vasut <marek.vasut@...il.com>,
        Richard Weinberger <richard@....at>,
        Cyrille Pitchen <cyrille.pitchen@...el.com>,
        devicetree@...r.kernel.org, linux-mtd@...ts.infradead.org,
        Moritz Fischer <moritz.fischer@...us.com>
Subject: Re: [PATCH v2 1/2] doc: bindings: Add bindings documentation for
 mtd nvmem

On Wed, 15 Mar 2017 12:24:01 -0500
Rob Herring <robh@...nel.org> wrote:

> On Tue, Mar 07, 2017 at 09:26:03AM +0100, Alban wrote:
> > Config data for drivers, like MAC addresses, is often stored in MTD.
> > Add a binding that define how such data storage can be represented in
> > device tree.
> > 
> > Signed-off-by: Alban <albeu@...e.fr>
> > ---
> > Changelog:
> > v2: * Added a "Required properties" section with the nvmem-provider
> >       property
> > ---
> >  .../devicetree/bindings/nvmem/mtd-nvmem.txt        | 33 ++++++++++++++++++++++
> >  1 file changed, 33 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/nvmem/mtd-nvmem.txt
> > 
> > diff --git a/Documentation/devicetree/bindings/nvmem/mtd-nvmem.txt b/Documentation/devicetree/bindings/nvmem/mtd-nvmem.txt
> > new file mode 100644
> > index 0000000..8ed25e6
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/nvmem/mtd-nvmem.txt
> > @@ -0,0 +1,33 @@
> > += NVMEM in MTD =
> > +
> > +Config data for drivers, like MAC addresses, is often stored in MTD.
> > +This binding define how such data storage can be represented in device tree.
> > +
> > +An MTD can be defined as an NVMEM provider by adding the `nvmem-provider`
> > +property to their node. Data cells can then be defined as child nodes
> > +of the partition as defined in nvmem.txt.
> > +
> > +Required properties:
> > +nvmem-provider:	Indicate that the device should be registered as
> > +		NVMEM provider  
> 
> I think we should use a compatible string here (perhaps with a 
> generic fallback), and that can imply it is an nvmem provider. The 
> reason is then the compatible can also imply other information that 
> isn't defined in DT.

That would work for partitions but not for unpartitioned MTD as these
will already have a compatible string for the MTD hardware. I was also
under the impression that capabilities/services provided by devices
were represented with such properties, like interrupt-controller or
gpio-controller, and not with compatible strings.

There is also another problem with unpartitioned MTD, earlier MTD
partitions binding allowed to have partitions as direct child nodes
without any compatible strings. The current nvmem binding do the same
for the nvmem cells, so it wouldn't be clear if a child node of the MTD
is a partition using the old binding or an nvmem cell.

As I think this problem could happen with some other device types I
suggested to re-work the nvmem binding to be more like the current MTD
partitions. See these threads[1][2], but a short example would look like
this:

flash {
	compatible = "vendor,flash-device-model";
	...
	nvmem-provider;
	nvmem-cells {
		compatible = "nvmem-cells";
		#address-cells = <1>;
		#size-cells = <1>;

		cell@100 {
			label = "mac-address";
			reg = <0x100 0x6>;
		};
	};
};

or like this if the device is partitioned:

flash {
	compatible = "vendor,flash-device-model";
	...
	partitions {
		compatible = "fixed-partitions"
		...
		partition@...0 {
			...
			nvmem-provider;
			nvmem-cells {
				compatible = "nvmem-cells";
				#address-cells = <1>;
				#size-cells = <1>;

				cell@100 {
					label = "mac-address";
					reg = <0x100 0x6>;
				};
			};
		};
	};
};

Alban

[1]:https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1344899.html
[2]:https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1348295.html

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