Re: [PATCH v7,3/3] MIPS: dts: jz4780/ci20: Add NEMC, BCH and NAND device tree nodes

[Boris Brezillon <boris.brezillon@free-electrons.com>]

Paul, Harvey,

On Fri, 16 Oct 2015 11:48:48 +0100
Paul Burton <paul.burton@imgtec.com> wrote:

> On Fri, Oct 16, 2015 at 11:31:12AM +0100, James Hogan wrote:
> > > >> +
> > > >> +&nemc {
> > > >> +     status = "okay";
> > > >> +
> > > >> +     nand: nand@1 {
> > > >> +             compatible = "ingenic,jz4780-nand";
> > > >
> > > > Isn't the NAND a micron part? This doesn't seem right. Is the device
> > > > driver and binding already accepted upstream with that compatible
> > > > string?
> > > 
> > > This is the compatible string for the JZ4780 NAND driver, this patch
> > > is part of the series adding that. Detection of the NAND part is
> > > handled by the MTD subsystem.
> > 
> > Right (didn't spot that it was part of a series).
> > 
> > The node appears to describe the NAND interface itself, i.e. a part of
> > the SoC, so should be in the SoC dtsi file, with overrides in the board
> > file if necessary for it to work with a particular NAND part
> > (potentially utilising status="disabled"). Would you agree?
> 
> Hi James,
> 
> The "nemc" node there is for the Nand & External Memory Controller which
> is a hardware block inside the SoC. It has 6 banks (ie. 6 chip select
> pins, each associated with a different address range, that connect to
> different devices). NAND flash is one such possible device, but a board
> could connect it to any of the 6 chip selects, or banks. To represent
> that in the SoC dtsi you'd want to have 6 NAND nodes, each disabled by
> default, which doesn't make a whole lot of sense to me. Other, non-NAND
> devices can connect to the NEMC too - for example the ethernet
> controller on the CI20 is connected to one bank.
> 
> The NAND device nodes are sort of a mix of describing the NAND flash
> (ie. Micron part as you point out) and its connections & properties, the
> way the NEMC should be used to interact with it alongside the BCH block,
> and the configuration for the NEMC such as timing parameters.
> 
> I imagine the most semantically correct means of describing it would
> probably be for the compatible string to reflect the Micron NAND part,
> and the NEMC driver to pick up on the relevant properties of its child
> nodes for configuring timings, whether the device is NAND etc. However
> the handling of registering NAND devices with MTD would probably then
> have to be part of the NEMC driver, which feels a bit off too.

Another solution would be to describe both the NAND controller and the
NAND chip in the DT (with the NAND chip being a chip of the NAND
controller).
Actually this is already what other binding are doing [1][2]. I know
those bindings are representing NAND controllers which can interface
with more than one NAND chip, but I think that even in the 1:1 case it
would make it clearer to represent both the NAND chip and the NAND
controller.

In your case this would give the following representation

+&nemc {
+	status = "okay";
+
+	nandc: nand-controller@1 {
+		compatible = "ingenic,jz4780-nand";
+		reg = <1 0 0x1000000>;
+		#address-cells = <1>;
+		#size-cells = <0>;
+
+		ingenic,bch-controller = <&bch>;
+
+		nand@0 {
+			nand-ecc-mode = "hw";
+			nand-on-flash-bbt;
+			nand-ecc-size = <1024>;
+			nand-ecc-strength = <24>;
+
+			#address-cells = <2>;
+			#size-cells = <2>;
+
+			partition@0 {
+				label = "u-boot-spl";
+				reg = <0x0 0x0 0x0 0x800000>;
+			};
+			/* ... */
+
+		};
+	};
+};

Best Regards,

Boris

[1]http://lxr.free-electrons.com/source/Documentation/devicetree/bindings/mtd/brcm,brcmnand.txt#L119
[2]http://lxr.free-electrons.com/source/Documentation/devicetree/bindings/mtd/sunxi-nand.txt#L28

> 
> Thanks,
>     Paul
> 
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-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com