Subject: Representation of external memory-mapped devices in DT (gpmc)

From: [email-masked] (Daniel Mack)
Date: Mon, 29 Oct 2012 15:39:15 +0100

Hi,

we're currently working on a DT binding for the GPMC bus that is found
on SoCs by TI. The implementation is based on CS lines and an 8, 16 or
32 bit parallel interface. That IP is quite flexible, and it can for
example be used for physmap flash, external peripherals or even NAND.

Depending on which CS is used to control the device, different memory
regions are reserved, and there's code to calculate the location and
size of them, given a CS number (see arch/arm/mach-omap2/gpmc.c).

The binding will use one top-level node to describe the GPMC controller
itself and register the actual devices as sub-nodes to it. The NAND type
is the only one that is currently supported. This is how it currently looks:

	gpmc: gpmc at 50000000 {
		compatible = "ti,gpmc";
		ti,hwmods = "gpmc";
		reg = <0x50000000 0x2000>;
		interrupt-parent = <&intc>;
		interrupts = <100>;
		#address-cells = <1>;
		#size-cells = <0>;

		nand at 0 {
			reg = <0>; /* CS0 */
			nand-bus-width = <16>;
			nand-ecc-mode = "soft";

			gpmc,sync-clk = <0>;
			gpmc,cs-on = <0>;
			gpmc,cs-rd-off = <44>;
			gpmc,cs-wr-off = <44>;
			gpmc,adv-on = <6>;
			gpmc,adv-rd-off = <34>;
			gpmc,adv-wr-off = <44>;
			gpmc,we-off = <40>;
			gpmc,oe-off = <54>;
			gpmc,access = <64>;
			gpmc,rd-cycle = <82>;
			gpmc,wr-cycle = <82>;
			gpmc,wr-access = <40>;
			gpmc,wr-data-mux-bus = <0>;

			#address-cells = <1>;
			#size-cells = <1>;

			partition at 0 {
				label = "1st";
				reg = <0x0 0x20000>;
			};
			/* more partitions ... */
		};
	};

The question is where the resource location and sizes should be
described so that the code that does the magic run-time calculations can
be removed eventually. I would clearly prefer not to have them in the
child, as the only thing these nodes really care about is the chip
select index the hardware is wired to.

Should the "reg" property in the parent be augmented to hold such details?

Once we got that sorted out, I'll do a re-spin of the series and copy
devicetree-discuss on the patch that adds the bindings.

Thanks,
Daniel