Device tree bindings for GPMC connected NANDs

GPMC connected NAND (found on OMAP boards) are represented as child nodes of
the GPMC controller with a name of "nand".

All timing relevant properties as well as generic gpmc child properties are
explained in a separate documents - please refer to
Documentation/devicetree/bindings/memory-controllers/omap-gpmc.txt

For NAND specific properties such as ECC modes or bus width, please refer to
Documentation/devicetree/bindings/mtd/nand-controller.yaml


Required properties:

 - compatible:	"ti,omap2-nand"
 - reg:		range id (CS number), base offset and length of the
		NAND I/O space
 - interrupts:	Two interrupt specifiers, one for fifoevent, one for termcount.

Optional properties:

 - nand-bus-width: 		Set this numeric value to 16 if the hardware
				is wired that way. If not specified, a bus
				width of 8 is assumed.

 - ti,nand-ecc-opt:		A string setting the ECC layout to use. One of:
		"sw"		1-bit Hamming ecc code via software
		"hw"		<deprecated> use "ham1" instead
		"hw-romcode"	<deprecated> use "ham1" instead
		"ham1"		1-bit Hamming ecc code
		"bch4"		4-bit BCH ecc code
		"bch8"		8-bit BCH ecc code
		"bch16"		16-bit BCH ECC code
		Refer below "How to select correct ECC scheme for your device ?"

 - ti,nand-xfer-type:		A string setting the data transfer type. One of:

		"prefetch-polled"	Prefetch polled mode (default)
		"polled"		Polled mode, without prefetch
		"prefetch-dma"		Prefetch enabled DMA mode
		"prefetch-irq"		Prefetch enabled irq mode

 - elm_id:	<deprecated> use "ti,elm-id" instead
 - ti,elm-id:	Specifies phandle of the ELM devicetree node.
		ELM is an on-chip hardware engine on TI SoC which is used for
		locating ECC errors for BCHx algorithms. SoC devices which have
		ELM hardware engines should specify this device node in .dtsi
		Using ELM for ECC error correction frees some CPU cycles.
 - rb-gpios:	GPIO specifier for the ready/busy# pin.

For inline partition table parsing (optional):

 - #address-cells: should be set to 1
 - #size-cells: should be set to 1

Example for an AM33xx board:

	gpmc: gpmc@50000000 {
		compatible = "ti,am3352-gpmc";
		ti,hwmods = "gpmc";
		reg = <0x50000000 0x36c>;
		interrupts = <100>;
		gpmc,num-cs = <8>;
		gpmc,num-waitpins = <2>;
		#address-cells = <2>;
		#size-cells = <1>;
		ranges = <0 0 0x08000000 0x1000000>;	/* CS0 space, 16MB */
		elm_id = <&elm>;
		interrupt-controller;
		#interrupt-cells = <2>;

		nand@0,0 {
			compatible = "ti,omap2-nand";
			reg = <0 0 4>;		/* CS0, offset 0, NAND I/O window 4 */
			interrupt-parent = <&gpmc>;
			interrupts = <0 IRQ_TYPE_NONE>, <1 IRQ_TYPE NONE>;
			nand-bus-width = <16>;
			ti,nand-ecc-opt = "bch8";
			ti,nand-xfer-type = "polled";
			rb-gpios = <&gpmc 0 GPIO_ACTIVE_HIGH>; /* gpmc_wait0 */

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

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

			/* partitions go here */
		};
	};

How to select correct ECC scheme for your device ?
--------------------------------------------------
Higher ECC scheme usually means better protection against bit-flips and
increased system lifetime. However, selection of ECC scheme is dependent
on various other factors also like;

(1) support of built in hardware engines.
	Some legacy OMAP SoC do not have ELM harware engine, so those SoC cannot
	support ecc-schemes with hardware error-correction (BCHx_HW). However
	such SoC can use ecc-schemes with software library for error-correction
	(BCHx_HW_DETECTION_SW). The error correction capability with software
	library remains equivalent to their hardware counter-part, but there is
	slight CPU penalty when too many bit-flips are detected during reads.

(2) Device parameters like OOBSIZE.
	Other factor which governs the selection of ecc-scheme is oob-size.
	Higher ECC schemes require more OOB/Spare area to store ECC syndrome,
	so the device should have enough free bytes available its OOB/Spare
	area to accommodate ECC for entire page. In general following expression
	helps in determining if given device can accommodate ECC syndrome:
	"2 + (PAGESIZE / 512) * ECC_BYTES" <= OOBSIZE"
	where
		OOBSIZE		number of bytes in OOB/spare area
		PAGESIZE	number of bytes in main-area of device page
		ECC_BYTES	number of ECC bytes generated to protect
		                512 bytes of data, which is:
				'3' for HAM1_xx ecc schemes
				'7' for BCH4_xx ecc schemes
				'14' for BCH8_xx ecc schemes
				'26' for BCH16_xx ecc schemes

	Example(a): For a device with PAGESIZE = 2048 and OOBSIZE = 64 and
		trying to use BCH16 (ECC_BYTES=26) ecc-scheme.
		Number of ECC bytes per page = (2 + (2048 / 512) * 26) = 106 B
		which is greater than capacity of NAND device (OOBSIZE=64)
		Hence, BCH16 cannot be supported on given device. But it can
		probably use lower ecc-schemes like BCH8.

	Example(b): For a device with PAGESIZE = 2048 and OOBSIZE = 128 and
		trying to use BCH16 (ECC_BYTES=26) ecc-scheme.
		Number of ECC bytes per page = (2 + (2048 / 512) * 26) = 106 B
		which can be accommodated in the OOB/Spare area of this device
		(OOBSIZE=128). So this device can use BCH16 ecc-scheme.