am335x: update README for BCH16

updates documentation with explanation on how to select ECC schemes.

Signed-off-by: Pekon Gupta <pekon@ti.com>

doc/README.nand

@@ -249,6 +249,48 @@ Platform specific options
 		8-bit BCH code with
 		- ecc calculation using GPMC hardware engine,
 		- error detection using ELM hardware engine.
+	OMAP_ECC_BCH16_CODE_HW
+		16-bit BCH code with
+		- ecc calculation using GPMC hardware engine,
+		- error detection using ELM hardware engine.
+
+	How to select ECC scheme on OMAP and AMxx platforms ?
+	-----------------------------------------------------
+	Though higher ECC schemes have more capability to detect and correct
+	bit-flips, but still selection of ECC scheme is dependent on following
+	- hardware engines present in SoC.
+		Some legacy OMAP SoC do not have ELM h/w engine thus such
+		SoC cannot support BCHx_HW ECC schemes.
+	- size of OOB/Spare region
+		With higher ECC schemes, more OOB/Spare area is required to
+		store ECC. So choice of ECC scheme is limited by NAND oobsize.
+
+	In general following expression can help:
+		NAND_OOBSIZE >= 2 + (NAND_PAGESIZE / 512) * ECC_BYTES
+	where
+		NAND_OOBSIZE	= number of bytes available in
+				OOB/spare area per NAND page.
+		NAND_PAGESIZE	= bytes in main-area of NAND 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 to check for BCH16 on 2K page NAND
+		NAND_PAGESIZE = 2048
+		NAND_OOBSIZE = 64
+		2 + (2048 / 512) * 26 = 106 > NAND_OOBSIZE
+		Thus BCH16 cannot be supported on 2K page NAND.
+
+		However, for 4K pagesize NAND
+		NAND_PAGESIZE = 4096
+		NAND_OOBSIZE = 64
+		ECC_BYTES = 26
+		2 + (4096 / 512) * 26 = 210 < NAND_OOBSIZE
+		Thus BCH16 can be supported on 4K page NAND.
+
 
 NOTE:
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