mtd: nand: omap: add CONFIG_SYS_NAND_BUSWIDTH_16BIT to indicate NAND device bus-width

GPMC controller needs to be configured based on bus-width of the NAND device
connected to it. Also, dynamic detection of NAND bus-width from on-chip ONFI
parameters is not possible in following situations:
SPL:    SPL NAND drivers does not support ONFI parameter reading.
U-boot: GPMC controller iniitalization is done in omap_gpmc.c:board_nand_init()
        which is called before probing for devices, hence any ONFI parameter
        information is not available during GPMC initialization.

Thus, OMAP NAND driver expected board developers to explicitely write GPMC
configurations specific to NAND device attached on board in board files itself.
But this was troublesome for board manufacturers as they need to dive into
lengthy platform & SoC documents to find details of GPMC registers and
appropriate configurations to get NAND device working.

This patch instead adds existing CONFIG_SYS_NAND_BUSWIDTH_16BIT to board config
hich indicates that connected NAND device has x16 bus-width. And then based on
this config GPMC driver itself initializes itself based on NAND bus-width. This
keeps board developers free from knowing GPMC controller specific internals.

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

1 files changed, 18 insertions, 0 deletions

diff --git a/doc/README.nand b/doc/README.nand
index b91f1985d1..2bc5b391f8 100644
--- a/doc/README.nand
+++ b/doc/README.nand
@@ -190,6 +190,24 @@ Configuration Options:
 	This is used by SoC platforms which do not have built-in ELM
 	hardware engine required for BCH ECC correction.
 
+   CONFIG_SYS_NAND_BUSWIDTH_16BIT
+	Indicates that NAND device has 16-bit wide data-bus. In absence of this
+	config, bus-width of NAND device is assumed to be either 8-bit and later
+	determined by reading ONFI params.
+	Above config is useful when NAND device's bus-width information cannot
+	be determined from on-chip ONFI params, like in following scenarios:
+	- SPL boot does not support reading of ONFI parameters. This is done to
+	  keep SPL code foot-print small.
+	- In current U-Boot flow using nand_init(), driver initialization
+	  happens in board_nand_init() which is called before any device probe
+	  (nand_scan_ident + nand_scan_tail), thus device's ONFI parameters are
+	  not available while configuring controller. So a static CONFIG_NAND_xx
+	  is needed to know the device's bus-width in advance.
+	Some drivers using above config are:
+	drivers/mtd/nand/mxc_nand.c
+	drivers/mtd/nand/ndfc.c
+	drivers/mtd/nand/omap_gpmc.c
+
 
 Platform specific options
 =========================