U－Boot 1.3.1是U－Boot目前的最新版本，和以前的版本相比，增加了对大页面NAND FLASH 的支持。早期版本基本上只能支持到512字节的页面大小，而现在市面上大多是2K字节页面大小的NAND FLASH，这样的FLASH容量大，价格低，性价比很高，很受电子设计工程师的青睐。
看源代码board目录下，有很多板子的配置。可以使用源代码阅读工具，搜索一下，可以看到其中有很多板子的目录下都有一个nand.c文件，文件中有一些nand flash必须的借口函数。下面逐一介绍。
board_nand_init函数
在这个函数可以定义NAND FLASH 命令／数据的地址，典型的可以看board/sc3/sc3nand.c文件：
sc3_io_base = (void *) CFG_NAND_BASE;
/* Set address of NAND IO lines (Using Linear Data Access Region) */
nand->IO_ADDR_R = (void __iomem *) sc3_io_base;
nand->IO_ADDR_W = (void __iomem *) sc3_io_base;
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还有一些接口也是必须的，如设置用户自定义的一些接口函数，比较典型的可以看board/prodrive/pdnp3/nand.c文件：
int board_nand_init(struct nand_chip *nand)
{
pdnb3_ndfc = (struct pdnb3_ndfc_regs *)CFG_NAND_BASE;
nand->eccmode = NAND_ECC_SOFT;
/* Set address of NAND IO lines (Using Linear Data Access Region) */
nand->IO_ADDR_R = (void __iomem *) ((ulong) pdnb3_ndfc + 0x4);
nand->IO_ADDR_W = (void __iomem *) ((ulong) pdnb3_ndfc + 0x4);
/* Reference hardware control function */
nand->hwcontrol = pdnb3_nand_hwcontrol;
/* Set command delay time */
nand->hwcontrol = pdnb3_nand_hwcontrol;
nand->write_byte = pdnb3_nand_write_byte;
nand->read_byte = pdnb3_nand_read_byte;
nand->write_buf = pdnb3_nand_write_buf;
nand->read_buf = pdnb3_nand_read_buf;
nand->verify_buf = pdnb3_nand_verify_buf;
nand->dev_ready = pdnb3_nand_dev_ready;
return 0;
}
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如果在这里定义了这些接口函数，则必须实现，用于替换u-boot的默认函数实现。
nand_chip数据结构
其实，这里面最重要的是nand_chip数据结构，请看文件include/linux/mtd/nand.h：
/**
* struct nand_chip - NAND Private Flash Chip Data
* @IO_ADDR_R: [BOARDSPECIFIC] address to read the 8 I/O lines of the flash device
* @IO_ADDR_W: [BOARDSPECIFIC] address to write the 8 I/O lines of the flash device
* @read_byte: [REPLACEABLE] read one byte from the chip
* @write_byte: [REPLACEABLE] write one byte to the chip
* @read_word: [REPLACEABLE] read one word from the chip
* @write_word: [REPLACEABLE] write one word to the chip
* @write_buf: [REPLACEABLE] write data from the buffer to the chip
* @read_buf: [REPLACEABLE] read data from the chip into the buffer
* @verify_buf: [REPLACEABLE] verify buffer contents against the chip data
* @select_chip: [REPLACEABLE] select chip nr
* @block_bad: [REPLACEABLE] check, if the block is bad
* @block_markbad: [REPLACEABLE] mark the block bad
* @hwcontrol: [BOARDSPECIFIC] hardwarespecific function for accesing control-lines
* @dev_ready: [BOARDSPECIFIC] hardwarespecific function for accesing device ready/busy line
* If set to NULL no access to ready/busy is available and the ready/busy information
* is read from the chip status register
* @cmdfunc: [REPLACEABLE] hardwarespecific function for writing commands to the chip
* @waitfunc: [REPLACEABLE] hardwarespecific function for wait on ready
* @calculate_ecc: [REPLACEABLE] function for ecc calculation or readback from ecc hardware
* @correct_data: [REPLACEABLE] function for ecc correction, matching to ecc generator (sw/hw)
* @enable_hwecc: [BOARDSPECIFIC] function to enable (reset) hardware ecc generator. Must only
* be provided if a hardware ECC is available
* @erase_cmd: [INTERN] erase command write function, selectable due to AND support
* @scan_bbt: [REPLACEABLE] function to scan bad block table
* @eccmode: [BOARDSPECIFIC] mode of ecc, see defines
* @eccsize: [INTERN] databytes used per ecc-calculation
* @eccbytes: [INTERN] number of ecc bytes per ecc-calculation step
* @eccsteps: [INTERN] number of ecc calculation steps per page
* @chip_delay: [BOARDSPECIFIC] chip dependent delay for transfering data from array to read regs (tR)
* @chip_lock: [INTERN] spinlock used to protect access to this structure and the chip
* @wq: [INTERN] wait queue to sleep on if a NAND operation is in progress
* @state: [INTERN] the current state of the NAND device
* @page_shift: [INTERN] number of address bits in a page (column address bits)
* @phys_erase_shift: [INTERN] number of address bits in a physical eraseblock
* @bbt_erase_shift: [INTERN] number of address bits in a bbt entry
* @chip_shift: [INTERN] number of address bits in one chip
* @data_buf: [INTERN] internal buffer for one page + oob
* @oob_buf: [INTERN] oob buffer for one eraseblock
* @oobdirty: [INTERN] indicates that oob_buf must be reinitialized
* @data_poi: [INTERN] pointer to a data buffer
* @options: [BOARDSPECIFIC] various chip options. They can partly be set to inform nand_scan about
* special functionality. See the defines for further explanation
* @badblockpos: [INTERN] position of the bad block marker in the oob area
* @numchips: [INTERN] number of physical chips
* @chipsize: [INTERN] the size of one chip for multichip arrays
* @pagemask: [INTERN] page number mask = number of (pages / chip) - 1
* @pagebuf: [INTERN] holds the pagenumber which is currently in data_buf
* @autooob: [REPLACEABLE] the default (auto)placement scheme
* @bbt: [INTERN] bad block table pointer
* @bbt_td: [REPLACEABLE] bad block table descriptor for flash lookup
* @bbt_md: [REPLACEABLE] bad block table mirror descriptor
* @badblock_pattern: [REPLACEABLE] bad block scan pattern used for initial bad block scan
* @controller: [OPTIONAL] a pointer to a hardware controller structure which is shared among multiple independend devices
* @priv: [OPTIONAL] pointer to private chip date
*/
struct nand_chip {
void __iomem *IO_ADDR_R;
void __iomem *IO_ADDR_W;
u_char (*read_byte)(struct mtd_info *mtd);
void (*write_byte)(struct mtd_info *mtd, u_char byte);
u16 (*read_word)(struct mtd_info *mtd);
void (*write_word)(struct mtd_info *mtd, u16 word);
void (*write_buf)(struct mtd_info *mtd, const u_char *buf, int len);
void (*read_buf)(struct mtd_info *mtd, u_char *buf, int len);
int (*verify_buf)(struct mtd_info *mtd, const u_char *buf, int len);
void (*select_chip)(struct mtd_info *mtd, int chip);
int (*block_bad)(struct mtd_info *mtd, loff_t ofs, int getchip);
int (*block_markbad)(struct mtd_info *mtd, loff_t ofs);
void (*hwcontrol)(struct mtd_info *mtd, int cmd);
int (*dev_ready)(struct mtd_info *mtd);
void (*cmdfunc)(struct mtd_info *mtd, unsigned command, int column, int page_addr);
int (*waitfunc)(struct mtd_info *mtd, struct nand_chip *this, int state);
int (*calculate_ecc)(struct mtd_info *mtd, const u_char *dat, u_char *ecc_code);
int (*correct_data)(struct mtd_info *mtd, u_char *dat, u_char *read_ecc, u_char *calc_ecc);
void (*enable_hwecc)(struct mtd_info *mtd, int mode);
void (*erase_cmd)(struct mtd_info *mtd, int page);
int (*scan_bbt)(struct mtd_info *mtd);
int eccmode;
int eccsize;
int eccbytes;
int eccsteps;
int chip_delay;
#if 0
spinlock_t chip_lock;
wait_queue_head_t wq;
nand_state_t state;
#endif
int page_shift;
int phys_erase_shift;
int bbt_erase_shift;
int chip_shift;
u_char *data_buf;
u_char *oob_buf;
int oobdirty;
u_char *data_poi;
unsigned int options;
int badblockpos;
int numchips;
unsigned long chipsize;
int pagemask;
int pagebuf;
struct nand_oobinfo *autooob;
uint8_t *bbt;
struct nand_bbt_descr *bbt_td;
struct nand_bbt_descr *bbt_md;
struct nand_bbt_descr *badblock_pattern;
struct nand_hw_control *controller;
void *priv;
};
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这个数据结构的注视非常明了，指出了那些函数可以被替换，可被替换的函数，用户可以根据自己的需求实现，一般在上面所讲的nand.c中实现。当然，在其他地方实现也可以，U-Boot源代码中也有这样的例子。
nand_base.c文件
nand_chip数据结构定义的函数，U-Boot的默认实现在drivers/mtd/nand/nand_base.c文件中。可以仔细看看源码。如果要进行替换，这些函数有一定的参考价值。
如果要为nand_chip数据结构增加某些接口函数或者某些成员，可以修改这两个文件。
nand Flash和nand_legacy
nand目录下的文件是能够支持2K大页面的NAND FLASH的实现，而nand_legacy目录下的文件只能支持到512字节的页面，在使用的时候需要加以区分。
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