之前在制作yaffs根文件系统的时候失败了，Failed to execute /linuxrc.Attempting defaults...现象是第一个文件都不能解析实行，原因是u-boot还不支持yaffs格式的读写，也不知道用nfs启动内核，本文实现u-boot烧写yaffs根文件系统映象，摘自《嵌入式Linux应用开发完全手册》韦东山著。
在上一篇笔记中，增加了NAND Flash 功能的 u-boot-1.1.6已经可以通过"nand write..."、"nand write.jffs2..."等命令来烧写cramfs、jffs2文件系统映象文件，下面增加"nand write.yaffs..."命令实现yaffs文件系统映象的烧写。
"nand write.yaffs..."命令中，"nand"是具体命令，"write.yaffs..."是参数。nand命令在 commom/cmd_nand.c中实现，在其中增加"nand write.yaffs..." 的使用说明，我们可以在 u-boot 下通过 ？nand 命令查看到所加信息，代码添加如下：
U_BOOT_CMD(nand, 5, 1, do_nand,
"nand - NAND sub-system\n",
"info - show available NAND devices\n"
"nand device [dev] - show or set current device\n"
"nand read[.jffs2] - addr off|partition size\n"
"nand write[.jffs2] - addr off|partiton size - read/write `size' bytes starting\n"
" at offset `off' to/from memory address `addr'\n"
"nand read.yaffs addr off size - read the `size' byte yaffs image starting\n"
" at offset `off' to memory address `addr'\n"
"nand write.yaffs addr off size - write the `size' byte yaffs image starting\n"
" at offset `off' from memory address `addr'\n"
"nand erase [clean] [off size] - erase `size' bytes from\n"
" offset `off' (entire device if not specified)\n"
"nand bad - show bad blocks\n"
"nand dump[.oob] off - dump page\n"
"nand scrub - really clean NAND erasing bad blocks (UNSAFE)\n"
"nand markbad off - mark bad block at offset (UNSAFE)\n"
"nand biterr off - make a bit error at offset (UNSAFE)\n"
"nand lock [tight] [status] - bring nand to lock state or display locked pages\n"
"nand unlock [offset] [size] - unlock section\n");
然后，在 nand 命令的处理函数 do_nand 中增加对"nand yaffs..."的支持。do_nand 函数仍在 commom/cmd_nand.c 中实现，代码修改如下：
else if ( s != NULL && !strcmp(s, ".yaffs")){ /*354*/
if (read) {
/* read */
nand_read_options_t opts;
memset(&opts, 0, sizeof(opts));
opts.buffer = (u_char*) addr;
opts.length = size;
opts.offset = off;
opts.readoob = 1;
opts.quiet = quiet;
ret = nand_read_opts(nand, &opts);
} else {
/* write */
nand_write_options_t opts;
memset(&opts, 0, sizeof(opts));
opts.buffer = (u_char*) addr;
opts.length = size;
opts.offset = off;
/* opts.forceyaffs = 1; */
opts.noecc = 1;
opts.writeoob = 1;
opts.blockalign = 1;
opts.quiet = quiet;
opts.skipfirstblk = 1;
ret = nand_write_opts(nand, &opts);
} /*379*/
NAND Flash 每一页大小为 (512+16)字节 (还有其他格式的NAND Flash ，比如每页大小为(256+8)、(2048+64)等，SAMSUNG_K9F1G08U0B就是(2048+64))，其中512字节就是数据存储区，16字节称为OOB(Out Of Band)区，在OBB区存放坏块标记、前面512字节的ECC校验码等。
上述代码中，opts.skipfirstblk是新增加的项，表示烧写时跳过第一个可用的逻辑块，这是由 yaffs 文件系统的特性决定的。下面给opts.skipfirstblk 新增加项重新定义 nand_write_options_t 结构，并在下面调用的 nand_write_opts 函数中对他进行处理。
a) 在 include/nand.h 中进行如下修改，增加 skipfirstblk 成员：
struct nand_write_options {
u_char *buffer; /* memory block containing image to write */
ulong length; /* number of bytes to write */
ulong offset; /* start address in NAND */
int quiet; /* don't display progress messages */
int autoplace; /* if true use auto oob layout */
int forcejffs2; /* force jffs2 oob layout */
int forceyaffs; /* force yaffs oob layout */
int noecc; /* write without ecc */
int writeoob; /* image contains oob data */
int pad; /* pad to page size */
int blockalign; /* 1|2|4 set multiple of eraseblocks
* to align to */
int skipfirstblk; /* if true, skip the first good block,
* set true when write the yaffs image,
* add by
*/
};
b) 在 drivers/nand/nand_util.c 修改 nand_write_opts 函数，增加对 skipfirstblk 成员的支持：
/* skip the first good block when wirte yaffs image, by */
if (skipfirstblk) { /*431行*/
mtdoffset += erasesize_blockalign;
skipfirstblk = 0;
continue;
}
进行上面移植后，u-boot 已经支持 yaffs 文件系统映象的烧写，由于前面设置"opts.noecc=1" 不使用ECC校验码，烧写时会提示很多提示信息，可以修改 drivers/nand/nand_base.c 文件918行的 nand_write_page 函数，将其注释掉。
case NAND_ECC_NONE:
//printk (KERN_WARNING "Writing data without ECC to NAND-FLASH is not recommended\n");
