基于mx27ads 的yaffs 文件系统释疑
Mx27ads bsp内核采用2.6.19, 选择文件系统中的yaffs2
File systems ---> Miscellaneous filesystems ---> <*> YAFFS2 file system support
创建测试的yaffs image
mkdir userfs
echo test > userfs/test
mkyaffsimage userfs userfs.yaffs
写入nand flash
nandwrite –a –o /dev/mtd/8 userfs.yaffs
挂载yaffs mtd
mount –t yaffs /dev/mtdblock/8 /mnt/rwfs
发现mount报错, 只有一个lost+found目录, 其余空空如也. 看样子只好分析yaffs和nand flash代码了.
mx27的使用的是8bit 512bytes+16bytes oob/page 的128M nand flash, 分析yaffs与nand flash驱动代码, 发现yaffs中调用yaffs_mtdif.c中的nandmtd_WriteChunkToNAND函数将它的chunk写入FLASH，包含一个512字节的数据与yaffs_Spare结构, 512字节数据对应nand flash 一page, 所以不需要关心他的512字节数据区; yaffs_Spare结构，在yaffs_guts.h中定义的
typedef struct {
__u8 tagByte0;
__u8 tagByte1;
__u8 tagByte2;
__u8 tagByte3;
__u8 pageStatus;/* set to 0 to delete the chunk */
__u8 blockStatus;
__u8 tagByte4;
__u8 tagByte5;
__u8 ecc1[3];
__u8 tagByte6;
__u8 tagByte7;
__u8 ecc2[3];
} yaffs_Spare;
正好是16字节, 那就是使用这16字节作为OOB. 其中ecc1与ecc2是用来计算ECC的, 只有使用yaffs自身的ECC时才用到, 我们这里使用mtd的硬件ECC, 可以忽略不计, 省下了YAFFS用来存放文件系统相关的信息（yaffs_Tags）8个bytes. 而mx27 nand flash 其 oob定义如下:
static struct nand_ecclayout nand_hw_eccoob_8 = {
.eccbytes = 5,
.eccpos = {6, 7, 8, 9, 10},
.oobfree = {{0, 5}, {11, 5}}
};
Oobfree有两块, {0,5}, {11,5}总共10个字节. 需要将这8个字节保存到OOB区中, 就需要一个转换. 继续分析yaffs_mtdif.c时,发现2.6.19内核在yaffs写入oob时先使用translate_spare2oob将yaffs_Spare转换为一个8bytes数据块,然后通过mtd->write_oob使用MTD_OOB_AUTO方式写入oob数据;
…
#if (LINUX_VERSION_CODE > KERNEL_VERSION(2,6,17))
__u8 spareAsBytes[8]; /* OOB */
//只有数据
if (data && !spare)
retval = mtd->write(mtd, addr, dev->nDataBytesPerChunk,
&dummy, data);
else if (spare) {
//使用nand 硬件ECC
if (dev->useNANDECC) {
//转换tag为8bytes数据块
translate_spare2oob(spare, spareAsBytes);
//使用MTD_OOB_AUTO方式将8bytes块写入到oobfree
ops.mode = MTD_OOB_AUTO;
ops.ooblen = 8; /* temp hack */
} else {
//使用yaffs自身ECC时, 直接将yaffs_Spare数据作为OOB
ops.mode = MTD_OOB_RAW;
ops.ooblen = YAFFS_BYTES_PER_SPARE;
}
ops.len = data ? dev->nDataBytesPerChunk : ops.ooblen;
ops.datbuf = (u8 *)data;
ops.ooboffs = 0;
ops.oobbuf = spareAsBytes;
retval = mtd->write_oob(mtd, addr, &ops);
}
#endif
…
继续深入分析, 发现mtd-write_oob实际上是调用的是nand_do_write_ops或nand_do_write_oob(都在driver/mtd/nand/nand_base.c), 在这两个函数中在处理oob数据时都调用了同一个函数nand_fill_oob:
static uint8_t *nand_fill_oob(struct nand_chip *chip, uint8_t *oob,
struct mtd_oob_ops *ops)
{
size_t len = ops->ooblen;
switch(ops->mode) {
case MTD_OOB_PLACE:
case MTD_OOB_RAW:
memcpy(chip->oob_poi + ops->ooboffs, oob, len);
return oob + len;
case MTD_OOB_AUTO: {
struct nand_oobfree *free = chip->ecc.layout->oobfree;
uint32_t boffs = 0, woffs = ops->ooboffs;
size_t bytes = 0;
for(; free->length && len; free++, len -= bytes) {
/* Write request not from offset 0 ? */
if (unlikely(woffs)) {
if (woffs >= free->length) {
woffs -= free->length;
continue;
}
boffs = free->offset + woffs;
bytes = min_t(size_t, len,
(free->length - woffs));
woffs = 0;
} else {
bytes = min_t(size_t, len, free->length);
boffs = free->offset;
}
memcpy(chip->oob_poi + boffs, oob, bytes);
oob += bytes;
}
return oob;
}
default:
BUG();
}
return NULL;
}
可以看出nand_fill_oob使用了2种方式来组织oob的处理方式: MTD_OOB_PLACE与MTD_OOB_RAW为一种, 直接将OOB数据复制到要写入oob的数据缓存chip->oob_poi; MTD_OOB_AUTO讲oob数据复制到要写入oob的数据缓存oobfree位置上. 这就是MTD_OOB_RAW与MTD_OOB_AUTO的最终解释了.
再来看mkyaffsimage的代码:
static int write_chunk(__u8 *data, __u32 objId, __u32 chunkId, __u32 nBytes)
{
yaffs_Tags t;
yaffs_Spare s;
error = write(outFile,data,512);
if(error < 0) return error;
memset(&t,0xff,sizeof (yaffs_Tags));
memset(&s,0xff,sizeof (yaffs_Spare));
t.chunkId = chunkId;
t.serialNumber = 0;
t.byteCount = nBytes;
t.objectId = objId;
if (convert_endian)
{
little_to_big_endian(&t);
}
yaffs_CalcTagsECC(&t);
yaffs_LoadTagsIntoSpare(&s,&t);
yaffs_CalcECC(data,&s);
nPages++;
return write(outFile,&s,sizeof(yaffs_Spare));
}
他在512字节之后是包含了16字节yaffs_Spare的，这个16字节的yaffs_Spare就是他的oob结构. 但是这个16字节并没有通过translate_spare2oob转换, 而是直接写入image中了.
再看通过nandwrite -a -o 写入mtd时的代码
if (!noecc) {
int i, start, len;
/*
* We use autoplacement and have the oobinfo with the autoplacement
* information from the kernel available
*
* Modified to support out of order oobfree segments,
* such as the layout used by diskonchip.c
*/
if (!oobinfochanged && (old_oobinfo.useecc == MTD_NANDECC_AUTOPLACE)) {
for (i = 0;old_oobinfo.oobfree[i][1]; i++) {
/* Set the reserved bytes to 0xff */
start = old_oobinfo.oobfree[i][0];
len = old_oobinfo.oobfree[i][1];
printf( "oob:[%d:%d]\n", start, len );
memcpy(oobbuf + start,
oobreadbuf + start,
len);
}
} else {
/* jffs2 or yaffs */
/* Set at least the ecc byte positions to 0xff */
start = old_oobinfo.eccbytes;
len = meminfo.oobsize - start;
memcpy(oobbuf + start,
oobreadbuf + start,
len);
}
}
可见nandwrite在写入oob时是也是通过MTD_NANDECC_AUTOPLACE(等同MTD_OOB_AUTO)方式写入的.
比较一下yaffs流程与mkyaffsimage流程:
yaffs流程是通过translate_spare2oob将8bytes的yaffs_tags转为8bytes数据块,然后通过write_oob将这8bytes写入到OOB的oobfree块区; 读出来的时候反过来translate_oob2spare, 就可以还原成yaffs_tags; 而mkyaffsimage创建yaffs image时却是直接将yaffs_Spare写入文件, 通过nandwrite -a -o 写入mtd时, 直接使用这块yaffs_Spare作为oob数据写入, 虽然使用方式也是MTD_OOB_AUTO; 这就造成yaffs读取chunk时无法读取正确的yaffs_Spare数据了;
由此可见只要在mkyaffsimage写入yaffs_Spare时, 只要将写入的数据转换为yaffs中写入flash之前一致的数据即可.
以下是修改过的 write_chunk
static int write_chunk(__u8 *data, __u32 objId, __u32 chunkId, __u32 nBytes)
{
yaffs_Tags t;
yaffs_Spare s;
__u8 oobdata[16];
error = write(outFile,data,512);
if(error < 0) return error;
memset(&t,0xff,sizeof (yaffs_Tags));
memset(&s,0xff,sizeof (yaffs_Spare));
t.chunkId = chunkId;
t.serialNumber = 0;
t.byteCount = nBytes;
t.objectId = objId;
if (convert_endian)
{
little_to_big_endian(&t);
}
yaffs_CalcTagsECC(&t);
yaffs_LoadTagsIntoSpare(&s,&t);
yaffs_CalcECC(data,&s);
nPages++;
#if 0
return write(outFile,&s,sizeof(yaffs_Spare));
#else
memset(oobdata,0xff,16);
translate_spare2oob( &s, oobdata )
//因为采用的是硬件ECC, 这里忽略了yaffs自身的ECC
return write(outFile, oobdata, 16);
#endif
}