轉：http://www.crifan.com/linux_system_in__dev__mtd_and__dev__mtdblock_distinction_character_devices_and_block_devices_mtd_difference/
1. /dev/mtdN 是Linux 中的MTD架構中，系統自己實現的mtd分區所對應的字符設備，其裏面添加了一些ioctl，支持很多命令，如MEMGETINFO，MEMERASE等。
而mtd-util中的flash_eraseall等工具，就是以這些ioctl爲基礎而實現的工具，實現一些關於Flash的操作。比如，mtd 工具中的 flash_eraseall中的：
if (ioctl(fd, MEMGETINFO, &meminfo) != 0) {
fprintf(stderr, "%s: %s: unable to get MTD device infon", exe_name, mtd_device);
return 1;
}
其中，MEMGETINFO，就是Linux MTD中的drivers/mtd/nand/mtdchar.c中的：
static int mtd_ioctl(struct inode *inode, struct file *file,
u_int cmd, u_long arg)
{
。。。。。
case MEMGETINFO:
info.type = mtd->type;
info.flags = mtd->flags;
info.size = mtd->size;
info.erasesize = mtd->erasesize;
info.writesize = mtd->writesize;
info.oobsize = mtd->oobsize;
/* The below fields are obsolete */
info.ecctype = -1;
info.eccsize = 0;
if (copy_to_user(argp, &info, sizeof(struct mtd_info_user)))
return -EFAULT;
break;
。。。
}
而/dev/mtdblockN，是Nand Flash驅動中，驅動在用add_mtd_partitions()添加MTD設備分區，而生成的對應的塊設備。
根據以上內容，也就更加明白，爲什麼不能用nandwrite,flash_eraseall,flash_erase等工具去對/dev/mtdblockN去操作了。因爲/dev/mtdblock中不包含對應的ioctl，不支持你這麼操作。
2. mtd char 設備的主設備號是90，而mtd block設備的主設備號是31：
# ls /dev/mtd? -l
crw-r—– 1 root root 90, 0 May 30 2007 /dev/mtd0
crw-r—– 1 root root 90, 2 May 30 2007 /dev/mtd1
crw-r—– 1 root root 90, 4 Jul 17 2009 /dev/mtd2
crw-r—– 1 root root 90, 6 May 30 2007 /dev/mtd3
crwxrwxrwx 1 root root 90, 8 May 30 2007 /dev/mtd4
crwxrwxrwx 1 root root 90, 10 May 30 2007 /dev/mtd5
crwxrwxrwx 1 root root 90, 12 May 30 2007 /dev/mtd6
crwxrwxrwx 1 root root 90, 14 May 30 2007 /dev/mtd7
crwxrwxrwx 1 root root 90, 16 May 30 2007 /dev/mtd8
crwxrwxrwx 1 root root 90, 18 May 30 2007 /dev/mtd9
# ls /dev/mtdblock? -l
brw-r—– 1 root root 31, 0 May 30 2007 /dev/mtdblock0
brw-r—– 1 root root 31, 1 May 30 2007 /dev/mtdblock1
brw-r—– 1 root root 31, 2 May 30 2007 /dev/mtdblock2
brw-r—– 1 root root 31, 3 May 30 2007 /dev/mtdblock3
brwxrwxrwx 1 root root 31, 4 May 30 2007 /dev/mtdblock4
brwxrwxrwx 1 root root 31, 5 May 30 2007 /dev/mtdblock5
brwxrwxrwx 1 root root 31, 6 May 30 2007 /dev/mtdblock6
brwxrwxrwx 1 root root 31, 7 May 30 2007 /dev/mtdblock7
brwxrwxrwx 1 root root 31, 8 May 30 2007 /dev/mtdblock8
brwxrwxrwx 1 root root 31, 9 May 30 2007 /dev/mtdblock9
此設備號，定義在/include/linux/mtd/mtd.h中 ：
#define MTD_CHAR_MAJOR 90
#define MTD_BLOCK_MAJOR 31
3. 其中，mtd的塊設備的大小，可以通過查看分區信息獲得：
# cat /proc/partitions
major minor #blocks name
31 0 1024 mtdblock0
31 1 8192 mtdblock1
31 2 204800 mtdblock2
31 3 65536 mtdblock3
31 4 225280 mtdblock4
上面中顯示的塊設備大小，是block的數目，每個block是1KB。
而每個字符設備，其實就是對應着上面的每個塊設備。即/dev/mtd0對應/dev/mtdblock0，其他以此類推。換句話說，mtdblockN的一些屬性，也就是mtdN的屬性，比如大小。
4。對每個mtd字符設備的操作，比如利用nandwrite去對/dev/mtd0寫數據，實際就是操作/dev/mtdblock0。
而這些操作裏面涉及到的偏移量offset，都指的是此mtd 分區內的偏移。比如向/dev/mtd1的offset爲0的位置寫入數據，實際操作的是物理偏移offset=/dev/mtd0的大小=1MB=0×100000。
5.mtd的字符設備和塊設備的命名規則，可以參考下表：
Table 7-1. MTD /dev entries, corresponding MTD user modules, and relevant device major numbers
|
/dev entry |
Accessible MTD user module |
Device type |
Major number |
|---|---|---|---|
|
mtdN |
char device |
char |
90 |
|
mtdrN |
char device |
char |
90 |
|
mtdblockN |
block device, read-only block device, JFFS, and JFFS2 |
block |
31 |
|
nftlLN |
NFTL |
block |
93 |
|
ftlLN |
FTL |
block |
44 |
Table 7-2. MTD /dev entries, minor numbers, and naming schemes
|
/dev entry |
Minor number range |
Naming scheme |
|---|---|---|
|
mtdN |
0 to 32 per increments of 2 |
N = minor / 2 |
|
mtdrN |
1 to 33 per increments of 2 |
N = (minor – 1) / 2 |
|
mtdblockN |
0 to 16 per increments of 1 |
N = minor |
|
nftlLN |
0 to 255 per sets of 16 |
L = set;[2] N = minor – (set – 1) x 16; N is not appended to entry name if its value is zero. |
|
ftlLN |
0 to 255 per sets of 16 |
Same as NFTL. |
上表相關內容，請參考附錄2.
【附錄】
1.最新的mtd-utils-1.3.0.tar.bz2 下載地址：ftp://ftp.infradead.org/pub/mtd-utils/mtd-utils-1.3.0.tar.bz2
2.7.1 MTD-Supported Devices