这个问题困扰我很久,root=/dev/mmcblk0p2如何被解析的,
疑问:
1)还没加载外部(mmc0的第二分区上,也就mmcblk0p2)跟文件系统时候,"/"目录和"/dev",如何提前存在:
答案:原来提前加载了虚拟的rootfs(rootfs在linux内核这里是专有名字),这在vfs_cache_init()里面实现,具体参考:
文件系统(五)—VFS初始化_rootfstype=tmpfs-CSDN博客
2)加载外部根文件系统时候,root=/dev/mmcblk0p2如何被解析的,
答案,分析启动打印,发现:
mmc0: SDHCI controller on samsung-hsmmc [s3c-sdhci.0] using ADMA
s3c-sdhci s3c-sdhci.1: clock source 0: hsmmc (133400000 Hz)
s3c-sdhci s3c-sdhci.1: clock source 1: hsmmc (133400000 Hz)
s3c-sdhci s3c-sdhci.1: clock source 2: sclk_mmc (47642857 Hz)
mmc0: new high speed MMC card at address 0001
mmcblk0: mmc0:0001 004GE0 3.68 GiB
mmcblk0: p1 p2 p3 p4
List of all partitions:
b300 3866624 mmcblk0 driver: mmcblk
b301 264495 mmcblk0p1
b302 124690 mmcblk0p2
b303 105798 mmcblk0p3
b304 3347751 mmcblk0p4
b308 15558144 mmcblk1 driver: mmcblk
b309 102400 mmcblk1p1
b30a 102400 mmcblk1p2
b30b 102400 mmcblk1p3
b30c 15230464 mmcblk1p4
VFS: Mounted root (ext3 filesystem) on device 179:2.
在内核源码上查询:
jimmy@ubuntu:~/news5pv210/Linux-Kernel-2.6.35/kernel$ grep "List of all partitions" -nR *
init/do_mounts.c:277: printk("List of all partitions:\n");
打开init/do_mounts.c: 发现上头有:
* Convert a name into device number. We accept the following variants:
* 1) device number in hexadecimal represents itself
* 2) /dev/nfs represents Root_NFS (0xff)
* 3) /dev/<disk_name> represents the device number of disk
* 4) /dev/<disk_name><decimal> represents the device number
* of partition - device number of disk plus the partition number
* 5) /dev/<disk_name>p<decimal> - same as the above, that form is
* used when disk name of partitioned disk ends on a digit.
* If name doesn't have fall into the categories above, we return (0,0).
* block_class is used to check if something is a disk name. If the disk
* name contains slashes, the device name has them replaced with
* bangs.
顿时豁然开朗:
这可以在系统启动后得到证实:
ls /dev/root
/dev/root
/ # ls /dev/root -alt
lrwxrwxrwx 1 0 0 9 Jan 1 12:22 /dev/root -> mmcblk0p2
/ # ls /dev/mmc* -alt
brw-r--r-- 1 0 0 179, 16 Jan 1 17:00 /dev/mmcblk2
brw-rw---- 1 0 0 179, 0 Jan 1 12:22 /dev/mmcblk0
brw-rw---- 1 0 0 179, 1 Jan 1 12:22 /dev/mmcblk0p1
brw-rw---- 1 0 0 179, 2 Jan 1 12:22 /dev/mmcblk0p2
brw-rw---- 1 0 0 179, 3 Jan 1 12:22 /dev/mmcblk0p3
brw-rw---- 1 0 0 179, 4 Jan 1 12:22 /dev/mmcblk0p4
brw-rw---- 1 0 0 179, 8 Jan 1 12:22 /dev/mmcblk1
brw-rw---- 1 0 0 179, 9 Jan 1 12:22 /dev/mmcblk1p1
brw-rw---- 1 0 0 179, 10 Jan 1 12:22 /dev/mmcblk1p2
brw-rw---- 1 0 0 179, 11 Jan 1 12:22 /dev/mmcblk1p3
brw-rw---- 1 0 0 179, 12 Jan 1 12:22 /dev/mmcblk1p4