SD卡挂载文件系统：VFS: Cannot open root device "mmcblk0p2"

使用initramfs文件系统，linux可以正常运行，也能正常读写SD卡。但挂载在SD 卡上，则出现无法挂载的现象，
bootargs为“console=ttySC0,115200 root=/dev/mmcblk0p2 init=/init rootfstype=ext3 rootwait”
相关信息为：

mmc0 at 0xd082c000 irq 119
usbcore: registered new interface driver usbhid
usbhid: USB HID core driver
No device for DAI AK4554
ALSA device list:
  No soundcards found.
TCP cubic registered
NET: Registered protocol family 17
VFP support v0.3: implementor 41 architecture 3 part 30 variant 9 rev 4
Waiting for root device /dev/mmcblk0p2...
mmc0: new high speed SDHC card at address b368
mmcblk0: mmc0:b368 00000 7.51 GiB (ro)
 mmcblk0: p1 p2 p3
VFS: Cannot open root device "mmcblk0p2" or unknown-block(179,2)
Please append a correct "root=" boot option; here are the available partitions:
b300         7879680 mmcblk0 driver: mmcblk
  b301          486266 mmcblk0p1
  b302          555768 mmcblk0p2
  b303         6831315 mmcblk0p3
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(179,2)
[] (unwind_backtrace+0x0/0xe4) from [] (panic+0x58/0xd8)
[] (panic+0x58/0xd8) from [] (mount_block_root+0x1c8/0x208)
[] (mount_block_root+0x1c8/0x208) from [] (prepare_namespace+0x120/0x174)
[] (prepare_namespace+0x120/0x174) from [] (kernel_init+0x120/0x168)
[] (kernel_init+0x120/0x168) from [] (kernel_thread_exit+0x0/0x8)

发现SD 为只读：mmcblk0: mmc0:b368 00000 7.51 GiB (ro)
注意该错误信息中，出现(ro)，以及后面会提示推荐的挂载分区：
Please append a correct "root=" boot option; here are the available partitions:... etc
而实际上SD 卡未上锁，由于硬件没法查看，因此临时在代码中修改：
将mmc驱动中关于判定读写保护的部分取消，直接return 0。
之后再运行，又出现新的错误：

Waiting for root device /dev/mmcblk0p2...
mmc0: new high speed SDHC card at address b368
mmcblk0: mmc0:b368 00000 7.51 GiB
 mmcblk0: p1 p2 p3
EXT3-fs: barriers not enabled
EXT3-fs (mmcblk0p2): warning: mounting unchecked fs, running e2fsck is recommended
kjournald starting.  Commit interval 5 seconds
EXT3-fs (mmcblk0p2): using internal journal
EXT3-fs (mmcblk0p2): mounted filesystem with ordered data mode
VFS: Mounted root (ext3 filesystem) on device 179:2.
Freeing init memory: 76K
Kernel panic - not syncing: Attempted to kill init!
[] (unwind_backtrace+0x0/0xe4) from [] (panic+0x58/0xd8)
[] (panic+0x58/0xd8) from [] (do_exit+0x68/0x608)
[] (do_exit+0x68/0x608) from [] (do_group_exit+0xa8/0xd8)
[] (do_group_exit+0xa8/0xd8) from [] (get_signal_to_deliver+0x378/0x3cc)
[] (get_signal_to_deliver+0x378/0x3cc) from [] (do_signal+0x68/0x680)
[] (do_signal+0x68/0x680) from [] (do_notify_resume+0x18/0x48)
[] (do_notify_resume+0x18/0x48) from [] (work_pending+0x1c/0x20)

从红色部分，可以看出，其实文件系统已经挂载上去了。应该是在加载init程序时出错。
后面查找过程中，发现该SD卡驱动部分代码依赖于DMA模块，且问题竟然是与驱动模块编译顺序有关。
这个问题实在太隐蔽。还好有大神帮忙。
在drivers/Makefile文件中，源代码默认的编译顺序是DMA在MMC模块之后，将MMC修改在DMA之前编译，问题解决。
注意该错误的信息：出现在Freeing init memory之后。
后面推测了下大致原因，由于MMC在DMA编译之前，当内核加载好MMC驱动之后，检测到SD卡分区，于是直接挂载mmcblk0p2。
但不知道为什么，DMA驱动还未加载完毕，这时候要读取SD卡里面的内容时无法正常读取，于是出错。