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

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

 

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)

[<c0027620>] (unwind_backtrace+0x0/0xe4) from [<c029c958>] (panic+0x58/0xd8)

[<c029c958>] (panic+0x58/0xd8) from [<c0008d74>] (mount_block_root+0x1c8/0x208)

[<c0008d74>] (mount_block_root+0x1c8/0x208) from [<c000902c>] (prepare_namespace+0x120/0x174)

[<c000902c>] (prepare_namespace+0x120/0x174) from [<c000845c>] (kernel_init+0x120/0x168)

[<c000845c>] (kernel_init+0x120/0x168) from [<c0022f90>] (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!

[<c0021844>] (unwind_backtrace+0x0/0xe4) from [<c023fe74>] (panic+0x58/0xd8)

[<c023fe74>] (panic+0x58/0xd8) from [<c0030d30>] (do_exit+0x68/0x608)

[<c0030d30>] (do_exit+0x68/0x608) from [<c0031378>] (do_group_exit+0xa8/0xd8)

[<c0031378>] (do_group_exit+0xa8/0xd8) from [<c003c47c>] (get_signal_to_deliver+0x378/0x3cc)

[<c003c47c>] (get_signal_to_deliver+0x378/0x3cc) from [<c001e7b4>] (do_signal+0x68/0x680)

[<c001e7b4>] (do_signal+0x68/0x680) from [<c001ede4>] (do_notify_resume+0x18/0x48)

[<c001ede4>] (do_notify_resume+0x18/0x48) from [<c001bed0>] (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。