mmc命令的帮助信息如下：
U-Bootmmc
mmc – MMC sub system
Usage:
mmc read addr blk# cnt
mmc write addr blk# cnt
mmc erase blk# cnt
mmc rescan
mmc part – lists available partition on current mmc device
mmc dev [dev] [part] – show or set current mmc device [partition]
mmc list – lists available devices
当系统启动后，系统会将mci0设置为当前设备。使用mmcinfo就可以看到插入到mmc controller 0的sdcard信息。
U-Bootmmcinfo
Device: mci
Manufacturer ID: 3
OEM: 5344
Name: SD04G
Tran Speed: 25000000
Rd Block Len: 512
SD version 2.0
High Capacity: Yes
Capacity: 3.8 GiB
Bus Width: 4-bit
如果SoC支持多个mmc controller并且使能，使用mmc list来查看使能了几个，可以通过mmc dev来查看当前是那个设备，或者通过mmc dev NO.来切换。
像我的系统支持两个mmc controller.
U-Bootmmc list(显示有几个mmc controller)
mci 0
mci 1
U-Bootmmc dev(显示当前是那个device)
mmc1 is current device
U-Bootmmc dev 0(设置0为当前设备)
mmc0 is current device
如果，换sdcard，则使用mmc rescan来重新获取sdcard信息
U-Bootmmc rescan(重新scan当前设备)
mci: setting clock 257812 Hz, block size 512
mci: setting clock 22000000 Hz, block size 512
mci: setting clock 257812 Hz, block size 512
gen_atmel_mci: CMDR 00001048 ( 8) ARGR 000001aa (SR: 0c100025) Command Failed
mci: setting clock 257812 Hz, block size 512
mci: setting clock 22000000 Hz, block size 512
U-Bootmmcinfo(列出当前设备的信息)
Device: mci
Manufacturer ID: 1
OEM: 5041
Name: SK512
Tran Speed: 25000000
Rd Block Len: 512
SD version 1.10
High Capacity: No
Capacity: 472.5 MiB
Bus Width: 4-bit