About bootup kernel from ramdisk

[SW C]

Dear all,

  My board is BBB. I have built the u-boot and kernel and copy the images into SD card. Also I download the official rootfs, but its' size is large, 2xxM bytes. I am trying to make a small one. My target is to boot up linux from SD card by manual method. So I key in some commands in the U-Boot Console as below,

fatload mmc 0:1 0x80200000 zImage

fatload mmc 0:1 0x80f00000 am335x-boneblack.dtb

fat load mmc 0:1 0x81000000 uRamdisk

setenv bootargs console=ttyS0,115200 initrd=/bin/sh

bootz 0x80200000 0x81000000 0x80f00000

in the .config of KERNEL, I enable some setting as below,

CONFIG_BLK_DEV_INITRD=y

CONFIG_BLK_DEV_RAM=y

CONFIG_DEV_RAM_COUNT=4

CONFIG_DEV_RAM_SIZE=32768

Some of my boot-up log

[    0.436868]  initrd_start[0x6aad8ae16aad8ae1]

[    0.436878] Trying to unpack rootfs image as initramfs...

[    0.437947] Detected gzip compressed data

[    0.437954] unpack_to_rootfs 501 

[    0.617054]  populate_rootfs 643 

[    0.619786] Freeing initrd memory: 2700K   (Note: I just make a small rootfs, create some folders and copy the systemd from offical rootfs                                                                                           into it)

[    2.542175] VFS: Cannot open root device "(null)" or unknown-block(0,0): error -6

[    2.549837] Please append a correct "root=" boot option; here are the available partitions:

[    2.558276] 0100           32768 ram0 

[    2.558282]  (driver?)

[    2.564465] 0101           32768 ram1 

[    2.564469]  (driver?)

[    2.570627] 0102           32768 ram2 

[    2.570630]  (driver?)

[    2.576787] 0103           32768 ram3 

[    2.576790]  (driver?)

[    2.582932] b300        30228480 mmcblk0 

[    2.582936]  driver: mmcblk

[    2.589784]   b301         1024000 mmcblk0p1 92439f51-01

[    2.589788] 

[    2.596635]   b302        29203456 mmcblk0p2 92439f51-02

[    2.596639] 

[    2.603475] b308         3735552 mmcblk1 

[    2.603480]  driver: mmcblk

[    2.610340]   b309         3731456 mmcblk1p1 f26b5473-01

[    2.610343] 

[    2.617192] b320             512 mmcblk1rpmb 

[    2.617195]  (driver?)

[    2.623946] b318            2048 mmcblk1boot1 

[    2.623950]  (driver?)

[    2.630787] b310            2048 mmcblk1boot0 

[    2.630790]  (driver?)

[    2.637639] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)

My question is:

1. Should I set root=xxx in the bootargs ?  Is the commands in U-boot console enough? 

2. I have blocked in this problem many days, Anyone is able give me a hint? 

I am appreciate your suggestion and hint. thanks a lot.