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uenv.md

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uEnv

booting sequence: MLO->u-boot.img->kernel->root file system Loading uEnv.txt after loading u-boot.img, U-boot allows the user a 2-3 seconds window to stop the boot process. Hit any key in the UART terminal application and the u-boot prompt will be displayed as shown below:

U-Boot#

There are several useful commands to remember here.

To display the environment variables, type printenv or just pri.

to display perticular virable, type env print VAR

to set virable, type setenv NAME VALUE

to get more help, type help or help COMMAND

To continue the boot from u-boot, simply type boot and the process will continue as if you did not stop the boot by hitting a key. This is also equivalent to typing run bootcmd. The bootcmd environment variable is actually a sequence of conditional statements (separated by semicolons) which perform checks on the hardware and software to complete the boot process.

These commands can do all the work, but we will need uEnv.txt if we want to make things work automaticaly.

Actually it works like GRUB, you will easily understand the whole process if you know GRUB well.

Let me show my uEnv.txt first:

#uEnv.txt boot kernel from TFTP, NFS or SD

#basic setting for booting from tftp
ipaddr=10.42.0.12
serverip=10.42.0.1
gatewayip=10.42.0.1
staticip=${ipaddr}:${serverip}:${gatewayip}:255.255.255.0:::off
rootpath=/home/longqi/BBB/rootfs

#setting for kernel loading
kloadaddr=0x82000000
fdtaddr=0x83000000

##Disable HDMI/eMMC
cape_disable=capemgr.disable_partno=BB-BONELT-HDMI,BB-BONELT-HDMIN,BB-BONE-EMMC-2G
#cape_enable=capemgr.enable_partno=BB-BONE-LCD7-01

#parameters for kernel
mmc_args=setenv bootargs console=ttyO0,115200n8 quiet root=/dev/mmcblk0p2 ro rootfstype=ext4 rootwait ${cape_disable} ${cape_enable}
nfs_args=setenv bootargs console=ttyO0,115200n8 root=/dev/nfs rw nfsroot=${serverip}:${rootpath} ip=${staticip} ${cape_disable} ${cape_enable}


#load kernel and device tree
load_u_kernel=tftp ${kloadaddr} ${rootpath}/boot/uImage
load_z_kernel=tftp ${kloadaddr} ${rootpath}/boot/zImage
loadfdt=tftp ${fdtaddr} ${rootpath}/boot/dtbs/am335x-boneblack.dtb


#booting options
boot_uimage=bootm ${kloadaddr} - ${fdtaddr}
boot_zimage=bootz ${kloadaddr} - ${fdtaddr}

#to boot
uenvcmd=run load_u_kernel; run loadfdt; run mmc_args; run boot_uimage

#uenvcmd=run load_z_kernel; run loadfdt; run mmc_args; run boot_zimage

format

NAME=VALUE in uEnv.txt is same with setenv NAME VALUE in u-boot prompt, they do the same thing:

define virables

${NAME} means call this variable

run NAME means run the NAME as command

parameters for booting

kloadaddr this is the loading address for the kernel fdtaddr this the loading addressfor the binary device tree initrd_addr this is the loading address for the initrd ramdisk

In computing, initrd (initial ramdisk) is a scheme for loading a temporary root file system
into memory in the boot process of the Linux kernel. initrd and initramfs refer to two
different methods of achieving this. Both are commonly used to make preparations before the
real root file system can be mounted.