本文笔者所使用的板子为nanopi-r4s，friendlyarm出的，芯片rk3399.

当我们在使用uboot命令行进行printenv操作时，可以看到如下的输出

altbootcmd=setenv boot_syslinux_conf extlinux/extlinux-rollback.conf;run distro_bootcmd
arch=arm
baudrate=1500000
board=evb_rk3399
board_name=evb_rk3399
boot_a_script=load ${devtype} ${devnum}:${distro_bootpart} ${scriptaddr} ${prefix}${script}; source ${scriptaddr}
boot_efi_binary=load ${devtype} ${devnum}:${distro_bootpart} ${kernel_addr_r} efi/boot/bootaa64.efi; if fdt addr -q ${fdt_addr_r}; then bootefi ${kernel_addr_r} ${fdt_addr_r};else bootefi ${kernel_addr_r} ${fdtcontroladdr};fi
boot_efi_bootmgr=if fdt addr -q ${fdt_addr_r}; then bootefi bootmgr ${fdt_addr_r};else bootefi bootmgr;fi
boot_extlinux=sysboot ${devtype} ${devnum}:${distro_bootpart} any ${scriptaddr} ${prefix}${boot_syslinux_conf}
boot_net_usb_start=usb start
boot_prefixes=/ /boot/
boot_script_dhcp=boot.scr.uimg
boot_scripts=boot.scr.uimg boot.scr
boot_syslinux_conf=extlinux/extlinux.conf
boot_targets=mmc1 mmc0 usb0 pxe dhcp sf0
bootcmd=run distro_bootcmd
bootcmd_dhcp=devtype=dhcp; run boot_net_usb_start; if dhcp ${scriptaddr} ${boot_script_dhcp}; then source ${scriptaddr}; fi;setenv efi_fdtfile ${fdtfile}; setenv efi_old_vci ${bootp_vci};setenv efi_old_arch ${bootp_arch};setenv bootp_vci PXEClient:Arch:00011:UNDI:003000;setenv bootp_arch 0xb;if dhcp ${kernel_addr_r}; then tftpboot ${fdt_addr_r} dtb/${efi_fdtfile};if fdt addr -q ${fdt_addr_r}; then bootefi ${kernel_addr_r} ${fdt_addr_r}; else bootefi ${kernel_addr_r} ${fdtcontroladdr};fi;fi;setenv bootp_vci ${efi_old_vci};setenv bootp_arch ${efi_old_arch};setenv efi_fdtfile;setenv efi_old_arch;setenv efi_old_vci;
bootcmd_mmc0=devnum=0; run mmc_boot
bootcmd_mmc1=devnum=1; run mmc_boot
bootcmd_pxe=run boot_net_usb_start; dhcp; if pxe get; then pxe boot; fi
bootcmd_sf0=busnum=0; run sf_boot
bootcmd_usb0=devnum=0; run usb_boot
bootdelay=2
cpu=armv8
distro_bootcmd=for target in ${boot_targets}; do run bootcmd_${target}; done
efi_dtb_prefixes=/ /dtb/ /dtb/current/
fdt_addr_r=0x01f00000
fdtcontroladdr=f1f2e9e0
fdtfile=rockchip/rk3399-nanopi-r4s.dtb
fdtoverlay_addr_r=0x02000000
kernel_addr_r=0x02080000
kernel_comp_addr_r=0x08000000
kernel_comp_size=0x2000000
load_efi_dtb=load ${devtype} ${devnum}:${distro_bootpart} ${fdt_addr_r} ${prefix}${efi_fdtfile}
loadaddr=0x800800
mmc_boot=if mmc dev ${devnum}; then devtype=mmc; run scan_dev_for_boot_part; fi
partitions=uuid_disk=${uuid_gpt_disk};name=loader1,start=32K,size=4000K,uuid=${uuid_gpt_loader1};name=loader2,start=8MB,size=4MB,uuid=${uuid_gpt_loader2};name=trust,size=4M,uuid=${uuid_gpt_atf};name=boot,size=112M,bootable,uuid=${uuid_gpt_boot};name=rootfs,size=-,uuid=B921B045-1DF0-41C3-AF44-4C6F280D3FAE;
pxefile_addr_r=0x00600000
ramdisk_addr_r=0x06000000
scan_dev_for_boot=echo Scanning ${devtype} ${devnum}:${distro_bootpart}...; for prefix in ${boot_prefixes}; do run scan_dev_for_extlinux; run scan_dev_for_scripts; done;run scan_dev_for_efi;
scan_dev_for_boot_part=part list ${devtype} ${devnum} -bootable devplist; env exists devplist || setenv devplist 1; for distro_bootpart in ${devplist}; do if fstype ${devtype} ${devnum}:${distro_bootpart} bootfstype; then run scan_dev_for_boot; fi; done; setenv devplist
scan_dev_for_efi=setenv efi_fdtfile ${fdtfile}; for prefix in ${efi_dtb_prefixes}; do if test -e ${devtype} ${devnum}:${distro_bootpart} ${prefix}${efi_fdtfile}; then run load_efi_dtb; fi;done;run boot_efi_bootmgr;if test -e ${devtype} ${devnum}:${distro_bootpart} efi/boot/bootaa64.efi; then echo Found EFI removable media binary efi/boot/bootaa64.efi; run boot_efi_binary; echo EFI LOAD FAILED: continuing...; fi; setenv efi_fdtfile
scan_dev_for_extlinux=if test -e ${devtype} ${devnum}:${distro_bootpart} ${prefix}${boot_syslinux_conf}; then echo Found ${prefix}${boot_syslinux_conf}; run boot_extlinux; echo SCRIPT FAILED: continuing...; fi
scan_dev_for_scripts=for script in ${boot_scripts}; do if test -e ${devtype} ${devnum}:${distro_bootpart} ${prefix}${script}; then echo Found U-Boot script ${prefix}${script}; run boot_a_script; echo SCRIPT FAILED: continuing...; fi; done
scan_sf_for_scripts=${devtype} read ${scriptaddr} ${script_offset_f} ${script_size_f}; source ${scriptaddr}; echo SCRIPT FAILED: continuing...
script_offset_f=0xffe000
script_size_f=0x2000
scriptaddr=0x00500000
sf_boot=if sf probe ${busnum}; then devtype=sf; run scan_sf_for_scripts; fi
soc=rk3399
stderr=serial,vidconsole
stdin=serial,usbkbd
stdout=serial,vidconsole
usb_boot=usb start; if usb dev ${devnum}; then devtype=usb; run scan_dev_for_boot_part; fi
vendor=rockchip

其中都是设置好的环境变量，其中的变量下次再细说。
关于如何设置环境变量，都在include文件夹下，以我使用的板子为例
设置在/include/configs/rk3399_common.h所定义的CFG_EXTRA_ENV_SETTINGS宏中，如下图所示

#define CFG_EXTRA_ENV_SETTINGS \
	ENV_MEM_LAYOUT_SETTINGS \
	"fdtfile=" CONFIG_DEFAULT_FDT_FILE "\0" \
	"partitions=" PARTS_DEFAULT \
	ROCKCHIP_DEVICE_SETTINGS \
	"boot_targets=" BOOT_TARGETS "\0" \
	"altbootcmd=" \
		"setenv boot_syslinux_conf extlinux/extlinux-rollback.conf;" \
		"run distro_bootcmd\0" 
#endif

以上时环境变量所设置的宏，这些都可以在printenv中找到。该宏会被/include/env-default.h所调用进行环境变量的配置。
根据此思路进行一些uboot的环境变量的添加，如下图所示

#define CFG_EXTRA_ENV_SETTINGS \
	ENV_MEM_LAYOUT_SETTINGS \
	"fdtfile=" CONFIG_DEFAULT_FDT_FILE "\0" \
	"partitions=" PARTS_DEFAULT \
	ROCKCHIP_DEVICE_SETTINGS \
	"boot_targets=" BOOT_TARGETS "\0" \
	"altbootcmd=" \
		"setenv boot_syslinux_conf extlinux/extlinux-rollback.conf;" \
		"run distro_bootcmd\0" \
	"helloworld=" \
		"hello world;" \
		"echo hello\0"
#endif

在环境变量中增加了一个helloworld
结果如下图所示，符合预期
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