Fixing Bad U Boot Environment Variables

Tonight I typed a bad uname_r for my Beaglebone Black’s /boot/uEnv.txt parameters, and it took me a long while to figure out how to fix it. These steps seem like ridiculous overkill, so there’s probably an easier way, but I’m writing these down so I don’t forget. (Note: this is with U-Boot 2015.10-rc3-00001-gfb375b5 (Sep 10 2015 - 10:07:48 -0500)).

Boot the BBB, stop in U-Boot.

=> printenv bootcmd
bootcmd=setenv umsmedia 0; gpio set 53; run findfdt; setenv mmcdev 0; setenv bootpart 0:1; run mmcboot;gpio clear 56; gpio clear 55; gpio clear 54; setenv mmcdev 1; setenv bootpart 1:1; run mmcboot;run failumsboot;
=> setenv umsmedia 0; gpio set 53; run findfdt; setenv mmcdev 0; setenv bootpart 0:1; run mmcboot;gpio clear 56; gpio clear 55; gpio clear 54; setenv mmcdev 1; setenv bootpart 1:1; run mmcboot
...bunch of stuff...
Running uname_boot ...
** Invalid partition 2 **
** Invalid partition 3 **
** Invalid partition 4 **
** Invalid partition 5 **
** Invalid partition 6 **
** Invalid partition 7 **
=> printenv
...bunch of stuff...
uname_r=4.1.4-ti-r15
...
=> set uname_r 4.1.6-ti-r16             //  or whatever you need it to be
=> set mmcpart 1                        //  Might be something different in your case
=> setenv bootpart ${mmcdev}:${mmcpart};
=> setenv mmcroot /dev/mmcblk${mmcdev}p${mmcpart} ro
=> run uname_boot

The full mmcboot command is this:

mmc dev ${mmcdev};
if mmc rescan;
	then gpio set 54;
	setenv bootpart ${mmcdev}:1;
	if test -e mmc ${bootpart} /etc/fstab;
		then setenv mmcpart 1;
	fi;
	echo Checking for: /uEnv.txt ...;
	if test -e mmc ${bootpart} /uEnv.txt;
		then if run loadbootenv;
			then gpio set 55;
			echo Loaded environment from ${bootenv};
			run importbootenv;
		fi;
		if test -n ${cape};
			then if test -e mmc ${bootpart} ${fdtdir}/${fdtbase}-${cape}.dtb;
				then setenv fdtfile ${fdtbase}-${cape}.dtb;
			fi;
			echo using: $fdtfile...;
		fi;
		echo Checking if uenvcmd is set ...;
		if test -n ${uenvcmd};
			then gpio set 56;
			echo Running uenvcmd ...;
			run uenvcmd;
		fi;
		echo Checking if client_ip is set ...;
		if test -n ${client_ip};
			then if test -n ${dtb};
				then setenv fdtfile ${dtb};
				echo using ${fdtfile} ...;
			fi;
			gpio set 56;
			if test -n ${uname_r};
				then echo Running nfsboot_uname_r ...;
				run nfsboot_uname_r;
			fi;
			echo Running nfsboot ...;
			run nfsboot;
		fi;
	fi;
	echo Checking for: /${script} ...;
	if test -e mmc ${bootpart} /${script};
		then gpio set 55;
		setenv scriptfile ${script};
		run loadbootscript;
		echo Loaded script from ${scriptfile};
		gpio set 56;
		run bootscript;
	fi;
	echo Checking for: /boot/${script} ...;
	if test -e mmc ${bootpart} /boot/${script};
		then gpio set 55;
		setenv scriptfile /boot/${script};
		run loadbootscript;
		echo Loaded script from ${scriptfile};
		gpio set 56;
		run bootscript;
	fi;
	echo Checking for: /boot/uEnv.txt ...;
	for i in 1 2 3 4 5 6 7 ;
		do setenv mmcpart ${i};
		setenv bootpart ${mmcdev}:${mmcpart};
		if test -e mmc ${bootpart} /boot/uEnv.txt;
			then gpio set 55;
			load mmc ${bootpart} ${loadaddr} /boot/uEnv.txt;
			env import -t ${loadaddr} ${filesize};
			echo Loaded environment from /boot/uEnv.txt;
			if test -n ${dtb};
				then setenv fdtfile ${dtb};
				echo Using: dtb=${fdtfile} ...;
			fi;
			echo Checking if uname_r is set in /boot/uEnv.txt...;
			if test -n ${uname_r};
				then gpio set 56;
				echo Running uname_boot ...;
				setenv mmcroot /dev/mmcblk${mmcdev}p${mmcpart} ro;
				run uname_boot;
			fi;
		fi;
	done;
fi;