Update ... feeling like I'm losing my mind now.

fw_env.config must be sensitive to spaces or tabs or something.

I messed around with it some more and it magically started working with my imx8mm-evk board with fw_env.config containing:

/dev/mmcblk1  0x400000    0x1000

I don't know why that didn't work before ... I tried all kinds of things to get this to work and really can't explain why it just started working.  To test, I used fw_setenv to set a variable and rebooted and stopped in u-boot and did printenv and sure enough the variable I set is there.

Regards,

Brian

So I think I figured out what was going on.  I flashed sdcard with fresh Yocto build.  Apparently the default u-boot environment is in ram and not in the eMMC until you stop in u-boot and do a saveenv ... then when you boot into Linux fw_printenv works.  Prior to doing that, fw_printenv will return the error I previously mentioned.

Hopefully this will help the next person thinking they are losing their mind messing with creating a fw_env.config file.

Regards,

Brian