Larry Baker baker at usgs.gov
Tue Jun 17 00:14:59 CEST 2014

Wolfgang, et al.,

A recent situation has awakened me to a a behavior of U-Boot that maybe should be changed.

I am experimenting with a BeagleBone Black (BBB) single-board computer.  Occasionally when I perform a reboot, the BBB never finishes booting Linux.  When I happen to have a console cable connected, I see the U-Boot prompt.  This explains why the BBB has not booted Linux.  But it is a mystery why U-Boot is waiting for a command when I did not request interrupting the autoboot sequence.

I found many others reporting the same experience -- and no solution.  Until I found Andrew Glen's 10/28/2013 post at https://groups.google.com/forum/#!topic/beagleboard/aXv6An1xfqI about U-Boot hangs likely due to noise on the UART0 port.  The solution he applied was to rebuild U-Boot to require specific text -- "uboot" -- to interrupt the autoboot sequence.  (BBB have no flash, so there is no opportunity to permanently alter any U-Boot environment variables, if this is settable.)

It occurs to me that this might be a more common occurrence on any number of circuit boards.

I have used many ARM single-board computers that have U-Boot.  As far as I recall, they all prompt "Hit any key to stop autoboot:".  It is probably not such a good idea that ANY character can interrupt the U-Boot autoboot sequence.  Perhaps the default should be to require an unusual character, like F2 or DEL for BIOS.  Or a multi-character sequence within a short period of time, like the "+++" to enter command mode on the old Hayes modems.

As I said, this awakening only occurred to me because of my recent experience with boot failures.  My realization is that U-Boot could probably do a little better job of protecting our (likely unattended, embedded) systems with a minimum of change to the code.  I plan to make it my practice to #define CONFIG_AUTOBOOT_DELAY_STR.  I'm thinking that should probably be the default.

Thank you for your valuable contributions (U-Boot and more).

Best regards,

Larry Baker
US Geological Survey
baker at usgs.gov

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