[U-Boot] Saving environment variables in MMC

[Detlev Zundel]

Hi Nitin,

>> It is rather common to write to the U-Boot environment in projects
>> for example to switch to a new set of kernel+file system after an
>> update from within linux for the next boot.
>> 
> My use case is exactly same, to switch to a new set of kernel+fs after
> an update for the next boot.
>
> I also have another usecase of updating the env variable 'bootargs' if
> required in the field. So this use-case combined with fw_env, what is
> your feedback?

It is doable of course.  Maybe if I did not mention it before, I advise
using a redundant environment for such procedures so that even a
powerloss during this upgrade will not brick the device.

> Could you give me some pointers on upgrading u-boot itself, but I
> don't have a spare partition for that. I would have to replace working
> copy itself?

I would not recommend upgrading U-Boot in the field.  As it is not
possible to build in redundancy for U-Boot (on most systems I know),
there is always the possibility to kill the device with such an update.

> I would wanted to have more info(in addition to what I have
> implemented) regarding the failsafe upgrade mechanisms for
> embedded-linux apps and kernel? Could you please point me to right
> forums regarding this. I understand that this is not specific to
> u-boot, but just give me some pointers.

I'm sorry that I cannot point you to a ready to use recipe here, as this
really depends on your strategy regarding upgrades, i.e. will you do the
upgrade from within Linux? (judging by your questions, you will...)  Do
you have enough ressources to keep two self-contained "program images"
(at least kernel+dtb+rootfs) so you can always update "the other half"?
If not, you will probably want to build a non-upgradeable fallback
system which is only capable to update "the other part".

As you see, solving your problem really requires you to define your
problem more rigorously first.

In order to protoect against interrupts during the update, you may very
well want to have a watchdog on your system and use the "bootcount"
(grep the documentation for it) feature of U-Boot to detect failing boot
attempts.

I hope this is enough to get you started.

Cheers
  Detlev

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