[U-Boot] a few questions about saving bootcount in the environment

[Wolfgang Denk]

Dear Tom,

In message <20160725135754.GN14698 at bill-the-cat> you wrote:
> 
> > Right, it is only a last resort when you cannot find any better place
> > to storeit (in a hardware register that survives resets).
> 
> That's not strictly true.  One of the things I noticed recently is that
> Mender uses bootcount, in environment, as a least common denominator.
> And thrown in a file in a filesystem, in so far as you trust the
> underlying black box to be good about reads/writes and wear levelling,
> it's robust enough (for certain values of robust and enough).  We're
> dipping into one of those areas where experts have varying opinions on
> what's good enough, hence all the qualifiers.  But it is a useful
> option.  And neatly circumvents the need for a "driver" to clear the
> count too.

Agreed.  Let me rephrase the warning, then: it is always a good idea
to minimize the number of writes to the environment, especially when
done automagically.  One should at least avoid to re-write it on every
boot, especially when storage is some (flash based) storage device
with a limited number of erase/write cycles.  And even on other stoage
each write access includes the risk of errors.

Best regards,

Wolfgang Denk

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