[U-Boot] [RFC] Review of U-Boot timer API

[Wolfgang Denk <wd@denx.de>]

Dear Graeme Russ,

In message <BANLkTi==emvE+2Xu-R=hzh3TgFJXs9DnmA@mail.gmail.com> you wrote:
> 
> Well, I think we can leave NTP out of the picture for U-Boot - What I am
> really getting at is are the time bases for udelay() and get_timer() the
> same, and are they always the most accurate which is readily available
> to U-Boot.

Please assume "no" to both questions.

Even more important: stop worrying about this.  The implementation is
noit supposed to make ANY such assumptions.

> I mean udelay() clocked by a tick counter and get_timer() clocked by
> an interrupt versus both being clocked by the same phyiscal tick counter
> (even though both are clocked by the same on-board xtal)

You really should not care about those low-level details.  They can
and shall be ignored.

> > There are counters of many different sizes around. You cannot squeeze
> > a bigger counter into a smaller data ubit, but you can easily stick
> > smaller data into a bigger container. So it seems most natural to me
> > to chose that data type that fits all, i. e. a bigger one.
> 
> Putting the smaller counter into a larger bit space causes problems because
> it wraps to "All 0's" before it gets to "All 1's" - This makes a simple
> unsigned 'current - last' calculation fail at the rollover point. The same
> problem does not occur if you truncate a larger counter into a smaller bit
> space

But then you may significantly reduce the available time-span; you may
see the counter wrapping around every few seconds. You will not be
able to implement - say - a 10 second delay because you cannot know if
there were 1, 2, or more rollovers inbetween.


I see two options:

1) either we can have some signal when the timer wrapes around (like
   an interrupt); in this case it does not matter how big the counter
   itself is, we just increment our own local 64 bit timestamp.

2) Alternatively, I could imagine we use a 64 bit container plus a
   mask which specifiec how many bits are actually used - then we can 
   accommodate counters with any numbers of valid bits.



Best regards,

Wolfgang Denk

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