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

[Wolfgang Denk]

Dear "J. William Campbell",

In message <4DDC31EB.6040609 at comcast.net> you wrote:
...
> A tick is defined as the smallest increment of system time as measured by a
> computer system (seehttp://en.wikipedia.org/wiki/System_time):
> 
> 	System time is measured by a system clock, which is typically
> 	implemented as a simple count of the number of ticks that have
> 	transpired since some arbitrary starting date, called the
> 	epoch.
> 
> Unfortunately, this definition is obsolete, and has been for quite some 

Do you have any proof for such a claim?

> years.  When computers had a single timer, the above definition worked, 
> but it no longer does, as many (most?) computers have several hardware 
> timers. A "tick" today is the time increment of any particular timer of 
> a computer system. So, when one writes a function called get_ticks on a 
> PPC, does one mean read the decrementer, or does one read the RTC or 
> does one read the TB register(s) A similar situation exists on the 
> Blackfin BF531/2/3, that has a preformance counter, a real-time clock, 
> and three programmable timers. Which tick do you want? For each u-boot 

Please re-read the definition.  At least as far as U-Boot and Linux
are concerned, there is only a single clock source used to implement
the _system_time_.  And I doubt that other OS do different.

> implementation, we can pick one timer as the "master" timer, but it may 
> not be the one with the most rapid tick rate. It may be the one with the 
> most USEFUL tick rate for get_timer. If you take the above definition at 
> face value, only the fastest counter value has ticks, and all other 
> counters time increments are not ticks. If they are not ticks, what are 
> they?

Clocks, timers?


Best regards,

Wolfgang Denk

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