[U-Boot] [RFC][Timer API] Revised Specification - Implementation details

[Simon Glass]

On Fri, May 27, 2011 at 12:40 AM, Wolfgang Denk <wd at denx.de> wrote:
> Dear Simon Glass,
>
> In message <BANLkTinxp1wuA9+_EvC0ppK+7Uj89UkN-g at mail.gmail.com> you wrote:
>>
>> >> I guess you cannot, at least not in general.  In worst case that would
>> >> mean we have to process 1e6 interrupts per second, which leaves little
>> >> time for anything useful.
>>
>> Sorry Wolfgang I don't really understand this. We would only process
>> when we read it, and then hopefully only a simple multiple or shift,
>> after compiler optimizations kick in. Probably I am just missing what
>> you are saying.
>
> You assume that there is a counter register that can be read.  This
> may not be the case. You may have just a timer which fires an
> interrupt every X time units, so you can implement a counter in the
> ISR.  This is for examole how the tick is implemented on PPC now:  we
> get an interrupt every millisecond and increment a counter then.
>
> For a microsecond tick you need in such a setup one million
> interrupts per second.

I thought PPC had a performance counter? But if not, then it will just
have to live with a millisecond timer.

>
>> I hope we can avoid integer division in the microsecond case. Someone
>> stated that time delays are the main use for the timer, but some of us
>> have performance-monitoring plans.
>>
>> Re the atomicity of handling 64-bit numbers, how about just
>> disable/enable interrupts around this? I think 64-bit is overkill but
>> at least it is simple, and prefer a u64 to a struct { u32 lo, hi; }.
>
> Enabling and disabling interrupts is not exactly performance-neutral
> either.

Unfortunately I know very little about PPC but at least on ARM UP this
is not expensive. We can compare that against just reading the 64-bit
counter until it doesn't change...

> Best regards,
>
> Wolfgang Denk

Regards,
Simon

>
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>