Dear Reinhard Meyer,
In message <4d3c9bfc.1010...@emk-elektronik.de> you wrote:
>
> >> get_timer() returns a monotonous upward counting time stamp with a
> >> resolution of milliseconds. After reaching ULONG_MAX the timer wraps
> >> around to 0.
>
> Exactly that wrap makes the situation so complicated, since the simple code
> u32 get_timer(void)
> {
> return (ticks * 1000ULL) / tickspersec;
> }
> won't do that wrap.
Do you have a better suggestion?
> >> The get_timer() implementation may be interrupt based and is only
> >> available after relocation.
>
> Currently it is used before relocation in some places, I think I have
> seen it in NAND drivers... That would have to be changed then.
Indeed. It is unreliable or even broken now.
> This is already implemented functionally very closely (apart from factoring
> and the
> get_timer(void) change) to this in AT91, the only (academic) hitch is that it
> will
> burp a few billion years after each reset :)
> What bothers me is the need for 64 bit mul/div in each loop iteration, for
> CPUs without
> hardware for that this might slow down data transfer loops of the style
>
> u32 start_time = get_timer();
> do {
> if ("data_ready")
> /* transfer a byte */
> if (get_timer() - start_time > timeout)
> /* fail and exit loop */
> } while (--"bytestodo" > 0);
>
> since get_timer() will be somewhat like:
>
> return (tick * 1000ULL) / tickspersec;
>
> As I stated before, tickspersec is a variable in, for example, AT91. So the
> expression cannot be optimized by the compiler.
I don't think this is the only way to implement this. How does Linux
derive time info from jiffies?
Best regards,
Wolfgang Denk
--
DENX Software Engineering GmbH, MD: Wolfgang Denk & Detlev Zundel
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