Clock Source in hrtimer

Clock Source in hrtimer

[wilbur.chan]

I am planning to use  linux 2.6.24  with hrtimer enabled and with
CONFIG_NO_HZ  on mips xlr 732.


As we know, a   monotomic increasing Clock Source is required to
support hrtimer,  whose cycles could be retrieved  from
clocksource->read function.

However  on  xlr 732 ,there is only a 32 bits counter register, which
would overflow in 4s ( 2^32 / 1GHZ = 4).

How to solve this ?


Thanks in advance

Re: Clock Source in hrtimer

[wu zhangjin]

On 8/15/10, wilbur.chan <wilbur512@gmail.com> wrote:
> I am planning to use  linux 2.6.24  with hrtimer enabled and with
> CONFIG_NO_HZ  on mips xlr 732.
>
>
> As we know, a   monotomic increasing Clock Source is required to
> support hrtimer,  whose cycles could be retrieved  from
> clocksource->read function.
>
> However  on  xlr 732 ,there is only a 32 bits counter register, which
> would overflow in 4s ( 2^32 / 1GHZ = 4).
>
> How to solve this ?
>

don't worry about it, the timekeeper solves it:

kernel/time/timekeeping.c

The r4k timer in most of the MIPS variants also only has a 32bits
counter register, no problem with it:

arch/mips/kernel/csrc-r4k.c
arch/mips/kernel/cevt-r4k.c

does  xlr 732 also use such a timer (with MIPS count & compare
registers of coprocessor0)?

If you need to get the time with high resolution, please use:
getnstimeofday(), this function will return a linear time with the
help of timekeeper(timekeeping_get_ns).

Regards,
Wu Zhangjin

Re: Clock Source in hrtimer

[wilbur.chan]

Hi wu,

2010/8/15 wu zhangjin <wuzhangjin@gmail.com>:

>
> don't worry about it, the timekeeper solves it:
>
> kernel/time/timekeeping.c
>
> The r4k timer in most of the MIPS variants also only has a 32bits
> counter register, no problem with it:
>
> arch/mips/kernel/csrc-r4k.c
> arch/mips/kernel/cevt-r4k.c
>
> does  xlr 732 also use such a timer (with MIPS count & compare
> registers of coprocessor0)?
>

yes, xlr732  use these two registers to generate timer interrupt,
with a frequency of 1GHZ



> If you need to get the time with high resolution, please use:
> getnstimeofday(), this function will return a linear time with the
> help of timekeeper(timekeeping_get_ns).
>


1)  So hrtimer is based on  xtime,

If CONFIG_NO_HZ is set , would xtime be updated on time and correctly ?


2) some question about __get_nsec_offset :

               /* read clocksource: */
	cycle_now = clocksource_read(clock);

	/* calculate the delta since the last update_wall_time: */
	cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;



if   cycle_now  is smaller than clock->cycle_last , say , cycle_now =
3 ,    cycle->cycle_last = 2^32-1 ,  then

cycle_delta  = 3 - (2^32-1)  =  ?


> Regards,
> Wu Zhangjin
>