Re: using get_cycles() to get the real elapsed time

["Jose R. Santos" <jrs at us dot ibm dot com>]

Gui,Jian wrote:
> Roland McGrath writes:
> > sched_clock is used by the scheduler and POSIX CPU clocks/timers, which
> > have the same requirements of being accurate and cheap.  [...]
>
> Thanks. sched_clock is a good alternate for do_gettimeofday in LKET.
> But when I read 2.6.17.7 code, I found it still lacks the code to
> handle the CPU frequency variation in ppc64 arch.
>
> In 2.6.17.7/ppc64, it uses
>    mulhdu(get_tb(), tb_to_ns_scale) << tb_to_ns_shift
> to return current time in nanosec units.
>
> But in ppc64 code, tb_to_ns_scale and tb_to_ns_shift are set only once
> in time_init() and will be constants after that, while in i386 code
> similar scaling factors will be updated each time the CPU frequency
> is changed.
>
> Is this a ppc64 bug? or Power cpu doesn't have the cpu frequency 
> scaling feature? Thanks.
>   

Power5 and below does not support frequency scaling.  I think the newer 
versions of PowerPC 970 do support frequency scaling but I'm not sure if 
that feature is supported on Linux at the moment.  Its been a while 
since I've touched a 970 chip, but I'll try to find out.

-JRS