Re: [PATCH] MIPS: Octeon: Use non-overflowing arithmetic in sched_clock

[David Daney <ddaney@caviumnetworks.com>]

Ralf Baechle wrote:
> On Fri, Jan 08, 2010 at 02:47:36PM -0800, David Daney wrote:
> 
>> With typical mult and shift values, the calculation for Octeon's
>> sched_clock overflows when using 64-bit arithmetic.  Use 128-bit
>> calculations instead.
> 
> Applied though my first thought whenever I see extended precission math
> is gross - maybe we're going to find a better solution.  Hopefully!
> 
>   Ralf

I did have some apprehension myself.  However consider:

* For an 800MHz core clock, clocksource_set_clock() generates a shift 
value of 31.  This leads to overflow of 64-bit arithmetic approximately 
every 8 seconds.  This specific case could be reduced to a 2 bit shift, 
resulting in time to overflow of more than 100 years.  But one can 
imagine clock rates that would require large shifts.

* We need to return a 64-bit clock value, this will overflow in about 
500 years, Unless we are very careful with our arithmetic, we risk 
overflow in unacceptable short time periods.

* This is octeon specific and the 128-bit operation is cheap.  Probably 
cheaper than accounting for overflows in 64-bit arithmetic.

David Daney