Re: How to measure time accurately.

[Peter Chubb <peterc@gelato.unsw.edu.au>]

>>>>> "Chris" == Chris Friesen <cfriesen@nortel.com> writes:

Chris> krishna wrote:
>> Hi All,
>> 
>> Can any one tell me how to measure time accurately for a block of C
>> code in device drivers.  For example, If I want to measure the time
>> duration of firmware download.

Chris> Most cpus have some way of getting at a counter or decrementer
Chris> of various frequencies.  Usually it requires low-level hardware
Chris> knowledge and often it needs assembly code.

As a device driver is inside the linux kernel (unless you're writein a
user-mode device driver :-)) you can use the getcycles() macro that's
defined for most architectures.  It provides a snapshot of the
cycle-counter.

Caveats:
	1.  If you're running with power management, the  cycle
	    counter ticks at a  variable rate.
	2.  If you're on a multiprocessor, the cycle counters of
	    different processors need not be synchronised.
-- 
Dr Peter Chubb  http://www.gelato.unsw.edu.au  peterc AT gelato.unsw.edu.au
The technical we do immediately,  the political takes *forever*
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