why kernel implement "udelay" by cpu instructions?

From: loody
Date: Sun Nov 01 2009 - 22:13:58 EST

Dear all:
I find the kernel use cpu instruction to implement the udelay function
as keeping decrease a big counter by 1.

If I search the right place in kernel, why kernel does so?
the precision will be different if cpu runs faster or slower, right?
appreciate your help,
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