linux-kernel - Re: why kernel implement "udelay" by cpu instructions?

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Message-ID: <4AEE63E2.9010003@redhat.com>
Date:	Sun, 01 Nov 2009 23:45:22 -0500
From:	Rik van Riel <riel@...hat.com>
To:	loody <miloody@...il.com>
CC:	linux-kernel@...r.kernel.org,
	Kernel Newbies <kernelnewbies@...linux.org>
Subject: Re: why kernel implement "udelay" by cpu instructions?

On 11/01/2009 10:13 PM, loody wrote:
> 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?

Because udelay is used in places where the kernel cannot
use other mechanisms, eg. because interrupts are blocked
or the current process cannot be scheduled out.

> the precision will be different if cpu runs faster or slower, right?

At bootup the kernel measures the delay loop speed of
each CPU.  CPU frequency scaling might make the loop
run slower at times, but that is okay because udelay
simply specifies a *minimum* delay.

This is true even on systems without frequency scaling,
because the udelay loop could be interrupted by an
interrupt, an NMI or by having the CPU trap into SMM
mode and execute code there.

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