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Re: kernel sources?

To: "Kevin D. Kissell" <kevink@mips.com>
Subject: Re: kernel sources?
From: Ralf Baechle <ralf@oss.sgi.com>
Date: Sun, 16 Jan 2000 02:20:45 +0100
Cc: Ralf Baechle <ralf@oss.sgi.com>, Geert Uytterhoeven <geert@linux-m68k.org>, Linux/MIPS <linux@cthulhu.engr.sgi.com>
In-reply-to: <005e01bf5f7d$fc1cf490$0ceca8c0@satanas.mips.com>
References: <005e01bf5f7d$fc1cf490$0ceca8c0@satanas.mips.com>
Sender: owner-linuxmips@oss.sgi.com
On Sat, Jan 15, 2000 at 06:28:40PM +0100, Kevin D. Kissell wrote:

> As Ralf says, it's very accurate *if* you have an accurate picture of
> the CPU's clock frequency.  The usual mechanism of measuring
> the progress of the count register against elapsed time on a TOD
> clock has been known to be inaccurate on some platforms due
> to skew between the visibility to software and the actual timing
> event, so be careful.

The only machine where we actually calibrate the CPU counter is the Indy
where due to a hardware bug we cannot use the external timers as source
of interrupts.

In fact the current Linux kernel would even panic if it ever would find
an Indy working as per spec ;-)

We could improve that meassurement by rounding to the nearest crystal
known to be used on IP22 CPU modules plus maybe add an eventual user /
xntpd suplied fudge factor for deviations of the exact crystal frequency.

  Ralf

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