linux-kernel - Re: [RFC][PATCH 3/3] Try to convert non-trivial clocksources to clocksource_register

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Message-ID: <20100511082050.4b11e5f3@mschwide.boeblingen.de.ibm.com>
Date:	Tue, 11 May 2010 08:20:50 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	john stultz <johnstul@...ibm.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mundt <lethal@...ux-sh.org>,
	Jim Cromie <jim.cromie@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC][PATCH 3/3] Try to convert non-trivial clocksources to
 clocksource_register_hz

On Mon, 10 May 2010 12:19:10 -0700
john stultz <johnstul@...ibm.com> wrote:

> On Mon, 2010-05-10 at 17:53 +0200, Martin Schwidefsky wrote:
> > On Fri, 30 Apr 2010 18:36:25 -0700 John Stultz <johnstul@...ibm.com> wrote:
> > The s390 conversion to clocksource_register_khz looks good - at least
> > the kilo hertz value is correct. Bit 2^12 of the TOD value are
> > microseconds, which makes one millisecond (1000<<12). 
> 
> Thanks for the review!
> 
> However, is there a more straightforward frequency value that we could
> use? Is 4096000000 hz really the correct value or is there something
> more precise?

>From the principles of operation manual 4-37:
The TOD clock nominally is incremented by adding a one in bit position
51 every microsecond. In models having a higher or lower resolution, a
different bit position is incremented at such a frequency that the rate
of advancing the clock is the same as if a one were added in bit
position 51 every microsecond. The resolution of the TOD clock is such
that the incrementing rate is comparable to the instruction-execution
rate of the model.

Remember that the bit numbering in IBM manuals starts with 0 for the
most significant bit (bit 51 == 2^12 for a 64 bit value). The quality
of the TOD clock is very good, with ETR or STP it will be drifted to an
external time source with a precision in the range of microseconds
(you could think of it as NTP in hardware / microcode).
If there is a clocksource register function that takes a microsecond
value we could improve the precision.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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