linux-kernel - clocksource.c "margin"

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Message-ID: <4ED6C5CE.2030105@gmail.com>
Date:	Wed, 30 Nov 2011 19:09:50 -0500
From:	"Yang Honggang(Joseph)" <ganggexiongqi@...il.com>
To:	johnstul@...ibm.com
CC:	linux-kernel@...r.kernel.org
Subject: clocksource.c "margin"

Hi John Stultz,
I'm very sorry to trouble you. A problem puzzled me when I reading your 
code in kernel/time/clocksource.c.
In the following functions, your said in the comments that >>5 was used 
to leave a margin of 12.5%.
I don't know how to get the result. I think is should be >> 3 in order 
to leave a margin of 12.5%.
The operation of >> 5 only left 3.125% margin.

__clocksource_updatefreq_scale
...
sec = (cs->mask - (cs->mask >> 5));
...
clocksource_max_deferment
...
     /*
      * To ensure that the clocksource does not wrap whilst we are idle,
      * limit the time the clocksource can be deferred by 12.5%. Please
      * note a margin of 12.5% is used because this can be computed with
      * a shift, versus say 10% which would require division.
      */
     return max_nsecs - (max_nsecs >> 5);
...

Best regards,
Joseph
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