linux-kernel - RTC subsystem and fractions of seconds

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Message-ID: <20070106022941.GA32125@ingate.macqel.be>
Date:	Sat, 6 Jan 2007 03:29:41 +0100
From:	Philippe De Muyter <phdm@...qel.be>
To:	linux-kernel@...r.kernel.org
Subject: RTC subsystem and fractions of seconds

Hi all,

A comment in driver/rtc/hctosys says :

    /* IMPORTANT: the RTC only stores whole seconds. It is arbitrary
     * whether it stores the most close value or the value with partial
     * seconds truncated. However, it is important that we use it to store
     * the truncated value. This is because otherwise it is necessary,
     * in an rtc sync function, to read both xtime.tv_sec and
     * xtime.tv_nsec. On some processors (i.e. ARM), an atomic read
     * of >32bits is not possible. So storing the most close value would
     * slow down the sync API. So here we have the truncated value and
     * the best guess is to add 0.5s.
     */

I work with ST m41t81 rtc's.  Those rtc's actually have a 1/100th of second
register.  Should the generic rtc interface not support that ? 

Philippe
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