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Date: Tue, 18 Mar 2008 19:43:26 -0700 From: Andrew Morton <akpm@...ux-foundation.org> To: john stultz <johnstul@...ibm.com> Cc: lkml <linux-kernel@...r.kernel.org>, Roman Zippel <zippel@...ux-m68k.org> Subject: Re: [PATCH 2/2] Introduce CLOCK_MONOTONIC_RAW On Tue, 18 Mar 2008 15:13:40 -0700 john stultz <johnstul@...ibm.com> wrote: > Here's my CLOCK_MONOTONIC_RAW, including some suggested changes from > Roman. > > Andrew, if there are not major objections, could you add it to your > 2.6.26 pending list? > > thanks > -john > > > In talking with Josip Loncaric, and his work on clock synchronization > (see btime.sf.net), he mentioned that for really close synchronization, > it is useful to have access to "hardware time", that is a notion of time > that is not in any way adjusted by the clock slewing done to keep close > time sync. > > Part of the issue is if we are using the kernel's ntp adjusted > representation of time in order to measure how we should correct time, > we can run into what Paul McKenney aptly described as "Painting a road > using the lines we're painting as the guide". > > I had been thinking of a similar problem, and was trying to come up with > a way to give users access to a purely hardware based time > representation that avoided users having to know the underlying > frequency and mask values needed to deal with the wide variety of > possible underlying hardware counters. > > My solution is to introduce CLOCK_MONOTONIC_RAW. This exposes a > nanosecond based time value, that increments starting at bootup and has > no frequency adjustments made to it what so ever. > > The time is accessed from userspace via the posix_clock_gettime() > syscall, passing CLOCK_MONOTONIC_RAW as the clock_id. > > This patch depends on the mult_orig patch, just sent a moment ago. alpha: kernel/built-in.o: In function `posix_get_monotonic_raw': : undefined reference to `getrawmonotonic' kernel/built-in.o: In function `posix_get_monotonic_raw': : undefined reference to `getrawmonotonic' presumably busted for all CONFIG_GENERIC_TIME=n architectures. Couldn't see a quick fix so I dropped it. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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