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Date: Tue, 21 Oct 2014 12:19:55 +0200 (CEST) From: Thomas Gleixner <tglx@...utronix.de> To: "pang.xunlei" <pang.xunlei@...aro.org> cc: linux-kernel@...r.kernel.org, John Stultz <john.stultz@...aro.org> Subject: Re: [PATCH 1/2] time: Fix NTP adjustment mult overflow. On Thu, 9 Oct 2014, pang.xunlei wrote: First of all: Please use proper line breaks in the changelog. > The mult memember of struct clocksource should always be a large u32 > number when calculated through __clocksource_updatefreq_scale(). The > value of (cs->mult+cs->maxadj) may have a chance to reach very near > 0xFFFFFFFF. And what's the actual problem with reaching a value near 0xFFFFFFFF? > For instance, 555MHz oscillator: cs->mult is 0xE6A17102, > cs->maxadj is 0x195E8EFD, cs->mult+cs->maxadj is 0xFFFFFFFF. Such > oscillators would probably exist on some processors like MIPS which > use CP0 compare/count CPU clock as the clock source. Again, what's the problem? > Clocksource might encounter large frequency adjustment due to the > hardware unstability, environment temperature, software deviation, > NTP algorithm accuracy, etc. When NTP slewes the clock, kernel goes > through update_wall_time()->...->timekeeping_apply_adjustment(): > tk->tkr.mult += mult_adj; Unfortunately, tk->tkr.mult may overflow > after this operation, though such cases are next to impossible to > happen in practice. So you adding this just for correctness reasons, not because you observed the problem in practice? Thanks, tglx -- 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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