From: tglx@linutronix.de (Thomas Gleixner)
To: linux-arm-kernel@lists.infradead.org
Subject: CLOCK_TICK_RATE, was: Re: [PATCH V4 3/4] ARM: Xilinx: base header files and assembly macros
Date: Mon, 21 Feb 2011 22:51:00 +0100 (CET) [thread overview]
Message-ID: <alpine.LFD.2.00.1102212250290.2701@localhost6.localdomain6> (raw)
In-Reply-To: <201102212208.10231.arnd@arndb.de>
On Mon, 21 Feb 2011, Arnd Bergmann wrote:
> On Monday 21 February 2011, Russell King - ARM Linux wrote:
> > Eg, LOW_RES_NSEC is exported to userspace via the posix clocks interface.
> >
> > NSEC_PER_SEC and TICK_NSEC are used for cmos clock updates, so probably
> > don't matter too much there. TICK_NSEC is also used by the scheduler,
> > time conversions (timespec/timeval to/from jiffies) and profiling code.
> >
> > NSEC_PER_JIFFY is used by the jiffy clocksource code, which only matters
> > if you don't have your own clocksource.
> >
> > So, I feel very uneasy about saying that CLOCK_TICK_RATE doesn't matter
> > anymore given all the places which reference something that's derived
> > from it.
>
> All the calculations based off of CLOCK_TICK_RATE are derived from ACTHZ,
> which is either the correct value based on the underlying HW timer tick,
> or slightly off, when either the HW tick or the value of CLOCK_TICK_RATE
> is not a true multiple of HZ.
>
> In fact, I'm pretty sure that it's off on a lot of machines:
>
> arch/frv/include/asm/timex.h:#define CLOCK_TICK_RATE 1193180 /* Underlying HZ */
> arch/m68k/include/asm/timex.h:#define CLOCK_TICK_RATE 1193180 /* Underlying HZ */
> arch/mips/include/asm/timex.h:#define CLOCK_TICK_RATE 1193182
> arch/parisc/include/asm/timex.h:#define CLOCK_TICK_RATE 1193180 /* Underlying HZ */
> arch/s390/include/asm/timex.h:#define CLOCK_TICK_RATE 1193180 /* Underlying HZ */
> arch/sh/include/asm/timex.h:#define CLOCK_TICK_RATE 1193180
> arch/x86/include/asm/timex.h:#define CLOCK_TICK_RATE PIT_TICK_RATE
> arch/xtensa/include/asm/timex.h:#define CLOCK_TICK_RATE 1193180 /* (everyone is using this value) */
>
> None of these is actually using a PC-style PIT these days, the just copied the
> definition blindly from old i386. I think a simple
>
> #define ACTHZ (HZ << 8)
>
> would fix more than it can break, and most likely nobody would ever notice
> the difference. If we do that, CLOCK_TICK_RATE becomes unused.
Indeed.
tglx
next prev parent reply other threads:[~2011-02-21 21:51 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <1298052881-14591-1-git-send-email-john.linn@xilinx.com>
2011-02-18 18:14 ` [PATCH V4 1/4] ARM: Xilinx: Adding Xilinx board support John Linn
2011-02-28 11:06 ` Jamie Iles
2011-02-28 14:13 ` John Linn
[not found] ` <1298052881-14591-2-git-send-email-john.linn@xilinx.com>
2011-02-18 18:14 ` [PATCH V4 2/4] ARM: Xilinx: Adding timer support to the platform John Linn
2011-02-28 11:11 ` Jamie Iles
2011-02-28 14:14 ` John Linn
[not found] ` <1298052881-14591-3-git-send-email-john.linn@xilinx.com>
2011-02-18 18:14 ` [PATCH V4 3/4] ARM: Xilinx: base header files and assembly macros John Linn
2011-02-20 21:37 ` Arnd Bergmann
2011-02-21 0:18 ` John Linn
2011-02-21 8:36 ` Arnd Bergmann
2011-02-21 14:36 ` John Linn
2011-02-21 14:48 ` Arnd Bergmann
2011-02-21 15:04 ` John Linn
2011-02-21 15:17 ` Russell King - ARM Linux
2011-02-21 21:08 ` CLOCK_TICK_RATE, was: " Arnd Bergmann
2011-02-21 21:51 ` Thomas Gleixner [this message]
2011-02-28 11:18 ` Jamie Iles
2011-02-28 14:20 ` John Linn
2011-02-28 15:01 ` Jamie Iles
2011-02-28 11:20 ` Russell King - ARM Linux
2011-02-28 14:22 ` [PATCH V4 3/4] ARM: Xilinx: base header files and assemblymacros John Linn
[not found] ` <1298052881-14591-4-git-send-email-john.linn@xilinx.com>
2011-02-18 18:14 ` [PATCH V4 4/4] ARM: Xilinx: Adding Xilinx platform infrastructure support John Linn
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