From: Ingo Molnar <mingo@kernel.org>
To: John Stultz <john.stultz@linaro.org>
Cc: lkml <linux-kernel@vger.kernel.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Andrew Morton <akpm@linux-foundation.org>,
Richard Cochran <richardcochran@gmail.com>,
Prarit Bhargava <prarit@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 1/2] jiffies: Allow CLOCK_TICK_RATE to be undefined
Date: Thu, 19 Jul 2012 11:37:04 +0200 [thread overview]
Message-ID: <20120719093704.GB27086@gmail.com> (raw)
In-Reply-To: <1342660753-10382-2-git-send-email-john.stultz@linaro.org>
* John Stultz <john.stultz@linaro.org> wrote:
> From: Catalin Marinas <catalin.marinas@arm.com>
>
> CLOCK_TICK_RATE is a legacy constant that defines the timer
> device's granularity. On hardware with particularly coarse
> granularity, this constant is used to reduce accumulated
> time error when using jiffies as a clocksource, by calculating
> the hardware's actual tick length rather then just assuming
> it is 1sec/HZ.
>
> However, for the most part this is unnecessary, as most modern
> systems don't use jiffies for their clocksource, and their
> tick device is sufficiently fine grained to avoid major error.
>
> Thus, this patch allows an architecture to not define
> CLOCK_TICK_RATE, in which case ACTHZ defaults to (HZ << 8).
>
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> Acked-by: Arnd Bergmann <arnd@arndb.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Richard Cochran <richardcochran@gmail.com>
> Cc: Prarit Bhargava <prarit@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> [jstultz: commit log tweaks]
> Signed-off-by: John Stultz <john.stultz@linaro.org>
> ---
> include/linux/jiffies.h | 10 +++++++---
> 1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
> index 265e2c3..a2134be 100644
> --- a/include/linux/jiffies.h
> +++ b/include/linux/jiffies.h
> @@ -39,9 +39,6 @@
> # error Invalid value of HZ.
> #endif
>
> -/* LATCH is used in the interval timer and ftape setup. */
> -#define LATCH ((CLOCK_TICK_RATE + HZ/2) / HZ) /* For divider */
> -
> /* Suppose we want to divide two numbers NOM and DEN: NOM/DEN, then we can
> * improve accuracy by shifting LSH bits, hence calculating:
> * (NOM << LSH) / DEN
> @@ -54,8 +51,15 @@
> #define SH_DIV(NOM,DEN,LSH) ( (((NOM) / (DEN)) << (LSH)) \
> + ((((NOM) % (DEN)) << (LSH)) + (DEN) / 2) / (DEN))
>
> +#ifdef CLOCK_TICK_RATE
> +/* LATCH is used in the interval timer and ftape setup. */
> +#define LATCH ((CLOCK_TICK_RATE + HZ/2) / HZ) /* For divider */
> +
> /* HZ is the requested value. ACTHZ is actual HZ ("<< 8" is for accuracy) */
> #define ACTHZ (SH_DIV (CLOCK_TICK_RATE, LATCH, 8))
> +#else
> +#define ACTHZ (HZ << 8)
> +#endif
The ACTHZ naming ugliness slipped past me. 'ACT' can mean so
many things - please improve it to something more obvious, like
'REAL_HZ' or 'KERNEL_HZ'.
Also, we tend to write such #if/#else/#endif patterns as:
#if FOO
# define BAR
#else
# define BAZ
#endif
Thanks,
Ingo
next prev parent reply other threads:[~2012-07-19 9:37 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-07-19 1:19 [PATCH 0/2] Additional time changes for 3.6 John Stultz
2012-07-19 1:19 ` [PATCH 1/2] jiffies: Allow CLOCK_TICK_RATE to be undefined John Stultz
2012-07-19 9:37 ` Ingo Molnar [this message]
2012-07-23 19:37 ` John Stultz
2012-07-26 12:56 ` Ingo Molnar
2012-07-19 1:19 ` [PATCH 2/2] time: Cleanup offs_real/wall_to_mono and offs_boot/total_sleep_time updates John Stultz
2012-07-19 9:33 ` Ingo Molnar
2012-07-23 19:31 ` John Stultz
2012-07-26 12:57 ` Ingo Molnar
2012-07-19 12:37 ` Prarit Bhargava
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