linux-kernel - Re: [PATCH] watchdog: s3c2410_wdt: Only register for cpufreq on CPU_FREQ

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Message-ID: <5293DBEF.3010401@roeck-us.net>
Date:	Mon, 25 Nov 2013 15:23:27 -0800
From:	Guenter Roeck <linux@...ck-us.net>
To:	Doug Anderson <dianders@...omium.org>,
	Wim Van Sebroeck <wim@...ana.be>
CC:	Leela Krishna Amudala <l.krishna@...sung.com>,
	Olof Johansson <olof@...om.net>,
	Tomasz Figa <tomasz.figa@...il.com>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Ben Dooks <ben-linux@...ff.org>,
	linux-arm-kernel@...ts.infradead.org,
	linux-samsung-soc@...r.kernel.org, linux-watchdog@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] watchdog: s3c2410_wdt: Only register for cpufreq on CPU_FREQ_S3C24XX

On 11/25/2013 02:55 PM, Doug Anderson wrote:
> On modern SoCs the watchdog timer is parented on a clock that doesn't
> change every time we have a cpufreq change.  That means we don't need
> to constantly adjust the watchdog timer, so avoid registering for and
> dealing with cpufreq transitions unless we've actually got
> CPU_FREQ_S3C24XX defined.
>
> Note that this is more than just an optimization.  The s3c2410
> watchdog driver actually pats the watchdog on every CPU frequency
> change.  On modern systems these happen many times per second (even in
> a system where "nothing" is happening).  That effectively makes any
> userspace watchdog program useless (the watchdog is constantly patted
> by the kernel).  If we need CPU_FREQ_S3C24XX defined on a
> multiplatform kernel we'll need to make sure that kernel supports
> common clock and change this to user common clock framework.
>
> Signed-off-by: Doug Anderson <dianders@...omium.org>
> ---
>   drivers/watchdog/s3c2410_wdt.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/watchdog/s3c2410_wdt.c b/drivers/watchdog/s3c2410_wdt.c
> index 7d8fd04..4980f84 100644
> --- a/drivers/watchdog/s3c2410_wdt.c
> +++ b/drivers/watchdog/s3c2410_wdt.c
> @@ -264,7 +264,7 @@ static irqreturn_t s3c2410wdt_irq(int irqno, void *param)
>   	return IRQ_HANDLED;
>   }
>
> -#ifdef CONFIG_CPU_FREQ
> +#ifdef CONFIG_CPU_FREQ_S3C24XX
>
Where is the CPU_FREQ_S3C24XX configuration option defined ? I don't see it
in the current upstream kernel, so it appears that this depends on some
out-of-tree changes.

Thanks,
Guenter

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