linux-kernel - Re: [PATCH] clocksource: add enable() and disable() callbacks

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Date:	Fri, 12 Dec 2008 07:36:45 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Magnus Damm <magnus.damm@...il.com>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	johnstul@...ibm.com, lethal@...ux-sh.org, tglx@...utronix.de,
	mingo@...hat.com
Subject: Re: [PATCH] clocksource: add enable() and disable() callbacks


* Magnus Damm <magnus.damm@...il.com> wrote:

> +static inline int clocksource_enable(struct clocksource *cs)
> +{
> +	return cs->enable ? cs->enable(cs) : 0;
> +}

> +static inline void clocksource_disable(struct clocksource *cs)
> +{
> +	if (cs->disable)
> +		cs->disable(cs);
> +}

why have the two different styles? The first one should be:

	if (cs->enable)
		return cs->enable(cs);
	return 0;

> @@ -193,11 +193,16 @@ static void change_clocksource(void)
>  
>  	clocksource_forward_now();
>  
> -	new->raw_time = clock->raw_time;
> +	if (clocksource_enable(new))
> +		return;

that looks fragile to me: if the enable fails we'll return silently, 
while change_clocksource() assumes that things went fine. At least put a 
WARN_ON_ONCE() in there.

also, why does it have to fail? If a clocksource cannot be enabled it 
should not be offered as a clocksource.

> +	clocksource_disable(old);

i do agree with the core purpose here, to allow lowlevel code to 
deactivate unused clocksources.

John, Thomas, what's your take on this?

	Ingo
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