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

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Date:	Fri, 12 Dec 2008 16:18:38 +0900
From:	"Magnus Damm" <magnus.damm@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
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

Hi Ingo,

On Fri, Dec 12, 2008 at 3:36 PM, Ingo Molnar <mingo@...e.hu> wrote:
> * 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;

Sure, that's fine too.

>> @@ -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.

Yeah, John and I discussed this before. What we really want it to move
the failing clocksource out of the list of available clocksources.
That type of change is pretty intrusive though, and I rather see it as
a separate topic.

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

Right. I guess most clocksource drivers for embedded platforms will
tie in the clock framework and use clk_enable() and clk_disable().
clk_enable() returns an int.

>> +     clocksource_disable(old);
>
> i do agree with the core purpose here, to allow lowlevel code to
> deactivate unused clocksources.

That's good! I hope we can sort out the details then!

Cheers,

/ magnus
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