linux-kernel - Re: re-adding clocksource_get

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Message-ID: <4BC404C3.2090102@st.com>
Date:	Tue, 13 Apr 2010 07:44:35 +0200
From:	Giuseppe CAVALLARO <peppe.cavallaro@...com>
To:	john stultz <johnstul@...ibm.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: re-adding clocksource_get_next ?

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On 04/12/2010 08:50 PM, john stultz wrote:
> On Mon, 2010-04-12 at 08:02 +0200, Giuseppe CAVALLARO wrote:
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>> Hello John,
>>
>> john stultz wrote:
>>> On Thu, Apr 8, 2010 at 11:53 PM, Giuseppe CAVALLARO
>>> <peppe.cavallaro@...com> wrote:
>>>> Hello!
>>>> this function was removed in the past but I've not clear all the details.
>>>> While testing LLTng, I wrote the patch, in attachment, that re-adds it
>>>> again.
>>>> The clocksource_get_next returns the first clock source available
>>>> (without looking at the rating).
>>>> In my environment, a clock source is a TMU channel (e.g. TMU1) and,
>>>> indeed, clocksource_get_next works fine.
>>>
>>> Could you provide some more details as to why you need raw access to
>>> the clocksource, instead of using something like ktime_get(),
>>> getrawmonotonic() or sched_clock()?
>>
>> I wanted to directly read the value of the TCNT register of a TMU
>> channel registered as clocksource in an SH platform.
>> My idea was to get the first clocksource available and than use the read
>> hook to get its raw value.
>> Indeed, on SH4, the clocksource_get_next returns the TMU channel 1
>> (usually registered as clocksource); the clk->reads points to
>> sh_tmu_clocksource_read (drivers/clocksource/sh_tmu.c). This returns the
>> value of the timer count register (TCNT).
> 
> If you specifically want the function sh_tmu_clocksource_read(), why not
> call it directly instead of indirecting through the clocksource
> infrastructure?
> 
> Additionally, since the clocksource code is dynamic, you might get a
> different clocksource then you expect (such as jiffies) depending on
> user config or user actions.
> 
> So I'd advise against using the clocksource code here.

This means I should export the sh_tmu_clocksource_read (currently
static) and don't pass for the clocksource code at all.

Thanks for your prompt feedback.

Best Regards,
Giuseppe

> thanks
> -john
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