linux-kernel - Re: [RFC v2 1/1] clk: Add notifier support in clk_prepare/clk

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Message-ID: <51437BB6.3040607@wwwdotorg.org>
Date:	Fri, 15 Mar 2013 13:51:18 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Bill Huang <bilhuang@...dia.com>, mturquette@...aro.org,
	patches@...aro.org, linux-kernel@...r.kernel.org,
	linaro-dev@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC v2 1/1] clk: Add notifier support in clk_prepare/clk_unprepare

On 03/15/2013 11:45 AM, Russell King - ARM Linux wrote:
> On Thu, Mar 14, 2013 at 02:31:04AM -0700, Bill Huang wrote:
>> Add the below two notifier events so drivers which are interested in
>> knowing the clock status can act accordingly. This is extremely useful
>> in some of the DVFS (Dynamic Voltage Frequency Scaling) design.
>>
>> CLK_PREPARED
>> CLK_UNPREPARED
>>
>> Signed-off-by: Bill Huang <bilhuang@...dia.com>
>> ---
>>  drivers/clk/clk.c   |    3 +++
>>  include/linux/clk.h |    2 ++
>>  2 files changed, 5 insertions(+)
>>
>> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
>> index ed87b24..3292cec 100644
>> --- a/drivers/clk/clk.c
>> +++ b/drivers/clk/clk.c
>> @@ -550,6 +550,7 @@ void clk_unprepare(struct clk *clk)
>>  {
>>  	mutex_lock(&prepare_lock);
>>  	__clk_unprepare(clk);
>> +	__clk_notify(clk, CLK_UNPREPARED, clk->rate, clk->rate);
>>  	mutex_unlock(&prepare_lock);
>>  }
>>  EXPORT_SYMBOL_GPL(clk_unprepare);
>> @@ -598,6 +599,8 @@ int clk_prepare(struct clk *clk)
>>  
>>  	mutex_lock(&prepare_lock);
>>  	ret = __clk_prepare(clk);
>> +	if (!ret)
>> +		__clk_notify(clk, CLK_PREPARED, clk->rate, clk->rate);
> 
> So, on prepare, we notify after we've prepared the clock.  On unprepare,
> we notify after the clock has been shut down.  Are you sure that's the
> correct ordering?  Would it not be better to view it in a stack-like
> fashion, iow:

> get
> 	prepare
> 		notify-prepare
> 			enable
> 			disable
> 		notify-unprepare
> 	unprepare
> put

Yes, these should be stacked/nested better for consistency.

But for DVFS, the voltage needs to be raised before the prepare body is
run, so that if clk_prepare actually enables the clock, the voltage is
already at the safe level required by that clock. Similarly, for
unprepare, you can only lower the voltage after having turned off the
clock, which is guaranteed after the unprepare body. So, I think you
want to move the notifier for prepare in the code above (and rename it
to pre/before_prepare?), rather than the notifier for unprepare.

In order to cover more cases, you might have both
{pre,post}_{un,}prepare notifiers, although I'm not sure when you'd use
the other two options.

>> diff --git a/include/linux/clk.h b/include/linux/clk.h

>> +#define CLK_PREPARED			BIT(3)
>> +#define CLK_UNPREPARED			BIT(4)

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