Message-ID: <51400D9D.9060305@wwwdotorg.org>
Date: Tue, 12 Mar 2013 23:24:45 -0600
From: Stephen Warren <swarren@wwwdotorg.org>
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To: Bill Huang <bilhuang@nvidia.com>
CC: Russell King - ARM Linux <linux@arm.linux.org.uk>,
        "mturquette@linaro.org" <mturquette@linaro.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linaro-dev@lists.linaro.org" <linaro-dev@lists.linaro.org>,
        "linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
        "patches@linaro.org" <patches@linaro.org>
Subject: Re: [RFC 1/1] clk: Add notifier support in clk_prepare_enable/clk_disable_unprepare
References: <1363091861-21534-1-git-send-email-bilhuang@nvidia.com>  <20130312134032.GU4977@n2100.arm.linux.org.uk>  <1363139273.21694.11.camel@bilhuang-vm1> <514003B6.8020904@wwwdotorg.org> <1363151317.3311.9.camel@bilhuang-vm1>
In-Reply-To: <1363151317.3311.9.camel@bilhuang-vm1>
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On 03/12/2013 11:08 PM, Bill Huang wrote:
> On Wed, 2013-03-13 at 12:42 +0800, Stephen Warren wrote:
>> On 03/12/2013 07:47 PM, Bill Huang wrote:
>>> On Tue, 2013-03-12 at 21:40 +0800, Russell King - ARM Linux wrote:
>>>> On Tue, Mar 12, 2013 at 05:37:41AM -0700, Bill Huang wrote:
>>>>> Add the below four 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.
>>>>>
>>>>> PRE_CLK_ENABLE
>>>>> POST_CLK_ENABLE
>>>>> PRE_CLK_DISABLE
>>>>> POST_CLK_DISABLE
>>>>>
>>>>> Signed-off-by: Bill Huang <bilhuang@nvidia.com>
>>>>
>>>> NAK.  *Sigh* NO, this is the wrong level to be doing stuff like this.
>>>>
>>>> The *ONLY* thing that clk_prepare_enable() and clk_prepare_disable() should
>>>> *EVER* be doing is calling clk_prepare(), clk_enable(), clk_disable() and
>>>> clk_unprepare().  Those two functions are *merely* helpers for drivers
>>>> who don't wish to make the individual calls.
>>>>
>>>> Drivers are still completely free to call the individual functions, at
>>>> which point your proposal breaks horribly - and they _do_ call the
>>>> individual functions.
>>>
>>> I'm proposing to give device driver a choice when it knows that some
>>> driver might be interested in knowing its clock's enabled/disabled state
>>> change at runtime, this is very important for centralized DVFS core
>>> driver. It is not meant to be covering all cases especially for drivers
>>> which is not part of the DVFS, so we don't care if it is calling
>>> clk_enable/disable directly or not.
>>
>> I believe the point Russell is making is not that the idea behind this
>> patch is wrong, but simply that the function where you put the hooks is
>> wrong. The hooks should at least be in clk_enable/clk_disable and not
>> clk_prepare_enable/clk_disable_unprepare, since any driver is free to
>> call clk_prepare separately from clk_enable. The hooks should be
>> implemented in the lowest-level common function that all
>> driver-accessible paths call through.
> 
> Thanks, I know the point, but unfortunately there is no good choice for
> hooking this since those low level functions clk_enable/clk_disable will
> be called in interrupt context so it is not possible to send notify. We
> might need to come out a better approach if we can think of any.
> Currently I still think this is acceptable (Having all the drivers which
> are using our interested clocks call these function to enable/disable
> clock in their runtime_pm calls) though it's not perfect. 

No, that definitely won't work. Not all drivers use those APIs, nor
should they.

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