Re: [RFC 1/1] clk: Add notifier support in clk_prepare_enable/clk_disable_unprepare

[Stephen Warren <swarren@wwwdotorg.org>]

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.