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

[Ulf Hansson]

On 15 March 2013 10:39, Peter De Schrijver <pdeschrijver at nvidia.com> wrote:
> On Fri, Mar 15, 2013 at 06:22:47AM +0100, Stephen Warren wrote:
>> On 03/14/2013 07:20 PM, Bill Huang wrote:
>> > On Fri, 2013-03-15 at 01:54 +0800, Stephen Warren wrote:
>> >> On 03/14/2013 03:28 AM, Bill Huang wrote:
>> >>> On Thu, 2013-03-14 at 17:21 +0800, Peter De Schrijver wrote:
>> >>>> On Thu, Mar 14, 2013 at 03:15:11AM +0100, Bill Huang wrote:
>> >>>>
>> >>>>> I don't think deferring will work either, considering the usage of DVFS,
>> >>>>> device voltage is tightly coupled with frequency, when clock rate is
>> >>>>> about to increase, we have to boost voltage first and we can lower the
>> >>>>> voltage after the clock rate has decreased. All the above sequence have
>> >>>>> to be guaranteed or you might crash, so deferring not only make thing
>> >>>>> complicated in controlling the order but also hurt performance.
>> >>>>
>> >>>> But we could use notifiers in clk_prepare/clk_unprepare to set the voltage no?
>> >>>> As clk_prepare/clk_unprepare have to be called before clk_enable or after
>> >>>> clk_disable, the voltage can be raised to a safe level, before the clock
>> >>>> becomes active.
>> >>>
>> >>> Thanks Peter, actually I'm just about to propose my v2 RFC which add
>> >>> notifier in clk_prepare/clk_unprepare.
>> >>
>> >> Can't clk_set_rate() be called while the clock is prepared, or even
>> >> enabled? I don't see how your proposal would work.
>> >
>> > I think it works with just a little sacrifice on saving more power but
>> > that's related minor. Taking clk_prepare as an indicator on that clock
>> > will be enabled later, so we can raise the voltage to a safe level
>> > (according to the current rate or maybe default rate when clk_prepare is
>> > called, some time late when clk_set_rate() is called we can adjust again
>> > according to the requested rate change)
>>
>> Is clk_set_rate() only legal to call in non-atomic contexts then? The
>> header file doesn't say, although I guess since many other functions
>> explicitly say they can't, then by omission it can...
>
> Yes. Only clk_enable() and clk_disable() can be called in an atomic context.
>
> Cheers,
>
> Peter.
>
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Just wanted to add my reflection on this topic;

Some prerequisites; I think am in favor of using the clk API to
trigger DVFS changes and then I agree on that clk_prepare|unprepare
needs to be possible to track from a DVFS perspective. clk_set_rate is
not enough.

So if we decide to do the above (using the clk API to trigger DVFS
changes), I believe we should discuss two possible solutions;
- clk notifiers or..
- dvfs clock type.

I am trying to make up my mind of what I think is the best solution.
Have you considered "dvfs clock type"?
I put some comments about this for "[PATCH 2/5] clk: notifier handler
for dynamic voltage scaling" recently as well.

What could the advantages/disadvantages be between the two options?

Kind regards
Ulf Hansson