linux-kernel - Re: [PATCH V2] clk: Add composite clock type

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Message-ID: <51485443.6060603@nvidia.com>
Date:	Tue, 19 Mar 2013 17:34:19 +0530
From:	Prashant Gaikwad <pgaikwad@...dia.com>
To:	Tomasz Figa <t.figa@...sung.com>
CC:	Stephen Warren <swarren@...dotorg.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"sboyd@...eaurora.org" <sboyd@...eaurora.org>,
	"mturquette@...aro.org" <mturquette@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2] clk: Add composite clock type

On Wednesday 13 March 2013 10:00 PM, Tomasz Figa wrote:
> Hi Prashant,
>
> On Thursday 28 of February 2013 11:20:31 Stephen Warren wrote:
>> On 02/28/2013 12:58 AM, Prashant Gaikwad wrote:
>>> On Wednesday 06 February 2013 03:36 PM, Tomasz Figa wrote:
>>>> On Wednesday 06 of February 2013 08:34:32 Prashant Gaikwad wrote:
>>>>> On Tuesday 05 February 2013 03:45 PM, Tomasz Figa wrote:
>>>>>> Hi Prashant,
>>>>>>
>>>>>> Thank you for your patch. Please see some comments inline.
>>>>>>
>>>>>> On Monday 04 of February 2013 13:41:22 Prashant Gaikwad wrote:
>>>>>>> Not all clocks are required to be decomposed into basic clock
>>>>>>> types but at the same time want to use the functionality
>>>>>>> provided by these basic clock types instead of duplicating.
>>>>>>>
>>>>>>> For example, Tegra SoC has ~100 clocks which can be decomposed
>>>>>>> into Mux -> Div -> Gate clock types making the clock count to
>>>>>>> ~300. Also, parent change operation can not be performed on gate
>>>>>>> clock which forces to use mux clock in driver if want to change
>>>>>>> the parent.
>>>>>>>
>>>>>>> Instead aggregate the basic clock types functionality into one
>>>>>>> clock and just use this clock for all operations. This clock
>>>>>>> type re-uses the functionality of basic clock types and not
>>>>>>> limited to basic clock types but any hardware-specific
>>>>>>> implementation.
>>>>>>>
>>>>>>> diff --git a/drivers/clk/clk-composite.c
>>>>>>>
>>>>>>> +static u8 clk_composite_get_parent(struct clk_hw *hw)
>>>>>>> +{
>>>>>>> +     struct clk_composite *composite = to_clk_composite(hw);
>>>>>>> +     const struct clk_ops *mux_ops = composite->mux_ops;
>>>>>>> +     struct clk_hw *mux_hw = composite->mux_hw;
>>>>>>> +
>>>>>>> +     mux_hw->clk = hw->clk;
>>>>>> Why is this needed? Looks like this filed is already being initialized
>>>>>> in clk_register_composite.
>>>>> Some ops will get called during clk_init where this clk is not populated
>>>>> hence doing here. I have done it for all ops to make sure that any
>>>>> future change in clock framework don't break this clock.
>>>>> Now, why duplicate it in clk_register_composite? It is possible that
>>>>> none of these ops get called in clk_init.
>>>>> For example, recalc_rate is called during init and this ops is supported
>>>>> by div clock type, but what if div clock is not added.
>>>>>
>>>>> I hope this explains the need.
>>>> Sorry, I don't understand your explanation.
>>>>
>>>> I have asked why mux_hw->clk field has to be reinitialized in all the
>>>> ops.
>>>>
>>>> In other words, is it even possible that this clk pointer changes since
>>>> calling clk_register from clk_register_composite and if yes, why?
>>> Tomasz,
>>>
>>> calling sequence is as
>>>
>>> clk_register(struct clk_hw *hw)
>>>
>>>      clk_init(struct clk_hw *hw)
>>>      
>>>          .
>>>          .
>>>          hw->clk = clk;
>>>          clk->ops.recalc_rate(hw) => clk_composite_recalc_rate(hw) =>
>>>
>>> composite->div_ops.recalc_rate(div_hw) => clk_divider_recalc_rate(hw)
>>>
>>> Now if clk_divider_recalc_rate tries to access clk from hw then it will
>>> get NULL since this is not assigned yet and that is what I am doing in
>>> clk_composite_recalc_rate.
>>>
>>> I am doing it in all ops because I can not assume which one will get
>>> called first and always. If in future something changes the calling
>>> sequence in ccf framework then it will break this clock.
>> Surely the CCF core should be taking care of this as part of
>> clk_register() or clk_init()?
> Any news on this? It would be nice if this patch could be merged soon, because
> we'd like to rework Exynos clock code to use composite clocks before merge
> window, to have that merged for 3.10.
>
> If you don't have time to work on this, would you mind if I made any necessary
> fixes, added my sign-off next to yours and posted next version myself?

Tomasz,

Sorry for delayed reply. I will send the updated patch in next 2-3 days. 
After that if any rework required you can make the changes and add your 
sign-off.

Thanks for the patience!!

Regards,
PrashantG

> Best regards,

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