linux-kernel - Re: [PATCH v7 2/3] clk: introduce the common clock framework

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Date:	Tue, 20 Mar 2012 20:10:34 -0700
From:	Saravana Kannan <skannan@...eaurora.org>
To:	"Turquette, Mike" <mturquette@...com>
CC:	Shawn Guo <shawn.guo@...aro.org>, Paul Walmsley <paul@...an.com>,
	Russell King <linux@....linux.org.uk>,
	Linus Walleij <linus.walleij@...ricsson.com>,
	patches@...aro.org, Magnus Damm <magnus.damm@...il.com>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	linux-kernel@...r.kernel.org, linaro-dev@...ts.linaro.org,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	linux-arm-kernel@...ts.infradead.org,
	Arnd Bergman <arnd.bergmann@...aro.org>
Subject: Re: [PATCH v7 2/3] clk: introduce the common clock framework

On 03/20/2012 04:53 PM, Turquette, Mike wrote:
> On Tue, Mar 20, 2012 at 10:46 AM, Saravana Kannan
> <skannan@...eaurora.org>  wrote:
>> On Tue, March 20, 2012 7:02 am, Shawn Guo wrote:
>>> On Thu, Mar 15, 2012 at 11:11:19PM -0700, Mike Turquette wrote:
>>> ...
>>>> +struct clk_ops {
>>>> +    int             (*prepare)(struct clk_hw *hw);
>>>> +    void            (*unprepare)(struct clk_hw *hw);
>>>> +    int             (*enable)(struct clk_hw *hw);
>>>> +    void            (*disable)(struct clk_hw *hw);
>>>> +    int             (*is_enabled)(struct clk_hw *hw);
>>>> +    unsigned long   (*recalc_rate)(struct clk_hw *hw,
>>>> +                                    unsigned long parent_rate);
>>>
>>> I believe I have heard people love the interface with parent_rate
>>> passed in.  I love that too.  But I would like to ask the same thing
>>> on .round_rate and .set_rate as well for the same reason why we have
>>> it for .recalc_rate.
>>
>> In my case, for most clocks, set rate involves reparenting. So, what does
>> passing parent_rate for these even mean? Passing parent_rate seems more
>> apt for recalc_rate since it's called when the parent rate changes -- so,
>> the actual parent itself is not expected to change.
>
>  From my conversations with folks across many platforms, I think that
> the way your clock tree expects to change rates is the exception, not
> the rule.  As such you should just ignore the parent_rate parameter as
> it useless to you.
>
>> I could ignore the parameter, but just wondering how many of the others
>> see value in this. And if we do add this parameter, it shouldn't be made
>> mandatory for the platform driver to use it (due to other assumptions the
>> clock framework might make).
>
>  From my rough census of folks that actually need .set_rate support, I
> think that everyone except MSM could benefit from this.  Your concept
> of clk_set_rate is everyone else's clk_set_parent.

To clarify, MSM's set parent is a subset of steps needed to be done to 
finish set parent. So, it's not that we just randomly decided to swap 
the meanings of these two functions.

Also, I think don't think the difference in view of set_rate is due to 
the difference in the clock trees between platforms with complicated 
trees. I think it's more because of who actually decides the rates for 
the clock tree. It looks like some platforms pick a top-down approach to 
deciding the rates of the clock tre while others pick a bottom-up approach.

> Ignoring the new parameter should cause you no harm.

As long as this is guaranteed, I have no problems with Shawn's suggestion.

> It does make me
> wonder if it would be a good idea to pass in the parent rate for
> .set_parent, which is analogous to .set_rate in many ways.

I need to think a bit more about this.

-Saravana

-- 
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
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