linux-kernel - Re: possibility of parent clock selection through DT

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Message-ID: <CAPub149DJ21zG27FSLC-MeQFdkYXtim4TbxnZx3Bg7gtxK9BiA@mail.gmail.com>
Date:	Sun, 18 Nov 2012 09:44:54 +0530
From:	Shiraz Hashim <shiraz.linux.kernel@...il.com>
To:	Mike Turquette <mturquette@...com>
Cc:	Rob Herring <rob.herring@...xeda.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Viresh Kumar <viresh.kumar@...aro.org>,
	devicetree-discuss@...ts.ozlabs.org
Subject: Re: possibility of parent clock selection through DT

Hi Mike,

On Tue, Nov 13, 2012 at 2:52 AM, Mike Turquette <mturquette@...com> wrote:
> Quoting Shiraz Hashim (2012-11-06 22:36:10)
>> On Wed, Nov 7, 2012 at 11:42 AM, Shiraz Hashim
>> <shiraz.linux.kernel@...il.com> wrote:
>> > Hi Mike, Rob,
>> >
>> > Devices in a SoC can have multiple possible clock sources which is
>> > perfectly captured through clk framework.
>> >
>> > But the device itself may not be aware of the complex hierarchy above it.
>> > In this case how do you suggest a board (through DT) should select its
>> > preference.
>> >
>> > Is there some work already going on in this direction ?
>>
>> Just to make it clear, I already have referred the clock DT bindings and
>> Shawn Guo patch on removing clk look up registration from kernel code.
>>
>> Here I am talking about possibility of selecting desired clock hierarchy
>> by the boards about which device nodes are not aware.
>>
>
> One way to achieve this is to use clk_set_rate as a way to switch
> parents at run-time.  The OMAP CCF code currently does this when
> relocking PLLs and makes use of __clk_reparent to update the clock
> framework's representation of the hierarchy dynamically.
>
> Maybe something like the following is helpful to you:
> http://git.linaro.org/gitweb?p=people/mturquette/linux.git;a=blob;f=arch/arm/mach-omap2/dpll3xxx.c;h=f72dedb4eee892ce4cd5bdf22cc8c22510f3d526;hb=clk-omap-3.8#l542

Thanks for the information.

So the generic SoC part can be handled through set_rate but
what about boards preferences and restrictions. For e.g., it prefers
to route a clock from a particular clk source only and wants its own
fixed (as per its design) clk source selections.

-- 
regards
Shiraz Hashim
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