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

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Message-ID: <20121112212212.20034.78522@nucleus>
Date:	Mon, 12 Nov 2012 13:22:12 -0800
From:	Mike Turquette <mturquette@...com>
To:	Shiraz Hashim <shiraz.linux.kernel@...il.com>,
	Rob Herring <rob.herring@...xeda.com>
CC:	<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

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

Regards,
Mike

> -- 
> regards
> Shiraz Hashim
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