linux-kernel - Re: [RFC PATCH v2 0/2] clk: Support for DT assigned clock parents and rates

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Message-ID: <20140321014551.21989.57253@quantum>
Date:	Thu, 20 Mar 2014 18:45:51 -0700
From:	Mike Turquette <mturquette@...aro.org>
To:	Sylwester Nawrocki <s.nawrocki@...sung.com>,
	"Maxime Coquelin" <maxime.coquelin@...com>,
	linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org
Cc:	mark.rutland@....com, gregkh@...uxfoundation.org,
	t.figa@...sung.com, sw0312.kim@...sung.com,
	linux-kernel@...r.kernel.org, kyungmin.park@...sung.com,
	robh+dt@...nel.org, galak@...eaurora.org, grant.likely@...aro.org,
	linux@....linux.org.uk, m.szyprowski@...sung.com, t-kristo@...com
Subject: Re: [RFC PATCH v2 0/2] clk: Support for DT assigned clock parents and rates

Quoting Sylwester Nawrocki (2014-03-20 05:42:33)
> Hi Maxime,
> 
> On 06/03/14 14:45, Maxime Coquelin wrote:
> > Hi Sylwester,
> > 
> >       I like the principle of your implementation, but I have two questions:
> >       1 - How can we manage PM with this solution, as the parent/rate will be 
> > set only once at probe time?
> >       2 - How to set the parent of a parent clock (which can be shared with 
> > other devices)? Same question about the parent rates.
> 
> Thanks for your feedback and apologies for late reply.
> 
> IIUC your first concern is about a situation when clocks need to be
> reconfigured upon each resume from system sleep or runtime PM resume ?
> As I mentioned in v1 of the RFC I was considering having individual
> drivers calling explicitly the clocks set up routine. Presumably this
> would allow to resolve the power management related issue.
> One example I'm aware the approach as in this RFC wouldn't work is
> when a device in a SoC belongs to a power domain, which needs to be
> first switched on before we can start setting up and the clocks'
> configuration get lost after the power domain switch off.

I like Sylwester's approach of handling this one-time setup in the
driver core.

Any kind of fine grained power management should not be hidden by DT,
and by definition that logic belongs in the device driver. It can still
be nicely abstracted away by runtime pm[1].

Regards,
Mike

[1] Message-ID: <20140320114238.GQ7528@...00.arm.linux.org.uk>

> 
> OTOH I suspect devices for which one-time clocks setup is sufficient
> will be quite common. And for these there would need to be a single
> call to the setup routine in probe() I guess, since it wouldn't be
> possible to figure out just from the DT data when the actual call
> should be made.
> 
> For a global clocks configuration, I thought about specifying that
> in the clocks controller node, and then to have the setup routine
> called e.g. from of_clk_init(). I think that could work well enough,
> together with the patch [1], adding clock dependencies handling.
> But then the clock frequency set up function would need to be
> modified to respect the clock parent relationships, similarly as
> in patch series [2]. A just noticed [2] recently, after posting
> this RFC (adding Tero at Cc).
> 
> --
> Regards,
> Sylwester
> 
> [1] http://www.spinics.net/lists/arm-kernel/msg310507.html
> [2] http://www.spinics.net/lists/linux-omap/msg103069.html
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