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

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Date:	Mon, 24 Mar 2014 14:57:29 -0700
From:	Mike Turquette <mturquette@...aro.org>
To:	Maxime Coquelin <maxime.coquelin@...com>,
	"Sylwester Nawrocki" <s.nawrocki@...sung.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 Maxime Coquelin (2014-03-21 07:09:26)
> Hi Mike,
> 
> On 03/21/2014 02:45 AM, Mike Turquette wrote:
> > 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>
> How can I access this reference?

I was trying to provide a reference to this thread which is tangentially
related to this topic:

http://www.spinics.net/lists/arm-kernel/msg317016.html

But it hadn't hit any of the mail archives at the time of my writing so
I just copied Russell's Message-ID from his email. If you have a copy of
that email then you can search on Message-ID's for that and you will
find it. :-)

Regards,
Mike

> 
> Thanks,
> Maxime
> 
> >
> >>
> >> 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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