Message-ID: <1363827789.8815.56.camel@bilhuang-vm1>
Subject: Re: [PATCH 1/1] clk: Add notifier support in
 clk_prepare/clk_unprepare
From: Bill Huang <bilhuang@nvidia.com>
To: Mike Turquette <mturquette@linaro.org>
CC: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
        "patches@linaro.org" <patches@linaro.org>,
        "linaro-dev@lists.linaro.org" <linaro-dev@lists.linaro.org>
Date: Wed, 20 Mar 2013 18:03:09 -0700
In-Reply-To: <20130320144752.11073.653@quantum>
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On Wed, 2013-03-20 at 22:47 +0800, Mike Turquette wrote:
> Quoting Bill Huang (2013-03-19 21:39:44)
> > On Wed, 2013-03-20 at 11:31 +0800, Mike Turquette wrote:
> > > Quoting Bill Huang (2013-03-19 19:55:49)
> > > > On Wed, 2013-03-20 at 01:01 +0800, Mike Turquette wrote:
> > > > > I'm still not sure about this approach.  Based on feedback I got from
> > > > > Linaro Connect I am not convinced that scaling voltage through clk
> > > > > rate-change notifiers is the right way to go.  As I understand it this
> > > > > patch only exists for that single purpose, so if the voltage-notifier
> > > > > idea gets dropped then I will not take this patch in.
> > > > > 
> > > > Thanks Mike, actually we won't use your "clk: notifier handler for
> > > > dynamic voltage scaling" patch instead we are trying to port our DVFS
> > > > into Non-CPU DVFS framework "devfreq" which will need to hook those
> > > > notifiers, without the clock notifiers been extended the framework is
> > > > useless for us since we cannot do polling due to the fact that polling
> > > > is not in real time. If it ended up extending the notifiers cannot
> > > > happen then the only choice for us I think would be giving up "devfreq"
> > > > and implement them in Tegra's "clk_hw".
> > > 
> > > I'm familiar with the devfreq framework.  Can you explain further how
> > > you plan to use devfreq with the clock notifiers?  What does the call
> > > graph look like?
> > > 
> > The call graph will look like this, when any DVFS interested clock rate
> > changes (including enable and disable) happen -> Tegra devfreq clock
> > notifier is called -> call into update_devfreq if needed -> in
> > update_devfreq it will call into .get_target_freq in Tegra
> > "devfreq_governor" -> and then call into .target of tegra
> > devfreq_dev_profile to set voltage and done. More details are as below.
> > 
> > We'll create devfreq driver for Tegra VDD_CORE rail, and the safe
> > voltage level of the rail is determined by tens of clocks (2d, 3d,
> > mpe,...), all the frequency ladders of those clocks are defined in DT
> > also the operating points for VDD_CORE is declared in DT where its
> > frequency will be more of a virtual clock or index.
> > 
> >         operating-points = <
> >         /* virtual-kHz  uV */
> >         0       950000
> >         1       1000000
> >         2       1050000
> >         3       1100000
> >         4       1150000
> >         5       1200000
> >         6       1250000
> >         7       1300000
> >         8       1350000
> > 
> > Register a Tegra governor where the callback .get_target_freq is the
> > function to determine the overall frequency it can go to, and
> > the .target callback in "devfreq_dev_profile" will be the function
> > really do the voltage scaling.
> > 
> > Tegra devfreq driver will register clock notifiers on all its interested
> > clock and hence when any of those clock rate changes, disabled, enabled,
> > we'll specifically call update_devfreq in the notifier.
> 
> Thank you for the explanation.  Do you plan to use actual devfreq
> governors (like simple-ondemand, or something custom) for changing OPPs,
> or do you just plan to use the clock framework as a trigger for DVFS?
> 
I think the existing governors are all polling basis that do not work
for us, we have only single voltage rails and have tens of clocks which
are used by different device drivers, the voltage has to be boost before
the frequency is increasing, and it can only be lower after all the
interested clocks rate are allowing it to or we might crash. Clock
notifier seem to be the only choice to build association with all those
interested device driver (in real time) who might make changes on clock
rate either by using runtime PM or the similar mechanism.

We do use OPP in our devfreq driver, we just need to do some extra work
on checking all the interested clocks for making sure we will be
choosing the correct operating point for the power rail.

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