linux-kernel - Re: [PATCH 2/6] ARM: Tegra: Add CPU's OPPs for using cpufreq-cpu0 driver

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Date:	Wed, 07 Aug 2013 12:55:13 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
CC:	rjw@...k.pl, swarren@...dia.com, linaro-kernel@...ts.linaro.org,
	patches@...aro.org, cpufreq@...r.kernel.org,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	mturquette@...aro.org, linux-arm-kernel@...ts.infradead.org,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Ian Campbell <ian.campbell@...rix.com>,
	Mark Rutland <Mark.Rutland@....com>,
	Pawel Moll <pawel.moll@....com>,
	Rob Herring <rob.herring@...xeda.com>
Subject: Re: [PATCH 2/6] ARM: Tegra: Add CPU's OPPs for using cpufreq-cpu0
 driver

On 08/07/2013 12:06 PM, Viresh Kumar wrote:
> On 7 August 2013 23:12, Stephen Warren <swarren@...dotorg.org> wrote:
>> On 08/07/2013 08:46 AM, Viresh Kumar wrote:
>>> cpufreq-cpu0 driver needs OPPs to be present in DT which can be probed by it to
>>> get frequency table. This patch adds OPPs and clock-latency to tegra cpu0 node
>>> for multiple SoCs.
>>>
>>> Voltage levels aren't used until now for tegra and so a flat value which would
>>> eventually be ignored is used to represent voltage.
>>
>> This patch is problematic w.r.t. DT being an ABI.
> 
> :(
> 
>> We can certainly add new optional properties to a DT binding that enable
>> new features. However, a new version of a binding can't require new
>> properties to exist that didn't before, since that means that old DTs
>> won't work with new kernels that require the new properties.
> 
> To be honest I didn't get it completely. You meant operating-points
> wasn't present before? Its here:
> 
> Documentation/devicetree/bindings/cpufreq/cpufreq-cpu0.txt
> Documentation/devicetree/bindings/power/opp.txt
> 
> Or you meant, Tegra never required voltage levels and we are getting
> them in here.

The current Tegra *.dts files do not contain this property. The current
Tegra *.dts files must continue to work without modification in future
kernels.

>> As such, I believe we do need some Tegra-specific piece of code that
>> defines these OPP tables in the kernel, so that the operating-points
>> property is not needed.
> 
> Generic cpufreq driver depends on OPP library and so somebody has
> to provide them. Now you can do it by calling opp_add() for each OPP
> you have or otherwise.

Sure. That's what the Tegra-specific cpufreq driver should do. It should
be the top-level cpufreq driver. If parts of the code can be implemented
by library functions or a core parameterizable driver, then presumably
the Tegra driver would simply exist to provide those parameters and/or
callback functions to the generic driver.

> Btw, you must have some specific voltage level for each freq, we can
> get them here..

Yes, I'm sure we do, but I have no idea what they are:-( It may even be
board-specific or SoC-SKU-specific. I think we should defer this aspect
for now.
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