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

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Date:	Wed, 7 Aug 2013 23:36:52 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Stephen Warren <swarren@...dotorg.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 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.

> 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.

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

> Similarly, we can't put invalid voltages into the DT, since if a later
> kernel version starts actually using that field, the HW will no longer
> work correctly. Unless perhaps we put 0 into the DT and make the binding
> define that 0 means "you can't change the voltage at all away from the
> boot value"?

Hmm.. so if you help me in getting actual voltage levels for these freqs
then this problem will be resolved. Otherwise I can check what will happen
if we pass zero to voltage.

> Is the operating-points property documented in
> Documentation/devicetree/bindings/ somewhere?

Check above.

> (Also Cc'ing the DT mailing list and maintainers)

Thanks.
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