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

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Message-ID: <52028719.8070908@wwwdotorg.org>
Date:	Wed, 07 Aug 2013 11:42:49 -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 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.

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.

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"?

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

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

> diff --git a/arch/arm/boot/dts/tegra114.dtsi b/arch/arm/boot/dts/tegra114.dtsi
> index abf6c40..730e0d9 100644
> --- a/arch/arm/boot/dts/tegra114.dtsi
> +++ b/arch/arm/boot/dts/tegra114.dtsi
> @@ -438,6 +438,18 @@
>  			device_type = "cpu";
>  			compatible = "arm,cortex-a15";
>  			reg = <0>;
> +			operating-points = <
> +				/* kHz    ignored */
> +				 216000   1000000
> +				 312000   1000000
> +				 456000   1000000
> +				 608000   1000000
> +				 760000   1000000
> +				 816000   1000000
> +				 912000   1000000
> +				 1000000  1000000
> +			>;
> +			clock-latency = <300000>;
>  		};

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