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

Stephen Warren
2013-08-07T17:42:49Z

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>;
> };