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

Am Mittwoch, den 07.08.2013, 12:55 -0600 schrieb Stephen Warren:

On 08/07/2013 12:06 PM, Viresh Kumar wrote:

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On 7 August 2013 23:12, Stephen Warren [off-list ref] wrote:

quoted

On 08/07/2013 08:46 AM, Viresh Kumar wrote:

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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.

:(

quoted

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.

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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.

quoted

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.

From what I learned those voltage levels are dependent on both the

Speedo and the process ID of the specific Tegra processor. So you really
get a two dimensional mapping table instead of a single OPP.
Also you can not scale the CPU voltage on it's own, but have to make
sure the core voltage isn't too far away from. Then core voltage also
depends on the operating states of engines like GR2D or even display.

Regards,
Lucas
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