linux-kernel - Re: [PATCH v4 2/2] dt: power: st: Provide bindings for ST's OPPs

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Message-ID: <20150810132247.GH3249@x1>
Date:	Mon, 10 Aug 2015 14:22:47 +0100
From:	Lee Jones <lee.jones@...aro.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Stephen Boyd <sboyd@...eaurora.org>,
	Rob Herring <robherring2@...il.com>,
	Nishanth Menon <nm@...com>, kernel@...inux.com,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Dmitry Eremin-Solenikov <dbaryshkov@...il.com>,
	Rafael Wysocki <rjw@...ysocki.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Sebastian Reichel <sre@...nel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Arnd Bergmann <arnd.bergmann@...aro.org>,
	Ajit Pal Singh <ajitpal.singh@...com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v4 2/2] dt: power: st: Provide bindings for ST's OPPs

On Mon, 03 Aug 2015, Viresh Kumar wrote:

> On 31-07-15, 09:37, Stephen Boyd wrote:
> > For qcom platforms, the frequency is almost always constant.
> > There may be some tables where we have a couple higher
> > frequencies than others because the speed bin is different.
> > Otherwise the voltage/current is changing based on the silicon
> > characteristics. So the biggest duplication is the frequency
> > property.
> > 
> > As far as I know there isn't any algorithm to generate the
> > voltage values. It's all hand tuned tables based on the silicon
> > characterization, so we're left to store these tables in DT and
> > pick the right one at runtime. With regards to the table
> > explosion, on qcom platforms we haven't worried that we have ~40
> > tables, but I'm not opposed to expressing it in a smaller set of
> > nodes, tables, etc. if that's what's desired.
> > 
> > Do we need vendor specific properties for that though? Or do we
> > need some sort of extended frequency/voltage properties that are
> > arrays of values that we can index into based on some silicon
> > characteristics? I like the name based approach because it's
> > simple. Use this OPP table because it's called
> > x-y-z-characteristics and be done. Cramming the tables into less
> > lines may save us some typing and dtb space, but I'm not sure
> > what else it does.
> 
> What about something like this:
> 
> diff --git a/Documentation/devicetree/bindings/opp/opp.txt b/Documentation/devicetree/bindings/opp/opp.txt
> index 0cb44dc21f97..bad7a8299b9c 100644
> --- a/Documentation/devicetree/bindings/opp/opp.txt
> +++ b/Documentation/devicetree/bindings/opp/opp.txt
> @@ -74,6 +74,8 @@ This describes the OPPs belonging to a device. This node can have following
>    reference an OPP.
>  
>  Optional properties:
> +- opp-cuts: One or more strings, describing the versions of hardware the OPPs
> +  can support.

This isn't very generic.

I'm guessing some vendors my have quite a few ways to differentiate
between board versions/revisions/cuts etc.

How about another array where a vendor can choose to identify a piece
of hardware however they see fit.

Example 1 (simple version):

/* Version 1 */
opp-version = <1>;

Example 2 (using the kernel's versioning):

/* 2.6.32-rc1 */
opp-version = <2 6 32 1>;

Example 3 (using ST's versioning):

/* Major 2, Minor 0, Cut 2, All substrates */
opp-version = <2 0 2 0xff>;

Qcom (or anyone else wanting to use names to identify their revisions)
can continue to use their node name method, as it doesn't break any
convention.

>  - opp-shared: Indicates that device nodes using this OPP Table Node's phandle
>    switch their DVFS state together, i.e. they share clock/voltage/current lines.
>    Missing property means devices have independent clock/voltage/current lines,
> @@ -100,6 +102,9 @@ properties.
>    Entries for multiple regulators must be present in the same order as
>    regulators are specified in device's DT node.
>  
> +  If used with 'opp-cuts', then the number of entries present here must match
> +  the number of strings present in 'opp-cuts'.
> +
>  - opp-microamp: The maximum current drawn by the device in microamperes
>    considering system specific parameters (such as transients, process, aging,
>    maximum operating temperature range etc.) as necessary. This may be used to
> 

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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