linux-kernel - Re: [RFC PATCH 08/11] arm: dts: exynos: Add parents and #interconnect-cells to Exynos4412

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Date:   Wed, 31 Jul 2019 15:00:56 +0200
From:   Artur Świgoń <a.swigon@...tner.samsung.com>
To:     Krzysztof Kozlowski <krzk@...nel.org>
Cc:     devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        cw00.choi@...sung.com, myungjoo.ham@...sung.com,
        inki.dae@...sung.com, sw0312.kim@...sung.com,
        georgi.djakov@...aro.org, m.szyprowski@...sung.com,
        Bartłomiej Żołnierkiewicz 
        <b.zolnierkie@...sung.com>
Subject: Re: [RFC PATCH 08/11] arm: dts: exynos: Add parents and
 #interconnect-cells to Exynos4412

On Wed, 2019-07-24 at 21:24 +0200, Krzysztof Kozlowski wrote:
> On Tue, Jul 23, 2019 at 02:20:13PM +0200, Artur Świgoń wrote:
> > This patch adds two fields tp the Exynos4412 DTS:
> 
> tp->to

Fixed. Thanks for catching the typo :)

> >   - parent: to declare connections between nodes that are not in a
> >     parent-child relation in devfreq;
> 
> Is it a standard property?
> The explanation needs some improvement... why are you adding parent to a
> devices which are not child-parent?

This is not a standard property. I actually wanted to call it 'neighbor' at first. If you take a look at [1] and search for 'Exynos4x12', you will see that there are two power lines, and each has exactly one parent block (the rest are modelled in devfreq as its children). In [2], the parent of each child is indicated using the 'devfreq' property, e.g.,

&bus_display {
	devfreq = <&bus_leftbus>;
	status = "okay";
};

The single piece missing to make this topology a connected graph (for
interconnect QoS purposes) is the 'parent' property I proposed in this patch.
bus_leftbus is dependent on bus_dmc, therefore using the term 'parent' is
justified IMHO.

The explanation could be improved by adding what Chanwoo Choi <
cw00.choi@...sung.com> expressed in a parallel reply to this patch:
> - If 'devfreq' property is used between buses,
>   it indicates that there are requirement of sharing of power line.
> - If 'parent' property is used between buses,
>   it indicates that there are requirement of interconnect connection.

[1] Documentation/devicetree/bindings/devfreq/exynos-bus.txt
[2] arch/arm/boot/dts/exynos4412-odroid-common.dtsi (subject of this patch)

Best regards,
-- 
Artur Świgoń
Samsung R&D Institute Poland
Samsung Electronics