linux-kernel - Re: [PATCH] arm64: dts: imx8mn-var-som-symphony: add vcc supply for PCA9534

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Message-Id: <20231106163633.764707a8140dfe750f875dc3@hugovil.com>
Date:   Mon, 6 Nov 2023 16:36:33 -0500
From:   Hugo Villeneuve <hugo@...ovil.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        Hugo Villeneuve <hvilleneuve@...onoff.com>,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: dts: imx8mn-var-som-symphony: add vcc supply for
 PCA9534

On Mon, 6 Nov 2023 21:39:42 +0100
Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org> wrote:

> On 06/11/2023 19:57, Hugo Villeneuve wrote:
> > From: Hugo Villeneuve <hvilleneuve@...onoff.com>
> > 
> > The following warning is shown when probing device:
> > 
> >     pca953x 1-0020: supply vcc not found, using dummy regulator
> > 
> > Add fixed 3.3v regulator as vcc supply to silence the warning.
> > 
> 
> This is not really a warning. It's perfectly fine to use dummy
> regulator. Adding such fixed supply has sense in following cases:
> 1. Bindings require the supply,
> 2. This is in fact exactly that supply (verified on schematics).
> 
> Best regards,
> Krzysztof

Hi Krzysztof,
this is case #2, but looking more closely at the carrier board
(symphony) schematics, it is not exactly the "reg_3v3_fixed" supply I
mentioned in the patch, as "reg_3v3_fixed" is located on the SOM and
only powers SOM components.

However, "reg_3v3_fixed", which is an always-on regulator (no software
enable control), is used as the enable input to a mosfet switch (Q2) on
the carrier board to generate the 3.3V (label BASE_PER_3V3 on the
schematics) used by all peripherals on that carrier board.

I guess I could then define a new "always-on" regulator labeled as,
for example, base_per_3v3, and then assign the pca953x vcc-supply
property to it?

Hugo.