linux-kernel - Re: [PATCH v2 3/3] Revert "usb: common: usb-conn-gpio: Make VBUS supply optional"

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Date:   Tue, 25 May 2021 13:36:30 +0200
From:   Thierry Reding <treding@...dia.com>
To:     Chunfeng Yun <chunfeng.yun@...iatek.com>
CC:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Paul Cercueil <paul@...pouillou.net>,
        Lee Jones <lee.jones@...aro.org>, <linux-usb@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-mediatek@...ts.infradead.org>
Subject: Re: [PATCH v2 3/3] Revert "usb: common: usb-conn-gpio: Make VBUS
 supply optional"

On Mon, May 24, 2021 at 01:51:51PM +0800, Chunfeng Yun wrote:
> On Fri, 2021-05-21 at 15:20 +0200, Thierry Reding wrote:
> > On Wed, May 19, 2021 at 02:39:46PM +0800, Chunfeng Yun wrote:
> > > Vbus is already an optional supply, if the vbus-supply is not
> > > provided in DTS, will use a dummy regulator,
> > 
> > That statement is not entirely correct. The dummy regulator is
> > substituted only if the supply is in fact not optional. The idea behind
> > that is to allow DTS files that don't specify all required regulators to
> > get away with it, based on the assumption that the supply is one of
> > those always-on supplies that are often not described in DTS.
> Yes, you are right.
> But from the point of result, it indeed can help to handle the absent
> regulator.
> > 
> > > the warning log is as below:
> > > "supply vbus not found, using dummy regulator"
> > 
> > And the reason why we get that warning is to point out that the DTS has
> > a bug and that it should be fixed (by adding a proper regulator to take
> > the place of the dummy).
> > 
> > > This reverts commit 4ddf1ac79e5f082451cd549283d2eb7559ab6ca9.
> > 
> > But if you read the description of that commit, the purpose of that
> > patch was in fact to make the supply completely optional in the case
> > where we already have the VBUS supply specified for the USB port that
> > the connector is parented to.
> Could you please give an example you mentioned?

You can find examples of this in these:

	arch/arm64/boot/dts/nvidia/tegra210-p2597.dtsi
	arch/arm64/boot/dts/nvidia/tegra210-p3450-0000.dts
	arch/arm64/boot/dts/nvidia/tegra186-p2771-0000.dts

> It seems prefer to provide vbus supply in connector instead of port
> according to dt-binding

My recollection is that the above (or at least some of them) predate USB
connectors.

It's possible that we could convert the above to have the VBUS supply
listed in the connector instead of the port. However, since we have to
preserve backwards compatibility with older device trees, we can't
revert the commit anyway.

Thierry

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